I

d [Eng] 1. diameter of a dampener A device for progressively reduc­

D� valve inlet in inches. [Sci] 2.� In math, the abbreviation "deci-" for 10-1 (= 0.1). D 1. diameter of pipe 2. Derivative (rate) action in modulating process control in which the rate of change

(speed) of correcting a force is proportional to the error signal. 3. Thermocouple type for tungsten 3.percent rhenium versus tungsten 25 percent rhenium. .'

DAC Digital-to-analog converter, an elec­tronic device that converts a digital signal, often from a computer, into a proportional analog voltage or current.

daemon 1. A computing process that runs in the background, performing a specified operation at predefined times or in response to certain events. A UNIX term now used with other operating systems, from Greek mythology in which dcemons were guardian spirits. 2. A program mnning on a system providing network services that listens for requests to use a particular service. t

tDahlin's algorithm A digital control algo­j l rithm in which the requirement of minimum� ( response time used in the deadbeat algo-�

i:• I rithm is relaxed in order to reduce ringing in� ~. the system response. Widely used for paper

I machine control. fDAIS Database Access Integration Services;

profile for uniform access to computer data. : daisy chain 1. A serial interconnection of

devices. Signals are passed from one device 1·�

to another, generally in the order of high pri­ority to low priority. 2. A method for propa­gating signals along a bus, often used in applications in which devices are connected in series.

daisywheel printer A printer that provides slow but good quality print output, using a rotating wheel to type.

DAL Data access language; in computers, a database access standard that provides a transparent connection among a variety of databases and any applications depending on them.

Dalton's law A scientific principle stating that the total pressure exerted by a mixture of gases equals the sum of the partial pres­sures that would be exerted if each of the .'� individual gases present were to occupy the same volume by itself.

damped freq uency See frequency, damped. damped wave A wave in which the source

amplitude diminishes with each succeeding cycle.

ing the amplitude of spring oscillations after a load is abruptly applied or removed.

damper A device for introducing a variable resistance for the purpose of regulating the� volumetric flow of gas or air. The major types� are the (a) butterfly-type damper-a single­�blade damper that is pivoted about its center.� (b) curtain-type da~per~a damper, com­�posed of flexible material, that moves in a� vertical plane as it is.):'ol~ed. (c) flat-type� damper-a da'mrer that consists of one or� more blades, each pivoted about one edge.� (d) louver-type damper-a damper that con­�sists of several blades, each pivoted about its� cen~er and linked together for simultaneous� operation. (e) slide-type damper-a damper� that consists of a single blade that moves� substantially normal to the flow.�

damper loss The reduction in the static pres­sure of a·gas flowing across a damper.

damping� 1. (noun) The progressive reduction or suppression of oscillation in a device or system. [ANSI/ISA-51.1-1979 (RI993)] Damping is built into electrical circuits and mechanical systems to prevent rapid or excessive corrections, which may lead to instability or oscillatory conditions, for example, connecting a resistor on the termi­nals of a pulse transformer in order to remove natural oscillations or placing a mov­ing element in oil or sluggish grease in order to prevent mechanical overshoot of the mov­ing parts. 2. (adjective) Pertaining to or pro­ductive of damping. Note 1: The response to an abrupt stimulus is commonly said to be "critically damped" when the time response is as fast as possible without overshoot, "underdamped" when overshoot occurs, or "overdamped" when the response is slm-ver than critical. [ANSI/1SA-51.1-1979 (RI993)] 3. The energy-dissipating characteristic that,� together with natural frequency, determines� the limit of frequency response and the� response-time characteristics of a trans­�ducer. Note 1: In response to a step change of� the measurand, an underdamped (periodic)� system oscillates about its final steady value� before coming to rest at that value. An over­�damped (aperiodic) system comes to rest� without overshoot, and a critically damped� system is at the point of change between the� underdamped and overdamped conditions.� Note 2: Viscous damping uses the viscosity� of fluids (liquids or gases) to effect damping.� Note 3: Magnetic damping uses the current� induced in electrical conductors by changes� in magnetic flux to effect damping. [ISA­

127�

---

damping, relative / data�

37.1-1975 (R1982)]4. Reducing or eliminating vibrations, especially reducing noise or reverberations, by using sound-absorbing materials.

SP

PERCENT� SIGNAL� ---/~pv

F--'=-==-=:"=

TIME

DAMPING (A)

Overdamped Response

PV

PERCENT SP� SIGNAL�

.,:,.

TIME

DAMPING (B)

Underdamped Response

damping, relative For an underdamped sys­tem, a number that expresses the quotient of the actual damping of a second-order linear system or element by its critical damping. [ISA-37.1-1975 (R1982)]

damping factor 1. For the free oscillation of a second-order linear system, a measure of damping that is expressed (without sign) as the quotient of the greater by the lesser of a pair of consecutive swings of the output (in opposite directions) about an ultimate steady-state value. [ANSI/ISA-51.1-1979 (R1993)]2. In any damped oscillation, the ratio of the amplitude of any given half-cycle to the amplitude of the succeeding half-cycle.

damping fluid A fluid that is used to damp the single-degree-of-freedom spring/mass system, which usually surrounds the refer­ence side (transduction element side) of the sensing element. [ISA-37.6-1982 (R1995)]

damping integrity The ability of the acceler­ometer to produce a predicted output, with no transients, during or after changes in the attitude of the transducer because of bubbles, contamination, and so on. [ISA-37.5-1982 (R1995)]

damping magnet A permanent magnet that is used in conjunction with a moving con­

ductor to produce an opposing torque when there is relative motion between the magnet and the conductor. A secondary function of the damping magnet is to dissipate the kinetic energy resulting from eddy currents that may be induced in the moving conduc­tor.

damping ratio 1. The ratio of the actual damping to the damping that is required for critical damping. [ISA-37.1-1975 (R1982)]2. The ratio of the deviations of the indicator followingafl abr:upt change in the measur~

and in two consecutive swings from the posi­tion of equilibrium, the greater deviation being divided by the lesser. The deviations are expressed in angular measure.

DAQ Data acquisition; the gathering of dis­crete and analog parameters from a process. Sometimes incorrectly called SCADA; see SCADA.

dark current The current that flows in photo­sensitive detectors when there is no incident radiant flux (total darkness) .

d'Arsonval galvanometer A galvanometer that is made by suspending a light coil of wire on thin gold or copper ribbons in the field of a permanent magnet. when current is carried to the coil via the suspending rib­bons. The coil rotates, and the amount of rotation is indicated by reflecting a beam of light from a small mirror carried on the coil onto a fixed linear scale. Also known as a light-beam galvanometer.

d'Arsonval movement The mechanism of a permanent-magnet, moving-coil instrument such as a d'Arsonval galvanometer.

dashpot 1. A less preferred term for snubber. [ANSI/ISA-75.05.01-2000]2. A fluid-filled cylinder that contains a loose-fitting piston that is used to damp vibratory motion or to change the effect of a sharp change in load from an instantaneous change in position to a more gradual change.

dashpot relay A timing device that relies upon the restrictive action of an orifice upon a fluid to provide the delay. When the relay coil is energized, the armature piston moves against a reservoir of fluid, forcing it through a restriction. This slows down the action. Timing is achieved by variations in orifice size.

DAT Duration adjusting type of process con­trol output (time proportioning). Also, digital audio tape.

data 1. Information of any type. 2. A common term used to indicate the basic elements that can be processed or produced by a computer. 3. Factual information suitable for communi­

128

I

cation, interpretation, or processing. [ISA­TR50.02, Part 9-2000] 4. In computers and transmission, information that is represented in digital form, including voice, text, facsim­ile, and video.

data, skewed Data that has been biased and is not representative.

data abstraction In object-oriented program­ming (OOP), the reduction of an object description to unique and general character­

.- is tics that define a class. data access arrang"ement (DDA) Equipment

that is supplied by a common carrier to per­mit the attachment of privately owned equipment to the network.

data acquisition The function of obtaining data from sources external to a microproces­sor or computer system, converting it into binary form, and processing it.

data acquisition system A system that is " used for acquiring data from sensors via

amplifiers and multiplexers and any neces­sary analog-to-digital converters.

data averaging An optional mode of opera­tion for an automatic data logger, which allows readings from two or more da ta

t-, acquisition channels to be averaged in each ­il\' scan. Alternatively, it allows readings from

"i"

each of several channels to be averaged over \. a preselected number of successive scans.

data bank A comprehensive collection of data, for example, several automated files, a library, or a set of loaded disks. Synonymous with database.

database 1. Any body of information. 2. A specific set of information that is available to a computer. In computer systems, those stores of data that are placed into a system by the user and in distributed control systems (DeS) by the process being monitored. Data­bases usually consolidate many records into a common pool of data records that serves as a single central file. 3. A collection of interre­lated data that are stored together with con­trolled redundancy to serve one or more applications. The data are stored so they are independent of programs that use the data. A common and controlled approach is used to add new data and to modify and retrieve existing data within a database. A system is said to contain a collection of databases if they are disjointed in structure.

. database management A system that pro­vides meaningful information derived from the data included in a database.

data bus The highway that connects the vari­ous microcomputer components carrying the data signals.

129

data, skewed I data flow

data capture (logging) The systematic collec­tion of data to be used in a particular data processing routine, such as monitoring and recording temperature changes over a period of time.

data channel A bidirectional data path between I/O devices and the main memory of a digital computer. Data channels permit one or more I/O operations to proceed con­currently with SQmp.\ltqtion, thereby enhanc­ing computer perfoi'mahce,

data code A structured set of characters that are used to represent the data items of a data element, for example, the data codes "1,2, ... ,7" may be used to represent the data items "Sunday, Monday, . , . , Saturday."

data collection The act of bringing data from one or more points to a central point.

data communication The transmission of data from one point to another.

data communications equipment (DeE) The embodiment of the media, modulation, and coding-dependent portion of a fieldbus-con­nected device, which comprise the lower portions of the physical layer within the device, [ANSI/ISA-50.02,~Part2-1992]

datacompression The elimination of redun­dant data without loss of information. A few standard telemetry data compression algo­rithms are ZFN, ZVP, ZVA, FFN, FFP, FFA, FVP, and FVA.

data converter Any of numerous devices for transforming analog signals into digital sig­nals, or vice versa.

data directory A listing of data stored in a database.

data display module A device that stores computer output and translates this output into signals that are distributed to a pro­gram-determined group of lights, annuncia­tors, numerical indicators, and cathode ray tubes in operator consoles and remote sta­tions,

data distributor A manually or automatically controlled unit that is used to distribute spe­cific data channels to quick-look devices.

data element A unit of data that, in a certain context, is considered indivisible. [ISA­TR50,02, Part 9-2000]

data error A deviation from correctness in data, usually an error, that occurred before the data was processed.

data file The portion of computer memory that is allocated to a specific set of organized data, including the codes that identify file name and sometimes file type.

data flow A view of the system as a set of independent asynchronous processes, each

data gathering I data set ready (DSR)

communicating with each other via I/O streams. Coined by Tom DeMarco, Yourdon Press.

data gathering See data collection. datagram An interface that has no acknowl­

edge, no ordering, and no retransmission. data handling See data processing. data highway A communication link

between separate stations that are tied with a multidrop cable and/or optical connections. It eliminates the need for separate, indepen- , dently wired data links. Each station on a highway can function independently.

data input/output unit (OIlOU) A device that interfaces to the process for the sole pur­pose of acquiring or sending data.

data integrity A performance measure that is based on the rate of undetected errors.

data link (DL) 1. Approximately Layer 2 of the OSI model (ISO 7498). The network pro­cessing entity that establishes; maintains, and releases data link connections between (adjacent) elements in a network. Controls access to physical medium (layer 1). 2. Any serial data communications transmission path, generally between two adjacent nodes or devices and without intermediate switch­ing nodes. The data link includes the physi­cal transmission medium, the protocol, and associated devices and programs, so it is both a physical and logical link. 3. A fiber­optic signal transmission system that carries information in digital or analog form. The term data link usually refers to short-distance communications, spanning distances of less than a kilometer.

data link layer (DLL) The data link layer (DLL) controls the transmission of messages onto the fieldbus and manages access to the fieldbus through the Link Active Scheduler (LAS). The DLL used by FOUNDATION fieldbus is defined in IEC 61158 and ISA SP50. It includes Publisher /Subscriber, Cli­ent/Server, and Source/Sink services.

data logger 1. A system or subsystem that has the primary function of acquiring and storing data in a form that is suitable for later reduction and analYSis, such as com­puter-language tape. 2. A computer system that is designed to obtain data from process sensors and to provide a log of the data. Many data loggers can carry out some filter­ing and linearizing of the data.

data logging A recording of data about events that occur in time sequence. See also data collection.

data management A general term that collec­tively describes those functions of the control

program that provide access to data sets, enforce data storage conventions, and regu­la te the use of input/ output devices.

data mining The process of discovering meaningful new correlations, patterns, and trends by sifting through large amounts of data stored in repositories by using pattern­recognition technologies and statistical and mathematical technologies. Typically used in enterprise resource planning (ERP) systems to develop an optimum business plan.

dataphone digital senike Common carrier service in which data is transmitted in a digi­tal rather than analog· form.

data plate A less preferred term for nameplate. [ANSI/ISA-75.05.01-2000]

data processing The automatic or semiauto­matic organization of numerical data in a desired manner. Any system that can receive, store, or transmit <;lata as well as perform operations upon data and then present the results. The execution of a systematic sequence of operations performed upon data. Synonymous with information processing.

data processing system A network of machine components that is capable of accepting information, processing it accord­ing to plan, and producing the desired results.

data processor A device that is capable of performing data processing, for example, a desk calculator, punched-card machine, or computer.

data protection Any method for preserving computer data from destruction or misuse. Backing up computer files is one example of data protection.

data rate The average number of bits, charac­ters, or blocks of information per unit of time being transmitted, such as bits per second. It mayor may not be equal to Signal rate or baud rate.

data reduction The process of transforming masses of raw test or experimentally obtained data, usually gathered by automatic recording equipment, into useful, con­densed, or simplified intelligence.

data set (OS) 1. A collection of data in one of several prescribed arrangements to which the system has access. 2. A device that per­forms the modulation/demodulation and control functions that are necessary to pro­vide compatibility between data processing equipment and communications facilities. See also subset.

data set ready (OSR) A modern interface sig­nal defined in RS-232 that indicates to the

130

data signaling rate / dBmV "'1' • ~

attached terminal equipment that the modem is attached to the telephone circuit.

data signaling rate In communications, the data transmission capacity of a set of parallel channels. The data signaling rate is expressed in bits per second.

data sink In communications, a device that is capable of accepting data signals from a transmission device. It also may check these signals and originate error control signals. Contrast with data source.

data source In communications, a device that is capable of originating data signals for a transmission device. It also may accept error control signals. Contrast with data sink.

data storage Data from a computer that are usually stored on a magnetic medium such as disk or tape. Newer technology stores data on optical disks.

data stream The movement of a group of measurements in one multiplexer.

data streaming The capability of delivering time-based digital data, as on a audio or video cassette, as it's requested rather than requiring that all the data be downloaded first before it can be used (or played).

, "

data structure An aggregate whose elemen!s need not be of the same data type and whose elements may be uniquely referenced by an identifier. [ISA-TR50.02, Part 9-2000]

data structures The storage of related data in computer memory by using arrays, records, or data lists.

data table The organized form on which data is held in computer.

data terminal equipment (DTE) 1. The embodiment of the media, modulation, and coding-dependent portion of a fieldbus-con­nected device. It comprises the lower por­tions of the physical layer as well as all higher layers within the device. [ANSI/ISA­50.02, Part 2-1992]2. The device that pro­vides the data source or end point for a trans­mission link.

data terminal ready (DTR) A modern inter­face signal defined in RS-232 that indicates to the modem that the terminal equipment is ready for transmission.

data transmission The sending of data from one part of a system to another part.

data type 1. A set of values together with a set of permitted operations. [ISA-TR50.02, Part 9-2000] 2. Anyone of several different types of data, such as integer, real, double precision, complex, logical, and Hollerith. Each has a different mathematical signifi­cance and may be represented differently internally.

data unpacking The process of recovering individual items of data from packed infor­mation. [ISA-RP55.1-1975 (R1983)]

data warehouse A repository of extremely large amounts of data generated both inside and outside of an enterprise (corporation), generally for the purpose of developing some business plan to make complex opera­tions more profitable.

datum 1. A point, direction, or level that is used as a convenient,reference for measuring angles, distances, he'igh'ts, speeds, or similar attributes. 2. Any value that serves as a refer­ence for measuring other values of the same quantity.

datum plane A permanently established ref­erence level, usually average sea level, that is used to determine the value of a specific alti­tude, depth sounding, ground elevation, or water-surface elevation. Also known as "chart datum," "datum level," reference level, and "reference plane."

daughter A nuclide formed as a result of . nuclear fission or radioactive decay.

day A unit of time whose exact value depends on the system of time measurement being used-apparent solar time, mean solar time, apparent sidereal time, universal time, ephemeris time, or atomic time. Except for atomic time, the basis of the definition of day is the period during which the earth makes one rotation on its axis. For general pur­poses, one day equals 24 h or 86,400 s.

dB 1. Decibel, a logarithmic unit of ratio. The ratio can express the power, voltage, or cur­rent difference between two signals. In sig­nal-to-noise measurements, dB usually signifies the voltage difference between a desired signal and the root mean square (rms) value of noise. The higher the ratio, the less noise that is present in the signal. 2. A measurement developed to approximate the response of human senses, which is logarith­mic. That is, the doubling of signal power, voltage, or current does not double the vol­ume of the perceived sound or the brightness of the perceived light.

dBm The measured signal power compared with standard signal power of 0.001 watt into 600 ohms.

DBMS Database management system; a col­lection of hardware and software that orga­nizes and provides access to a database, simplifying the management of additions and rearrangements.

dBmV The measured signal voltage com­pared with 0.001 volts into 75 ohms.

131

I

DC (or de) / dead band, control valve

Ii·'

·f

DC (or de) 1. direct current; electric current flowing in one direction only and generally constant in value. 2. device control; in data communications, a category of control char­acters intended to turn other (usually subor­dinate) equipment on or off. 3. data communication

DCC Digital compact cassette; uses digital audio compression method (PSSC) for qual­itv similar to a CD but with less "data." Also, d~ty cycle control.

DCD Data carrier detect(ed); a modem inter­face control signal (typically, EIA RS-2321 422) indicating that a device is connected from DCE; is also called "received line signal detector" (RLSD). See DCE.

DCOM Distributed COQ1ponent Object Model; extends the Common Object Model (COM) for the network communication of independent, interoperable software objects. See COM.

D controller See controller, derivative (D). DC voltage A voltage that forces electrons to

move through a circuit in the same direction continuously, thereby producing a direct cur­rent.

DCX Intel file format for saving multiple PCX graphics in electronic memory; used for exchanging data between computers.

DD Device description; in Fieldbus Founda­tion context, the definition and description of function blocks and their parameters.

DDD Direct distance dialing; telephone ser­vice in North America that permits the sub­scriber to call other subscribers in different areas without operator assistance and per­mitting computers to do the same.

ODE Dynamic Data Exchange; Microsoft­developed interapplication communications protocol where data from one program auto­matically updates another; originally to move data from a spreadsheet to a word pro­cessor. ODE is the baseline protocol for OLE 1.0 but not for OLE 2.0 (it is supported there, however, to maintain upward compatibility). Has become more complex with the advent of Windows and Windows NT in industrial applications, causing wags to refer to it as "Different Dynamics to Everyone."

dd item Id A unique number that identifies the description of the associated object class, which is developed as part of its device description. [ISA-TR50.02, Part 9-2000]

DDL Data description language [ISA­TR50.02, Part 9-2000]; data definition lan­guage. Also, device description language from InterOperable Systems Project and HART, and now called VDS. See VDS.

DDP Distributed data processing; network of geographically dispersed, logically con­nected processors that share common resources.

DDR Digital data repeating; scheme on a sin­gle radio frequency that allows peer-to-peer communication between RTUs so as to share workload, compare output to inputs, moni­tor remote I/O, and relay data back and forth. Also known as digital disk recorders, stand- alone devices that connect to host computer via.Ethernet or SCSI interface SO "as to share files for video image processing and animation effects. See RTU, SCSI.

DDS Dataphone Digital Service; AT&T­developed communication service in which data is transmitted in digital rather than ana­log form, thus eliminating the need for mod(:ms:

DE Digitally enhanced; proprietary, digital multidrop instrument protocol by Honey­

. . well. DECnet Digital Equipment Corporation Net­

work; proprietary architecture to communi­cate between all of that company's

dd member Id A unique number that identi­fies the function of the associated object class, which is developed as part of its device description. [ISA-TR50.02, Part 9-2000]

de facto standard A standard that is widely adopted by the marketplace but has no offi­ciallegal status. Compare de jure standard.

de jure standard A standard whose recogni­tion has been extended among various orga­nizations, agreed upon by a consensus (substantial agreement, although not neces­sarily unanimous). Usually administered by official organizations, such as ISO IIEC, ANSI, lTU, IEEE, and so on. Compare de facto standard.

dead band In process instrumentation, the range through which an input signal may be varied, upon reversal of direction, without initiating an observable change in output sig­nal. Note 1: There are separate and distinct input-output relationships for increasing and decreasing signals. Note 2: Dead band produces phase lag between input and out­put. Note 3: Dead band is usually expressed in percentage of span. [ANSI/ISA-51.1-1979 (R1993)]

dead band, control valve The range through which a control valve's input signal may be varied, upon reversal of direction, without initiating an observable change in the posi­tion of the closure member. [ANSI/ISA­75.05.01-2000]

132

I', It "

J.

I "

, <

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dead band test Refer to ISA Standard ISA­RP55.1-1975 (R1983) for the dead band test procedure.

deadbeat algorithm A digital control algo­rithm that attempts to follow set-point changes in minimum time. It assumes that the controlled process can be modeled approximately as a first-order plus dead­time system.

dead ce~ter 1. Either of two positions of a crank at which the turning force between the crank and its connecting rod are zero. Dead ' center occurs when the centerline of the crank and the centerline of the connecting rod lie in the same plane. 2. A nonrotating center for holding a rotating workpiece.

dead-end shut off A nonstandard term used to rHer to control valve leakage. Refer to ANSI/FeI 70-2 and ANSI 816.104 for specifi­cation of leakage classifications. [ANSI/ISA­75.05.01-2000] :

dead-end tube A tube that has a closed end­for example, a tube in a porcupine boiler.

dead-front switchboard A switching panel that is constructed so that all of the live ter­minations are made on the rear of the panel.

deadman's brake A safety device that auto­matically stops a vehicle when the driver' does not have his or her foot on the pedal. It is also used on other operator-controlled mechanisms such as cranes and lift trucks.

deadman's handle A hand grip or handle that an operator must squeeze or press on continuously to keep a machine running.

dead man timer (DMT) A circuit that moni­tors the operation of the processor cards and signals if a failure occurs.

dead reckoning The process of determining a navigational position by constructing dis­tance vectors on a map or a similar represen­tation, starting from a known position and calculating the distance vectors from a log of headings and speeds versus time.

dead room See anechoic chamber. dead time 1. The interval of time between the

initiation of an input change or stimulus and the start of the resulting response. [ANSI/ ISA-51.1-1979 (R1993)]2. Any definite delay that is deliberately placed between two related actions in order to avoid overlap that might cause confusion or to permit a particu­lar different event, such as a control decision, switching event, or similar action to take place.

dead-time compensation The modification of a controller to allow for time delays between the input to a control system and the response to the signal.

dead band test / decalescence

dead-time correction A correction that is applied to an instrument reading to account for events or stimuli actually occurring dur­ing the instrument's dead time.

dead volume The total volume of the pres­sure port cavity of a transducer at room tem­perature and ambient barometric pressure. om barometric pressure applied. [ISA-37.1­1975 (R1982)]

dead zone Also called dead band. A range of va.1t;es around the set puint. When the con­

. ,- trolled variable is' wHhin this range, no con­trol action takes place..See zone, dead.

deadweight gauge A device that is used to generate accurate pressures for the purpose of calibrating pressure gauges. Freely bal­anced weights (dead weights) are loaded on a calibrated piston to give a static hydraulic pressure output.

deaeration Removing a gas-air, oxygen, or carbon dioxide, for example-from a liquid or semisolid substance, such as boiler feed­water or food.

debug 1. To locate and correct any errors in a computer program. 2. To detect and correct malfunctions in the computer itself. Related to diagnostic routine. 3. To submit a newly designed process, mechanism, or computer program to simulated or actual operating conditions for the purpose of detecting and eliminating flaws or inefficiencies.

debuggers 1. System programs that enable computer programs to be debugged. 2. Pro­grammers whose main task is the removal of bugs.

debugging The process of detecting, diag­nosing, and then correcting program faults.

debugging aid routine A routine to help pro­grammers debug their routines. Some typical aid routines are storage printout, tape print­out, and drum printout.

debugging on-line See on-line debugging. deburr To remove burrs, fins, sharp edges,

and the like from corners and edges of parts or from around holes, by any of several methods. Often involves the use of abrasives.

decade A group or assembly of 10 units, for example, a counter that counts to 10 in one column or a resistor box that inserts resis­tance quantities in multiples of powers of 10.

decade counter A counter that produces one output pulse for every 10 input pulses.

decade scaler A scaling device that produces one output pulse for each 10 input pulses.

decalescence The darkening of a metal sur­face as it undergoes a phase transformation when heated. The phenomenon is caused by

133�

decanting / decision instruction�

the isothermal absorption of the latent heat of transformation.

decanting The boiling or pouring off of liq­uid near the top of a vessel that contains two immiscible liquids or a liquid-solid mixture that has separated by sedimentation, without disturbing the heavier liquid or settled solid.

decarburizing The process of removing car­bon from the surface layer of a steel or other ferrous alloy by heating it in an atmosphere that reacts selectively with carbon. Atmo: spheres that are relatively rich in water vapor or carbon dioxide are typical deoxidizing atmospheres.

decay The spontaneous transformation of a nuclide into one or more other nuclides either by emitting one or more subatomic particles or gamma rays from its nucleus or by nuclear fission. Radioactive decay of a specific nuclide is characterized by its half life-the time it takes for one-half of the orig­inal mass to spontaneously transform.

decay ratio The ratio of the amplitude change of successive cycles of a damped oscillatory system.

DECAY RATIO =DIIDJ

0,

~I'" \.0 1----,1--7-~------:rC_-'--=.......,,==-----~.

ARBITRAAY RESPONSE LIMITS

'SE TIME

~ RESPONSE TIME ---+

DECAY RATIO

decay time The time in which a voltage or current pulse will decrease to one tenth of its maximum value. Decay time is proportional to the time constant of the circuit.

decelerating electrode An intermediate elec­trode in an electron tube that is maintained at a potential that induces decelerating forces

·;" on a beam of electrons. decelerometer An instrument for measuring

the rate at which speed decreases. decentralized 1. To distribute the functions

among several Cluthorities. 2. Decentralized maintenance distributes maintenance func­tions among areas of responsibilities or areas of the physical plant.

decibel (dB) 1. A unit of level, where Level in dB =10 log 10 P1/P ref PI =a power, or quan­tity directly proportional to power and Prd =

a reference power, or a corresponding refer­ence quantity proportional to power. [ANSI! ISA-51.1-1979 (R1993)] 2. A measure of mag­nitude ratio. Magnitude ratio is dB =20 10gl0 (magnitude ratio). [ISA-26-1968] 3. A unit for measuring the relative strength of a signal parameter, such as power, voltage, and so on. The number of decibels is twenty (10 for power ratio) times the logarithm (base 10) of the ratio of the measured quantity to the ref­erence level. Th~ reference level must always be indicated, such as 1 milliamp for current· ratio. See also power level.

decibel meter An instrument that is cali­brated in logarithmic steps and used for measuring power levels, in decibel units, of audio or communication circuits.

decimal 1. Pertaining to a characteristic or property that involves a selection, choice, or condition in which there are 10 possibilities. 2. Pertaining to the numeration system that has radix of 10. 3. See binary code decimal.

decimal balance A type of balance that has one arm that is 10 times as long as the other, so that heavy objects can be balanced with light weights.

decimal coded digit A digit or character that is defined by a set of decimal digits, such as a pair of decimal digits that specify a letter or special character in a system of notation.

decimal digit In decimal notation, one of the characters a through 9.

decimal notation A fixed radix notation, where the radix is 10. For example, in deci­mal notation, the numeral 576.2 represents the number 5 x 10 squared plus 7 x 10 to the first power, plus 6 x 10 to the zero power, plus 2 x 10 to the minus 1 power.

decimal number A number, usually consist­ing of more than one figure, that represents a sum, in which the quantity represented by each figure is based on the radix of 10. The figures used are a, I, 2, 3,4,5,6,7,8, and 9.

decimal numbering system A system of reckoning by 10 or the powers of 10 by using the digits 0-9 to express numerical quanti­ties.

decimal numeral A decimal representation of a number.

decimal point The radix point in decimal representation. See radix point.

decimal-to-binary conversion The process of converting a number written to the base la, or decimal, into the equivalent number writ­ten to the base two, or binary.

decision instruction An instruction that effects the selection of a branch in a program, for example, a conditional jump instruction.

134

I

decision table I default

decision table A table of all the contingencies decommutator Equipment for separating or that are to be considered in the description of demultiplexing commutated signals. a problem, together with the actions to be decompression Any method for relieving taken. Decision tables are sometimes used in pressure. place of flow charts for the purpose of describing and documenting problems.

deck A collection of cards, commonly a com­plete set, that have been punched for a defi­nite service or purpose.

deck scale A low-profile weighing device used for moderate to heavy loads-up to 20,000 lb. Because the load platform of such scales is 2 to 10 in. above floor level, loads must be lifted onto the scale or ramps must be provided to enable wheeled vehicles to move onto the platform and off again. The frame of a deck scale rests directly on the existing floor rather than in a pit, and most models can be moved to different locations as needed.

declaration As used in many programming languages, a statement indicating the type of data to be stored and the name to be used for that data.

declinometer An instrument that is similar to a surveyor's compass and is used to deter­mine the variation of magnetic directions . from true directions. The horizontal circle is constructed so that the line of sight can be aligned with the magnetic needle or with any other desired setting.

decode 1. To apply a code so as to reverse some previous encoding. 2. To determine the meaning of individual characters or groups of characters in a message. 3. To determine the meaning of instructions from the status of the bits that describe the instruction, com­mand, or operation to be performed.

decoder 1. A device that determines the meaning of a set of signals and initiates a computer operation based on them. 2. A matrix of switching elements that selects one or more output channels according to the combination of input signals present. Con­trast with encoder; clarified by matrix. 3. A device that is used to change computer data from one coded format into another.

decollate To separate multipart computer forms.

,: !:

decommissioning The permanent removal of a complete safety instrumented system (SIS) from active service. [ANSI/ISA-84.01-1996]

decommutation A reversal of the commuta­tion process. The separation of information in a commutated data stream into as many independent information channels as were originally commutated.

; .

decontamination Removing or neutralizing an unwanted chemical, biological, or radio­logical substance.

decoupling The technique of reducing pro­cess interaction by coordinating control l.oops.

decoupling con~~9J 'A technique in which interacting control loops are automatically compensated when anyone control loop takes a control action. .

2x2 DeCaf/pier

decrement 1. The quantity by which a vari­able is decreased. 2. A specific part of an instruction word in some binary computers, thus a set of digits.

decremeter An instrument for measuring the damping of a train of waves by determining its logarithmic decrement.

decryption Translating computer data from an unreadable format into a readable one.

dedicated In data processing, a device that performs only one function.

de-energize to trip Safety instrumented sys­tem circuits in which the outputs and devices are energized under normal operation. Removing the source of power (e.g., elec~ric­ity, air) causes a trip action. [ANSI/ISA­84.01-1996]

default 1. Value(s) or option(s) that are assumed by a system ."hen they are not par­ticularly specified. Usually important in very flexible systems that require the user to make so many selections that most decisions can be extremely confusing, time-consuming, or just unnecessary. Thus, the user is free to concentrate only on those selections appro­priate for the use at hand. 2. The value of an argument, operand, or field that a program assumes if the user does not supply a specific assignment. 3. The alternative that is assumed

135�

defect / degree of protection UP)�

when an identifier has not been declared to have one of two or more alternative attributes.

defect A departure of any quality characteris­tic from its specified or intended value that is severe enough to cause the object or service to be rejected.

definition 1. The resolution and sharpness of an image, or the extent to which an image is brought into sharp relief. 2. The degree with which a communication system reproduces sound images or messages.

deflashing Removing fins or protrusions from the parting line of a die casting or molded plastics part.

deflecting electrode An intermediate elec­trode in an electron tube whose surrounding electric field induces constant or variable deflecting forces on an electron beam.

deflecting force In a direct-acting recording instrument, the force produced at the mark­ing device, for any position of the scale, when its positioning mechanism acts in response to the electrical quantity being mea­sured.

deflecting yoke An assembly of one or more coils that induces a magnetic field to deflect an electron beam in a manner that is related to the oscillating frequency and magnitude of the current flov.... ing through the coils.

deflection 1. The movement of a pointer away from its zero or null position. 2. The elastic movement of a structural member

~ \ under load. 3. The shape change or change in.< '

'. "

diameter of a tubular member wi thou t frac­turing the material.

deflection factor The reciprocal of the instru­ment sensitivity.

deflection polarity In an oscilloscope, the relationship between the direction of elec­tron-beam displacement and the polarity of applied signal voltage.

deflectometer An instrument for determin­ing minute elastic movements that occur when a structure is loaded.

deflector A device for changing the direction of a stream of air or of a mixture of pulver­ized fuel and air.

defocus To cause a beam of electrons, light, X-rays, or other type of radiation to depart from accurate focus at a specific point in space, ordinarily the surface of a workpiece or test object.

defragmenting Moving the contents of files around on your hard disk so that each file is stored as one big chunk, for the purpose of speeding up the process by which the com­puter accesses the file.

defrost To remove ice from a surface, usually by melting or sublimation.

defuzzifier A function that calculates the output of a fuzzy controller.

degas To remove dissolved, entrained, or adsorbed gas from a solid or liquid.

degasification The process of removing gases from samples of steam taken for purity tests. Removing CO2 from water as in the ion exchange method of softening.

degasifier 1. An el~ment or compound that is added to mbHe"n metal to remove dissolved gases. 2. A process or type of vessel that removes dissolved gases from molten metal.

degenerate waveguide modes A set of waveguide modes that have the same propa­gation constant for all frequencies of interest.

degradation failure The gradual shift of an attribute or operating characteristic to a point where the device can no longer can ful­fill its intended purpose.

degreasing An industrial process for remov­ing grease, oil, or other fatty substances from the surfaces of metal parts, usually by expos­ing the parts to the condensing vapors of a polyhalogenated hydrocarbon solvent.

degree 1. -Anyone of several units for mea­suring the hardness of water, such as the English or Clark degree, the French degree, and the German degree. 2. One of the units used to measure specific gravity, such as the Baume scale. 3. A unit for measuring plane angles. 4. One of the units used to measure temperature or temperature difference in such scales such as Celsius (C) or Fahrenheit (F). Note: The units in the Kelvin scale are called kelvins (K). The symbol for degree is 0 •

degree of freedom 1. A number that is one less than the number of frequencies being tested with a chi-square test. 2. Anyone of the variables, including pressure, tempera­ture, composition, and specific volume, that must be specified in order to define the state of a system

degree of protection UP) A numerical classi­fication according to IEC 60529 preceded by the symbol IP applied to the enclosure of electrical apparatus to provide for 1. Protec­tion of persons against contact with or approach to live parts and against contact with moving parts (other than smooth rotat­ing shafts and the like) inside the enclosure. 2. Protection of the electrical apparatus against ingress of solid foreign objects and where indicated by the classification. 3. Pro­tection of the electrical apparatus against harmful ingress of water. Note: The enclo­sure which provides the degree of protection

136�

I

degree of protection of enclosure OP) I demand meter� r� I IP is not necessarily identical to the appara­I� tus enclosure for the types of protection

listed in clause 1. [ISA-12.00.01-1999 (IEC i,. 60079-0 Mod)]

degree of protection of enclosure UP) A numerical classification as defined by IEC standard 60529. It is preceded by the symbol IP and applied to the enclosure of electrical apparatus to provide for: (a) protection of persons against contact with or approach toj

I live and moving parts (other than smooth rotating shafts and the like) inside the enclo­sure, (b) protection of the electrical apparatus against ingress by solid foreign objects, and where indicated by the classification (c) pro­tection of the electrical apparatus against harmful ingress of water. [ISA-12.00.01-1999 (IEC 60079-0 Mod)]

dehumidification Reducing the moisture content of air, which increases its cooling power.

deicing The process of using heat, chemicals, or mechanical rupture to remove ice depos­its, especially those that form on motor vehi­cles and aircraft at low temperatures or high altitudes.

deionization time The time it takes for the grid in a gas tube to regain control of tube output after the anode current has been interrupted.

DEK Dansk Elektroteknisk Komite (Den­mark); the Danish member of IEC

delamination Separation of a material into layers, especially a material such as a bonded laminate.

delay The interval of time between a chang­ing signal and its repetition for some speci­fied duration at a downstream point of the signal path. The value L in the transform fac­tor exp (-L5). See time, dead. [ANSI/ISA-51.1­1979 (R1993)]

delay distortion A form of distortion in a transmitted radio wave that occurs when the rate of change of phase shift with frequency is not constant over the transmission-fre­quency range.

delayed combustion A continuation of com­bustion beyond the furnace. See also second­ary combustion.

delayed contacts Output contacts that trans­fer when the timer times out.

delay-interval timer A timing device that is electrically reset in order to delay the energi­zation or deenergization of a circuit for an interval of time up to 10 minutes after a spe­cific event such as restoring power after a power failure or turning a manual switch off.

delay line A transmission medium that delays a signal that passes through it by a known amount of time. Typically used in timing events.

delay-line memory A type of circulating memory that has a delay circuit as the chief element in the path of circulation.

delay-line register An acoustic or electric delay line, one or more words long, that is combined with appropriate input, output, and circulation circuits:

delay modulation A me'thod of data encod-. ing for the transmission or recording of serial data. A logic ONE (or ZERO) is represented by a signal transition at midbit time, and a logic ZERO (or QNE) followed by a logic ZERO (or ONE) is represented by a transi­tion at the end of the first ZERO (or ONE) bit.

delay-on-make timer A timing device that holds its main contacts open for a preset period'of time after it receives an initiating signal, then closes the contacts and allows current to flow in the main circuit. When the timer receives a stopping signal, the contacts open, and after a short interval the timer automatically resets so it can repeat the cycle.

delay on release A .mode of operation in which a delay timing period begins when a control switch is opened.

deletion punch A record elimination feature that is used on paper tape I/O devices to cause all tape channels to be punched. [ISA­RP55.1-1975 (R1983)]

delimiter A flag that separates and organizes items of data. Note: This definition is taken from IEEE standard 100-1984. [ANSI/ISA­50.02, Part 2-1992]

deliquescence The absorption of atmo­spheric water vapor by a crystalline solid until the crystal dissolves into a saturated solution.

delta network A set of three circuit branches that are connected in series, end to end, in order to form a mesh that has three nodes.

DEMKO Danmark Elektriske Materiellkon­troll (Denmark); Danish certification and testing laboratory for testing the equipment of different vendors to some common stan­dard.

demand A condition or event that requires the safety instrumented system (SIS) to take appropriate action so as to prevent a hazard­ous event from occurring or to mitigate the consequence of a hazardous event. [ANSI! ISA-84.01-1996]

demand meter Any of several types of instru­ments that are used to determine the amount of electricity used over a fixed period of time,

137�

demodulation / derivative action�

usually for the purpose of establishing a cus­tomer's bill.

demodulation The process of retrieving intel­ligence (data) from a modulated carrier wave. The reverse of modulation.

demodulator A device that recovers informa­tion from a carrier or subcarrier. A telemetry receiver has a demodulator. An FM discrimi­nator is a demodulator.

demultiplexer 1. The device that enables the telemetry operator to observe individuaI measurements from within a multiplexer. The opposite of a multiplexer. 2. A device that separates two or more signals that have been multiplexed together for the purpose of transmitting them through a single optical fiber. 3. A reverse multiplexer that allows the transfer of data from one microprocessor port to a number of output devices such as actuators.

densimeter An instrument for determinillg the density of a substance in absolute units, or for determining its specific gravity-that is, its relative density with respect to that of pure water. Also known as "density gauge," "density indicator," and "gravitometer."

densitometer An instrument for determin­ing the optical density of photographic or radiographic film by measuring the intensity of transmitted or reflected light.

density 1. The mass of a unit volume of a liq­uid at a specified temperature in units such as kilograms per meter3. The form of expres­sion for density is "density __ kg/m3 at __ kelvin." [ISA-RP31.1-1977]2. A physi­cal property of materials, measured as mass per unit volume. 3. The weight of a substance for a specified volume at a definite tempera­ture, for example, grams per cubic centimeter at 20°C. 4. Closeness of texture or consistency. 5. Degree of opacity, often referred to as "optical density."

density bottle See speCific-gravity bottle. density correction Any correction made to an

instrument reading to compensate for the deviation of density from a fixed reference value. The correction may be applied because the fluid being measured is not at standard temperature and pressure, because ambient temperature affects the density of the fluid in a fluid-filled instrument, or because of other similar effects.

density transmitter An instrument that is used to determine liquid density by measur­ing the buoyant force on an air-filled float immersed in a flowing liquid stream.

dependent uncertainty Uncertainty compo­nents are said to be dependent on each other

if they possess a significant correlation, for whatever cause, known or unknown. Typi­cally, dependencies form when effects share a common cause. [ISA-RP67.04.02-2000]

depolarizers Optical components that scram­ble the polarization of light passing through them, effectively turning a polarized beam into an unpolarized beam.

deposit 1. Any substance that is intentionally l<:id down on a surface by chemical, electri­cal, electrochemical, mechanical, vacuum, or vapor transfer methods. 2. A solid or semi­solid material that is accumulated by corro­sion or sedimentation on the interior of a tube or pipe.

.. deposited metal In a weldment, a filler metal added to the joint during welding.

deposit gauge Any instrument that is used to assess atmospheric quality by measuring the amount of particulate matter that settles out on a specific area during a defined period of time.

deposition rate 1. The amount of filler metal deposited per unit time by a specific welding procedure, usually expressed in pounds per hour. 2. The rate at which a coating material is deposited on a surface, usually expressed as weight per unit area per unit time, or as thickness per unit time.

deposition sequence The order in which increments of a weld deposit are laid down.

depth gauge An instrument or micrometer device that is capable of measuring distance below a reference surface to the nearest 0.001 inch. It is most often used to measure the depth of a blind hole, slot, or recess below the normal part surface that surrounds it, or to measure the height of a shoulder or projec­tion above the adjacent part surface.

depth of engagement The radial contact dis­tance between mating threads.

depth of thread The radial distance from crest to root of a screw thread.

derandomizer The circuit that removes the effect of data randomizing, thereby recover­ing data that had been randomized for tape storage.

derivative 1. Mathematically, derivative is the reciprocal of rate. 2. This control action will cause the output signal to change according to the rate at which input signal variations occur during a certain time inter­val.

derivative action A type of control system action in which a predetermined relation exists between the position of the final con­trol element and the derivative of the con­trolled variable with respect to time.

138�

I

derivative action gain / destructive testing�

derivative action gain See gain, derivative describing function For a nonlinear element action (rate gain)". [ANSI/ISA-51.1-1979 in sinusoidal steady state, the frequency (R1993)] response that is obtained by taking only the

derivative action (rate) A process control fundamental component of the output sig­response that speeds a correction based upon the deviation rate of change from set point, thereby eliminating overshoot in a system.

derivative action time See time derivative� action. [ANSI/lSA-51.1-1979 (R1993)]�

" derivative action time constant See time C01l­�i i stant, derivative action. [ANSI/ISA-51.1-1979

(R1993)]I

I derivative control Change in the output that

is proportional to the rate of change of the input. Also called rate control. See control action, derivative (D).

I ----------------- PV

: 10 PERCENT SP

SIGNAl.

OUT

. ! TIME

r Derivative Control Response to Step Change I .J

f derivative control action (rate i action) Control action in which the output

is proportional to the rate of change of the input. See control action and control action,., , derivative.

derivative controller See controller, derivative. derivative control mode A controller mode

in which controller output is directly propor­tional to the rate of change of the controlled variable error.

derivative time The time interval by which rate action advances the effect of propor­tional action on the final control element.

DES Data Encryption Standard; data security scheme approved by NBS and specified by FIPS. Also, distribution execution system; software packages for such functions as mar­keting and sales.

descaling The process of removing adherent deposits from a metal surface, such as thick oxide from hot rolled or forged steel or inor­ganic compounds from the interior of boiler tubes. Descaling may be done by chemical attack, mechanical action, electrolytic disso­lution, or other means, alone or in combina­tion.

descender In typography, that part of a lower­case letter that extends below the main body

I, I of the letter, as in the characters p and g.

; ,

nal. The describing function depends on the� frequency and amplitude of the input signal� or only on the amplitude of the input signal.�

design capacity The maximum weight load that a scale is designed to weigh in one hour within the desi/SI1atedclass accuracy. It is custo!TIarily 125 percent of normal capacity. Also known as '''scal~ capacity." [ISA­RP74.01-1984]

design control An approach to design that ensures that the initial simulator design and any subsequent changes to it are carried out in a systematic, controlled, and documented manner. [ANSI/ISA-77.20-1993]

design load The load for which a steam-gen­erating unit is deSigned. Design load is con­sidered to be the maximum load that a unit should carry.

design pressure The maximum allowable working pressure that is permitted under the rules of the ASME Construction Code. See pressure, design.

design steam temperature The temperature of the steam for which a boiler is designed.

design stress The maximum permissible load per unit area that a given structure can with­stand in service, including all allowances for such things as unexpected or impact loads, corrosion, dimensional variations during fabrication, and possible underestimations of service loading.

design thickness The sum of the thickness required to support service loads. This method of specifying material thickness is used particularly for designing boilers, chemical process equipment, and metal structures that will be exposed to atmo­spheric environments, soils, or seawater.

desired value See value, desired. desk top publishing The merging of text and

graphics on a computer in order to produce manuals and leaflets.

desorption The process of removing adsorbed material.

destination field In digital communications, a field in the message header that contains the address of the station to which the mes­sage is being directed.

destructive testing Any method for deter­mining a material property, functional attribute, or operational characteristic that renders the test object unsuitable for further use or severely impairs its intended service life.

139�

I

desuperheater / device

desuperheater 1. Equipment that is used to reactors to detect the concentration of deute­remove superheat from steam, usually by rium nuclei present. injecting water. 2. See attemperator. [ANSI! developed boiler horsepower The boiler ISA-77.44-1995] horsepower that is generated by a steam­

detectability The quality of a measured vari­able in a specific environment. The variable is determined by its relative freedom from interfering energy or other characteristics of the same general nature as the measured variable.

detector 1. The part of an analyzer that is used to lietect the presence and / or concentration of a component or components in a sample. Also see sensor or transducer. 2. A device that detects light and generates an electrical signal that can be measured or otherwise processed. 3. The stage in a receiver at which demodula­tion takes place. 4. An apparatus or system that is used to detect the presence of an object, radiation, chemical compound, or such. It may be quantitative and/or qualitative. 5. An optoelectronic transducer that is used in fiber optics to convert optical power into electric current. In fiber optics, a detector is usually a photodiode.

detector-amplifier A device in which an opti­cal detector is packaged together with elec­tronic amplification circuitry.

detector head The gas-responsive portion of a gas-detection instrument. The detector head is located in the area where sensing the presence of gas is desired. Its location may be integral to or remote from its control unit. Note: The detector head may incorporate the gas-sensing element and additional circuitry such as signal-processing or amplifying com­ponents or circuits. [ISA-92.02, 03, 04 and 06.01-1998]

detent A catch or lever that initiates or pre­vents movement in a mechanism, especially an escapement.

detergent A natural material or synthetic substance that has the quality of soap of being able to emulsify oil and remove soil from a surface.

deterioration The decline in the quality of a device, mechanism, or structure over time because of environmental effects, corrosion, wear, or gradual changes in material proper­ties. If allowed to continue unchecked, dete­rioration often leads to degradation failure.

deterministic model A mathematical model that, given a set of input data, will produce a single output or a single set of outputs (i.e., will act in only one very predictable way).

deutron detector A type of specialized radia­tion detector that is used in some nuclear

generating unit. development system A system that is used to

develop both the hardware and software for a microcomputer system. The development system may contain an editor, assembler and/or a high-level language, a compiler, and debugging.anp ir:cdrcuit emulation facilities. '. ~~.

deviation 1. Any depa~ture from a desired value or expected value or·pattern. [ANSI! ISA-51.1-1979 (R1993)]2. The difference between the loop set point and the process variable. [ANSI! ISA-77.44-1995] 3. In quality control, any departure of a quality character­istic from its specified value. 4. A statistical quantity that measures the random error that can be expected in numerous independent measurements of the same value under the' same conditions of measurement.

deviation, steady-state The system deviation after transients have expired. See offset. [ANSI!ISA-51.1-1979 (R1993)]

deviation, system In process instrumenta­tion, the instantaneous value of the directly controlled variable minus the set point. See also signal, actuating error. [ANSI!ISA-51.1­1979 (R1993)]

deviation, transient In process instrumenta­tion, the instantaneous value of the directly controlled variable minus its steady-state value. [ANSI/ISA-51.1-1979 (R1993)]

deviation alarm 1. An alarm that is set when­ever the deviation exceeds the preset limits. 2. An alarm that is caused when a variable departs from its desired value by a specified amount.

deviation amplitude Total variation from the desired set point of a system. Sometimes called "closeness in control" and expressed as follows: "closeness of control is ±2°C" or "system bandwidth of 4°C."

deviation controller A type of automatic con­trol device that acts in response to any differ­ence between the value of a process variable and the instrument set point, independent of their actual values.

deviation ratio The ratio given by M =f/fmav wherefis the maximum frequency difference between the modulated carrier and the unmodulated carrier andfmax is the maxi­mum modulation frequency.

device� 1. A physical entity connected that is to the fieldbus, which is composed of at least one communication element (the network

140�

I

device control character / dewatering

element) and may also have a control ele­ an electrical insulator and covered by wick­ment and/or a final element (transducer, ing that has been wetted with an aqueous actuator, etc.). [A SI/ISA-50.02, Part 2-1992] solution containing an excess of LiCL The 2. An apparatus for performing a prescribed dew point of the surrounding atmosphere is function. [A SI/ISA-51.1-1979 (R1993)]3. A component or assembly that is designed to perform a specific function by harnessing mechanical, electrical, magnetic, thermal, or chemical energy. 4. An apparatus for per­forming a prescribed function. See ANSI/ . ISA-S51.1-1979 (R1993). [ANSI/ISA-67.14.01J 2000]5. A component in a control system, such as a primary element, transmitter, con­troller, recorder, or final control element. 6. A physical entity that is capable of performing one or more specified functions in a particu­lar context and that is delimited by its inter­faces. [ISA-TR50.02, Part 9-2000]

device control character One of a class of control characters that is designed to control the peripheral devices associated with a data processing or telecommunications system, usually for switching such devices "on" or "off."

device controller A hardware unit that elec­tronically supervises one or more of the same types of devices. It acts as the link between the CPU and I/O devices.

Device Description (00) A device descrip­tion (OD) provides an extended description of each object in the Virtual Field Device (VFD) and includes information that a con­trol system or host needs to understand the

',' T

.• 1 meaning of data in the VFD. \' device driver A program or routine that con­�

trols the physical hardware activities on a� peripheral device such as a printer, mouse,� monitor, and the like. A device driver is gen­�erally the device-dependent software inter­�face between a device and the common,� device-independent I/O code in an operating�

.' system. device flags One-bit registers that record the

current status of a device. device handler A program or routine that

drives or services an I/O device. A device handler is similar to a device driver but pro­vides more control and interfacing functions.

device independence The ability to request input/output operations without regard to the characteristics of the input/output devices.

OeviceNet An "open" network on top of Controller Area Network. Created by Allen Bradley now owned and operated by Open

" DeviceNet Vendors Association.� , dew cell An instrument that consists of two�

bare electrical wires wound spirally around�

141

determined by passing an electric current� between the two wires. This raises the tem­�perature of the LiC! solution until its vapor� pressure is the same as that of the ambient� atmosphere.�

dew poil1t 1. The temperature, referred to as "specific pr!"ssure,".~t.~ hich water vapors condense. [ISA-7l:02-1991]2. That tempera­ture at which moisture from the vapor phase begins to condense.

dew point

dew point (at line pressure) The dew point value of the air at line pressure of the com­pressed air system (usually measured at the outlet of the dryer system or at any instru­ment air supply source before pressure is reduced). When presenting or referencing dew point, the value should be given in terms of the line pressure, for example, -40°C (_40°F) dew point at 100 psig.

dew point recorder An instrument that determines dew point temperature by alter­nately heating and cooling a metal plate. It uses a photocell to automatically detect and record the temperature as that condensed moisture appears and disappears on the tar­get. Also known as "mechanized dew point meter."

dew point temperature The temperature, referred to as a "specific pressure," at which water vapor condenses. [ANSI/ISA-7.0.01­1996]

dewars Insulated, thermoslike containers for cryogenic liquids, which can be designed to house detectors or lasers that require cooling.

dewatering 1. The process of removing water from solid or semisolid material-for instance, by centrifuging, filtering, settling, or evaporation. 2. Removing water from a riverbed, pond, caisson, or other enclosure through pumping or evaporation.

1

II

dewetting / diamond-pyramid hardness�

dewetting Generally, the loss of surface bleshooting. Synonymous with malfunction attraction between a solid and a liquid. Spe­ routine and related to debug. cifically, the flow of solder away from a sol­ diagnostics Information concerning known dered joint when it is reheated. failure modes and their characteristics. Such

DFA Digital fault analysis; design for assem­bly; the discipline of synchronizing and opti­mizing fabrication and assembly techniques.

DFM Design for manufacturing; the disci­pline of synchronizing and optimizing fabri­cation and assembly techniques.

Dfs Distributed file system; network version of UNIX that provides transparent file trans­fer capability by allowing files and other resources attached to one system on a net­work to be used by other systems as though they were local resources. Also, Design for service; the design of equipment to economi­cally allow easy access and repair.

DGIS Direct Graphics Interchange Standard proposed and created by Graphics Software Systems, Inc. (GSS).

DI-iCP Dynamic Host Configuration Proto­col; network protocol that allows a worksta­tion to be assigned an IP address dynamically from a centrally managed con­figuration server; used in Windows™ NT Advanced Server.

DHW Data highway for digital communica­tions; see data highway.

diagnostic 1. Pertaining to the detection and isolation of a malfunction or mistake. 2. A program or other system feature that is designed to help identify malfunctions in the system. An aid to debugging.

diagnostic alarm An alarm that is set when­ever the diagnostic program reports a mal­function.

diagnostic coverage For a safety instru­mented system (SIS) that has active fault­detection capabilities, the ratio of detectable faults to the total number of faults. [ANSI! ISA-84.01-1996]

diagnostic function test (DFT) A program for testing overall system reliability.

diagnostic message An error message in a programming routine to help the program­me; identify the error.

diagnostic programs 1. A troubleshooting aid for locating hardware malfunctions in a system. 2. A program to help users locate coding errors in newly developed programs. 3. Computer programs that isolate equip­ment malfunctions or programming errors.

diagnostic routine A routine that is used to locate a malfunction in a computer or to aid in locating mistakes in a computer program. Thus, in general, any routine that is specifi­cally designed to aid in debugging or trou­

information can be used in troubleshooting and failure analysis to help pinpoint the cause of a failure and to help define suitable corrective measures.

diagnostics, or diagnostic software The pro­gram by which a computer or other pro­grammable deyic:e.orsystem can literally "check itself," 'iri'C1rder to diagnose any defects that may be present.

diagnostic test The running of a machine program or routine for the purpose of dis­covering a failure or a potential failure of a machine element and to determine its loca­tion or its potential location.

diagonal stay A brace that is used in fire-tube boilers between a flat head or tube sheet and the shell.

dial 1. Generally, any circular scale. 2. The graduated scale adjacent to a control knob that is used to indicate the value or relative position of the c.ontrol setting.

dial indicator 1. Any meter or gauge that includes a graduated circular face and a piv­oted pointer to indicate the meter or gauge's reading. 2. A type of measuring gauge that is used to determine fine linear measurements, such as the radial or lateral runout of a rotat­ing member. A feeler is rested against a sur­face, and the change in position of a pivoted pointer relative to the calibrated gauge face is noted as the part is rotated, The gauge can also be adapted to other setups where precise relative position is to be determined.

dial network A network shared among many users that can establish a connection with another.

dialog box A video screen pop-up window within a selected view that carries a message appropriate to the actions that are occurring and to which a response can usually be made.

diamagnetic material A substance whose specific permeability is less than 1.00 and is therefore weakly repelled by a magnetic field.

diamond-pyramid hardness A material hardness that is determined by indenting a specimen with a diamond-pyramid indenter, which has a 136° angle between opposite faces. A hardness number is then calculated by dividing the indenting load by the pyra­midal area of the impression. Also known as "Vickers hardness."

142�

diamond-turned mirror / dictionary�

diamond-turned mirror A mirror in which I ..__ ._--..-.._....._..-4--'1 7~g:u~~v~

I PNEUMATICALLY i ELECTRICALLY.

the surface has been formed by machining away material "vith a diamond tool.

diaphragm 1. A sensing element that consists of a thin, usually circular, plate that is deformed by preSStlre differential applied across the plate. [ISA-37.1-1975 (R1982)] See illustration. 2. A thin, flexible disk that is sllpported around the edges and whose cen­ter is allowed to move in a direction perpen­diclilar to the plane of the disk. A diaphragm is used for a wide variety of purposes, such as detecting or reproducing sound waves, keeping two fluids separate while transmit­ting pressure or motion between them, or producing a mechanical or electrical signal that is proportional to the deflection pro­duced by differential pressure across the dia­phragm. 3. A partition of metal or other material placed in a header, duct, or pipe to separate portions thereof.

i PROCESS

Diaphragrn Sensor

diaphragm actuator A fluid-powered device in which the fluid acts upon a flexible com­ponent, called the diaphragm. [ANSI/ISA­75.05.01-2000]

diaphragm motor A diaphragm mechanism that is used to position a pneun1atically oper­ated control element in response to the action of a pneumatic controller or pneumatic posi­tioning relay.

diaphragm seal A thin flexible sheet of mate­rial that is clamped between two body halves to form a physical barrier bet\veen the instru­ment and process fluid.

diaphragm valve See bodYI Iueir type. [ANSI/ ISA-75.05.01-2000]

DIAT Direction Impulse Adjllsting Type pro­cess control output (ptl1sed signal).

DIB Device-independent bitmap format for saving graphics in electronic memory; used

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for exchanging data between computers.� Also l directory information base, for direc­�tory services.�

dibit Group of two bits providing four possi­ble states: 00, aI, 10, 11.

dibit A grouping of t\VO bits (i.e., 00, 01, 10, 11).

dichroic filter A filter that selectively trans­mits some wavelengths of light and reflects others. Typically, such filters are based on multilayer il1terference coatings.

dichromate treatment A technique for pro­ducing a corrosion-resistant conversion coat­ing on magnesium parts by boiling them in a sodium d.ichromate solution.

dictionary A list of code names used in a computer rOlltine or system together with

143�

I

die / differential pressure

their intended meaning in that routine or the point where the controller is triggered off system. and the point where it is triggered back on.

die A tool, usually containing at least one Also known as control dead band or neutral cavity, that imparts shape to either solid, zone. If the differential is too narrow, it can molten, or powdered metal or to elastomers or plastics, primarily because of the shape of the tool itself. A die is used together with a punch or a matching die in such operations as stamping, forging, forming, blanking, die casting, plastics molding, and coining. In cer­tain operations-die casting, powder metal­lurgy, and plastics forming, for instance­dies are sometimes referred to as "molds."

die casting 1. A casting process in which mol­ten metal is forced under pressure into the cavity of a metal mold. 2. A part made through this process.

dielectric An insulating material, or a mate­rial that can sustain an electric field with very little dissipation of power.

dielectric absorption The persistence of elec­tric polarization in certain dielectrics after the discharge of a capacitor.

dielectric coating An optical coating that is made up of one or more layers of dielectric (nonconductive) materials. The layer struc­ture determines what fractions of incident light at various wave lengths are transmitted and reflected.

dielectric constant A material characteristic that is expressed as the capacitance between two plates when the intervening space is filled with a given insulating material divided by the capacitance of the same plate arrangement when the space is filled with air or is evacuated.

dielectric strength See breakdown voltage rat­ing and insulation resistance.

DIF Data Interchange Format; method of sav­ing data to electronic memory expressly for exchanging data between computers.

difference equation An equation that expresses a functional relationship among one or more independent variables, one or more functions dependent on these vari­ables, and the successive differences between these functions.

difference limen The increment in a stimulus that is barely noticed in a specified fraction of independent observations where the same increment is imposed.

differential 1. Any arrangement of epicyclic gears that allows two driven shafts to revolve at different speeds. The speed of the main driving shaft is the algebraic mean of the speeds of the driven shafts. Also known as "differential gear." 2. For an ani off controller, differential refers to the difference between

cause "chatter,'~ which will cause high wear on components. If too wide, it causes poor control. With proportional control, the differ­ential effect is usually averaged out and is of little consequence.

differential amplifier ~ A device that com­pares two input signals and amplifies the dif­ference between them.

differential analyzer 'A computer (usually analog) that is designed and used primarily to solve many types of differential equations.

differential delay The difference between the maximum and the minimum frequency delays occurring across a band.

differential gain The amplification factor of a differential amplifier for a signal between its inputs.

differential gap The smallest increment of change in a controlled variable that is required to cause the final control element in a two-position control system to move from one position to its alternative position.

differential gap control See control, differen­tial gap.

differential input 1. The difference between the instantaneous values of two voltages that are biased by a common mode voltage. 2. To a signal conditioner: an input in which both sides are isolated from the chassis and power supply ground. The signal is applied as a dif­ferential voltage across the two sides. 3. Dif­ferential input allows an analog-to-digital converter to measure the difference between two input signals.

differential instrument Any instrument that has an output signal or indication that is pro­portional to the algebraic difference between two input Signals.

differential mode interference See interfer­ence, normal mode.

differential modulation A type of signal modulation in which the value that is assigned to any bit or dibit is dependent on the value of the previous signal element.

differential motion A mechanism in which the net motion of a single driven element is the difference behveen motions that would be imparted by each of two driving elements acting alone.

differential pressure 1. The difference in static pressure between two identical pres­sure taps at the same elevation, which are located in two different locations in a device. 2. The difference in pressure between two

144

'­

differential pressure gauge / diffuse-field response

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t

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points of measurement. 3. The static pressure difference that is generated by the primary device when there is no difference in elevation between the upstream and downstream pres­sure taps.

differential pressure gauge Any of several instruments that is designed to measure the difference in pressure between two enclosed spaces, independent of their absolute pres­sures.

differential pressure transmitter Any of sev­• .....eral transducers designed to measure the ~

pressure difference between two points in a process and to transmit a signal proportional to this difference, without regard to the abso­lute pressure at either point.

differential pressure-type liquid level meter Any of several devices designed to measure the head of liquid in a tank above some minimum level and to produce an indi­cation that is proportional to this value. Alternatively, the meter can measure and similarly display the head below some maxi­mum level.

differential producer A measuring element that is inserted into a process flow path and used to create a pressure drop that is propor­tional to the square of the volumetric flow rate. [ANSI/ISA-77.13.01-1999j

differential quantum efficiency Used to describe the quantum efficiency in devices that have nonlinear outputlinput character­istics. The slope of the characteristic curve is the differential quantum efficiency.

differential screw A type of compound screw that produces a motion that is equal to the difference in motion between the two components of the compound screw.

differential windlass A windlass that has a barrel consisting of two sections of different diameter. The pulling rope passes around one section, then through a pulley and around the other section. The pulley is attached to the load.

differentiation 1. The act of taking a deriva­tive. 2. A mathematical process for convert­ing displacement into velocity and velocity into acceleration. In a fast Fourier transform (FFT) spectrum analyzer, a single differentia­tion is represented by multiplying by jOJ.

differentiator A device whose output func­tion is proportional to the derivative, that is, the rate of change, of its input function with respect to one or more variables (usually with respect to time).

diffracted beam In X-ray crystallography, a beam of radiation that is composed of a large

number of scattered rays mutually reinforc­ing one another.

diffracted wave The wave component that exists in the primary propagation medium after an interaction between the wave and a discontinuity or a second medium. The dif­fracted wave coexists in the primary medium with incident waves and with waves that are reflected from suitable plane boundaries.

diffraction 1. The deviation of light from the paths and foci that are prescribed by rectilin­ear propagation. The phenomenon that is responsible for the bright and dark bands found within a geometrical shadow. 2. A phenomenon that is associated with the scat­tering of waves when they encounter obsta­cles whose size is about the same orde'r of magnitude as the wavelength. In effect, each scattering point produces a secondary wave that is superimposed on the unscattered por­tion of the incident wave. The intensity of the scattered wavevar:ies with the direction from the scattering p6int. Diffraction effects form the basis of X-ray crystallography, and they also tend to produce aberrations that must be dealt with in designing and constructing high-quality acoustical and optical systems.

diffraction grating An array of fine, parallel, equally spaced reflecting or transmitting lines that diffract light into a direction that is characteristic of the spacing of the lines and the wavelength of the diffracted light.

diffraction-limited beamA beam with a far-field spot size that is dependent only on the theoretical diffraction limit. This limit is the function of output wavelength divided by the diameter of the output aperture.

diffraction X-ray machine An apparatus that consists of an X-ray tube, power supply, con­trols, and auxiliary equipment and used in the study of crystals, semiconductors, and polymeric materials.

diffused-semiconductor strain gauge A component used in manufacturing transduc­ers (principally, diaphragm-type pressure transducers) that consists of a slice of silicon about 2.5 to 22 rom in diameter, into which an impurity element such as boron has been diffused. Modem photolithographic-mask­ing techniques make it possible to simulta­neously produce hundreds of full four-arm Wheatstone bridge patterns, complete with leadwire soldering pads, on a single slice of silicon about 50 to 75 mm (2 to 3 in.) in diam­eter.

diffuse-field response A frequency response of a piezoelectric sound-pressure transducer

145

I

I

II

diffuser / digitalization

in which the sound emanates from random directions. [ISA-37.10-1982 (RI995)]

diffuser 1. A duct, chamber, or enclosure in which a low-pressure, high-velocity flow of a fluid, usually air, is converted into high-pres­sure, low-velocity flow. 2. As applied to oil or gas burners, a metal plate with openings placed so as to protect the fuel spray from high-velocity air while admitting sufficient air to promote the ignition and combustion of fuel. Sometimes termed impeller.

diffusion 1. A process by which the atmo­sphere being monitored is transported by natural random molecular movement to and from the gas-sensing element. [ANSI/ISA­92.02.01, Part 1-1998; ANSI/ ISA-92.03.01­1998; ANSI/ISA-92.04.01, Part 1-1996; ISA­92.06.01-1998] 2. The conversion of gas-flow velocity into static pressure, as in the diffuser casing of a centrifugal fan. 3. The movement of ions from a point of high concentration to a point of low concentration. 4. The spontane­ous migration of atoms, molecules, or ions, under the driving force of compositional dif­ferences, in which only the energy of thermal excitation is used to cause atom movements.

diffusion pump A vacuum pump in which a stream of heavy particles such as oil or mer­cury vapors carries gas molecules out of the vacuum chamber

digit 1. A character that is used to represent one of the nonnegative integers smaller than the radix. For example, in decimal notation, one of the characters from 0 to 9. [ISA­RP55.1-1975 (RI983)] Synonymous with "numeric character." 2. See binary digit, equiv­alent binary digits, sign position, and significant digits.

digital 1. Referring to communications proce­dures, techniques, and equipment by which information is encoded as either binary "1" or "0." The representation of information in discrete binary form, discontinuous in time. 2. A term applied to a signal or device that uses binary digits to represent continuous val­ues or discrete states. 3. Pertaining to data that is in the form of digits. Contrast with analog. [ISA-RP55.1-1975 (RI983)] 4. A method of measurement that uses precise quantities to represent variables. 5. Binary. 6. A reference to

special-purpose digital logic when the com­puter system fails.

digital computer 1. A computing device that uses numerical digits to represent all vari­ables discretely. 2. A computer in which dis­crete representation of data is mainly used. 3. A computer that operates on discrete data by performin~ arithmetic and logic processes on them. Contrast with analog computer.

digital control Automatic process control loaps performed by digital control devices,

• c:. such as a compu.ter, microcomputer, or microprocessor. Compare with analog control.

digital controller A control device that con­sists of a microprocessor together with asso­ciated AID input converters and DI A output converters. A digital controller receives one.?r. more analog inputs that are related to current process variables, uses the digitized information to compute an output signal using a predetermined control algo­rithm, and converts!he result into an analog signal that operates the final control element. The digital controller device also may be adapted to furnish additional outputs such as alarms, totalizer signals, and displays.

digital data Data that is represented in dis­crete discontinuous form, as contrasted with analog data, which is represented in continu­ous form. Digital data is usually represented by means of coded characters, for example, numbers, signs, symbols, and so on.

Digital Data Communications Message Proto­col (DDCMP) A character-oriented com­munications protocol standard.

digital delay generator An electronic instru­ment that can be programmed digitally to delay a signal by a specific interval time­delay generator.

digital differential analyzer 1. An incremen­tal computer in which the principal type of computing unit is a digital integrator, which operates much like an integrating mecha­nism. 2. A differential analyzer that repre­sents analog quantities digitally.

digital filter An algorithm that reduces undesirable frequencies in the signal.

digital indicator A device that displays the value of a measured variable in digitized form. In most instances, the measurement

II1\

J, ,

j 1

the representation of data by discrete pulses, range is not displayed simultaneously, which I J

as in the presence or absence of a signal level to indicate the Is and Os of binary data. 7. A type of readout in which the data is displayed as discrete, fully informed alphanumeric char­acters.

digital backup An alternate method of digi­tal process control that is initiated by using

is considered an inherent disadvantage. digital input A number value input. See

input, digital. digitalization The process of transforming an

analog (video) signal into digital informa­tion.

146 J

I

digital logic / dimensional stability

digital logic A signal level is represented as a digital subset See data set. number value with a most signi.:icant and digital tachometer Any of several instru­least significant bit. Binary digital logic uses ments that are designed to determine rota­numbers that consist of strings of Is and Os. tional speed and display the indication in

digital manometer A manometer that is equipped with a sonar device that measures column height and produces a digitized dis­play.

digital motor See stepping motor. digital multimeter (OMM) An instrument

that measures resistance, AC and DC volt­age, and current in several selectable ranges and displays the results in numeric form.

digital multiplexer A data selection device that permits multiple groups of digital devices to share a common information path, such as from a computer CPU to any of sev­eral groups of digital output devices.

digital output Transducer output that repre­sents the magnitude of the measurand through a series of discrete quantities coded in a system of notation. Note: Digital output is distinguished from analog output. [ISA­37.1-1975 (R1982]

digital panel meter (OPM) A panel-mounted instrument that displays the value of a single

i type of measurement in numeric form. tdigital readout An electrically powered

device that interprets a continuously variable signal and displays its amplitude, or another signal attribute, as a series of numerals or other characters. These numerals or charac­ters correspond to the measured value and can be read directly. The accuracy of mea­surement is limited by the decimal position of the rightmost character in the display rather than by characteristics of the measure­ment circuit alone.

digital recorder An instrument that records data in digital form.

digital resolution The value of the least sig­I. nificant digit in a digitally coded representa­tion.

digital scrubbing In video development, the process of reading back and forth over a small portion of audio to locate a precise edit point. Same as "audio scrubbing."

digital service unit The interface between a user's data terminal equipment and a digital data service, usually through a channel ser­vice unit.

digital signal See signal, digital. digital speed transducer See digital tachome­

ter. digital storage oscilloscope (050) An oscil­

loscope that digitizes an input signal and stores the data in memory for later display or analysis.

digital form. digital-to-analog converter (O/A or DAC) 1.

A device or subsystem that converts binary (digital) data into continuous analog data, as, for example, to drive actuators of various types, motor-speed controllers, and so on. 2. An electronic device that converts a binary-coded word'ihto an analog voltage that is proportional to the binary value of that word.

digital valve A single valve casing that con­tains multiple solenoid valves whose flow capacities vary in binary sequence (1, 2,4,8, 16, ... ). To regulate flow, the control device sends operating signals to various combina­tions of the solenoids. Applications of digital valves are limited to very clean fluids at moderate temperatures and pressures, but within these limitations precise flow control and rapid response are possible. An eight­element valve, for example, yields flow reso­lution of 0.39 percent (1 part in 256).

digitize To convert an analog measurement of a physical variable into a numerical value, thereby expressing the quantity in digital form. See analog-to-digital converter.

digitized signal The representation of infor­mation by a set of discrete values, in accor­dance with a prescribed law. Every discrete value represents a definite range of the origi­nal undigitized signal. See analog-to-digital converter.

digitized typesetting In typographic imag­ing, the process of creating typographic char­acters and symbols by arranging pixels.

digitizer A device that converts an analog measurement into digital form.

dilatant substance A material that flows under low shear stress but whose rate of flow decreases with increasing shear stress.

dilatometer An apparatus for accurately measuring the thermal expansion of materi­als.

dilution 1. Adding solvent to a solution to lower its concentration. 2. Melting low-alloy base metal or previously deposited weld metal into high-alloy filler metal so as to pro­duce a weld deposit of intermediate compo­sition.

dimensional stability The ability of a mate­rial to retain its size and shape over an extended period of time under a defined set of environmental conditions, especially tem­perature.

147

I

dimetcote / direct current plasma display panel (DCPDP)

dimetcote An inorganic zinc coating that is DIP switches Miniature circuit switches in a composed of two materials--a reactive liquid DIP (dual in-line) configuration that are and a finely divided powder-which are mounted on circuit boards. mixed together. The mixture reacts in place dip tube See bubble tube. with a steel surface to form an insoluble coat­ing.

diminished radix complement A number that is obtained by subtracting each digit of the given number from one less than the radix. Typical examples are the nines-com­plement in decimal not~tion and the ones-complement in binary notatio·n.

DIN Deutsches Industrie Norms; Deutsches Institut fur Normung e.Y. (Germany). Ger­man industrial standards often used interna­tionally.

diode Any electronic device that has only two electrodes and a voltage characteristic that allows AC current to pass only in the forward direction (as in a flow check valve).

diode laser A laser in which stimulated emis-­sion is produced at a p-n junction in a semi­conductor material. Only certain materials are suited for diode-laser operation, among them gallium arsenide, indium phosphide, and certain lead salts.

diode laser array A device in which the out­put of several diode lasers is brought together in one beam. The lasers may be inte­grated into the same substrate, or discrete devices may be coupled optically and elec­tronically.

diopter A measurement of the refractive power of a lens that is equal to the reciprocal of the focal length in meters. A lens with a 20-centimeter focal length has the power of five diopters, while one with a 2-meter focal length has a power of 0.5 diopter.

DIP Dust-ignition-proof; dual in-line pack­age, standard for or package for integrated circuits ranging in size from 8 to 48 pins.

dip brazing Producing a brazed joint by immersing the assembly in a bath of hot mol­ten chemicals or hot metal. A chemical bath may provide the brazing flux. Molten metal may provide the brazing alloy.

dip coating Covering the surface of a part by immersing it in a bath that contains the coat­ing material.

dip needle A device for indicating the angle, in a vertical plane, between a magnetic field and the horizontal plane.

dipole antenna A center-fed antenna that is approximately half as long as the wave­length of the radio waves it is primarily intended to transmit or receive.

dip soldering A process that is similar to dip brazing but uses a lower-melting filler metal.

direct access The retrieval or storage of data by referring to its location on a volume, rather than by its location relative to the pre­viously retrieved or stored data.

direct-access device See random access device. direct-access storage device (DASD) A data . storage uhit cnwhich data can be accessed

directly aF randqm without having to progress-through a serial file such as tape. A disk unit is a direct~access storage device.

direct-acting controller See controller, direct­acting.

direct-acting recorder A recorder in which the pen or other writing device is directly connected to, or directly operated by, the pri­mary sensor.

": direct acting valve A valve that travels to the closed position when the signal increases. [ANSl/ISA-7S.0S.01-2000]

direct action 1. A controller in which the value of the output signal increases as the value of the input (measured variable or con­trolled variable) increases. 2. An actuator that extends the actuator stem when the power supply increases.

direct address An address that indicates the location at which the referenced operand is to be found or stored with no reference to an index register. Synonymous with first-level address.

direct addressing An addressing mode in which the instruction operand specifies the location of the data to be used.

direct code A code that specifies the use of actual computer command and address con­figurations.

direct-connected An arrangement whereby a meter or other driving mechanism is con­nected to a driven mechanism without inter­vening gears, pulleys, or other speed-changing devices.

direct coupling The act of associating two circuits through the capacitance, resistance, or self-inductance that is common to them.

direct-current amplifier An amplifier that is designed to amplify signals of infinitesimally small frequency.

direct current plasma display panel (DCPDP) A type of liquid crystal display (LCD) that operates through the emission of photons from gas that has been ionized by electric charge when electrodes are exposed to working gas. DCPDPs have lower driving voltages than alternating current plasma dis­

148

direct digital control (DOC) / direct numerical control (DNC)

plays (ACPDPs), but their luminescence diminishes over time because of electrode deterioration. Compare with alternating ClIr­rent plasma display panel (ACPDP).

direct digital control (DOC) 1. A computer control technique in which the final control element's position is set directly by the com­puter output. 2. A control system in which the computer carries out the functions that are normally performed by conventional controllers, for example, three-term control. 3. A term that is llsed to imply that a digital controller is connected directly to a final con­trol element or actuator in a manufacturing process. An example would be a valve in a process stream or an electric drive motor that is mechanically operating on a process. The term direct digital control is used to distin­guish this control from analog control. 4. A method of control in which all control out­puts are generated by the computer directly, with no other intelligence between the cen­tral computer and the process being con­trolled. See analog control.

direct distance dialing (DOD) A telephone system that allows users to call subscribers

! outside their local area without operator,. . ~: assIstance. ;direct drive Any powered mechanism whose , driven portion is on the same shaft as the

driving portion or is coupled directly to the driving portion.

:;. direct entry 1. In computing and data pro­. cessing, inputting data directly to computer

memory and disk. This contrasts with to ear­lier methods of keying to punched cards, which were then read into a computer. 2. In engineering, a method for connecting an electrical apparatus to the external circuits by means of the connecting facilities inside the main enclosure or in a terminal compartment that has a free opening to the main enclosure. (lEV 426-04-07) [ANSI/ISA-12.22.01-1998 (IEC 60079-7 Mod)]

direct extensions A device that indicates the flow rate when the user views the position of the extension of the metering float within a glass extension tube.

direction The act of having the person who is qualified to perform a task phYSically present when the task is performed or in con­tinuous communication with the person who is performing the task. [ANSI/ISA-67.14.01­2000]

directional control valve A valve whose chief function is to control the direction of flow within a fluid system.

directional coupler A device for separately sampling either the forward or backward oscillations in a transmission line. A fiber­optic coupler is directional if it preferentially transmits light in one direction.

directional gyroscope A navigational instru­ment for indicating direction. It contains a free gyroscope that holds its position in azi­muth, thus allowing the instrument scale to indicate deviation.from the reference direc­tion. ,.

directional property Any mechanical or physical property of a. material whose value varies with the orientation of the test axis within the test specimen.

• direction of polarization The direction of the . electric field vector of an electromagnetic

wave. direction of propagation The direction of

average energy flow with respect to time at . any point in a ho.mogeneous, isotropic

medium. . directive An operator command that is rec­

ognized by computer software. directivity The solid angle, or the angle in a

specified plane, over which sound or radiant energy on a transducer is measured within specified tolerances in a specified band of measurand frequencies. [ISA-37.1-1975 (R1982)]

directivity characteristic (directional response pattern) A plot of the sensitivity level of a piezoelectric sound-pressure transducer ver­sus the angle of sound incidence on that transducer's sensing element relative to tl.le sensitivity level in a specified direction and at a specified frequency. [ISA-37.10-1982 (R1995)]

directly controlled system See system, directly controlled.

directly controlled variable See variable, directly controlled.

direct memory access (DMA) 1. A method of fast data transfer between the peripherals and the computer memory. The transfer does not involve the CPU. 2. Direct memory access pertains to hardware that enables data to be entered into computer memory without involving the CPU. This is the method used by most telemetry and computer systems.

direct multiplex control See control, direct multiplex.

direct numerical control (DNC) A distrib­uted numerical control system in which the supervisory computer controls several CNC (computer numerical control) or NC (numer­ical control) machines.

, , 149

directory / discrete increment

directory 1. A file that contains the layout for each field of the record that it describes. 2. The layout of a record within a file. 3. A table that contains the names of, and pointers to, files on a mass-storage device.

directory device A mass-storage retrieval device such as disk that contains a directory of the files stored on the device.

directory service The network management function that provides all the addressing information required to access an application process. See PSAP address.

direct power generation Any method for producing electric power directly from ther­mal or chemical energy without first convert­ing it into mechanical energy. Examples of direct power generation include thermopiles, primary batteries, and fuel cells.

direct process piping The piping between the process and the control center that con­tains process fluid. [ISA-RP60.9-1981]

direct-reading g:lUge Any instrument that indicates a measured value directly rather than by inference. Examples include indicat­ing liquid level by means of a sight glass partly filled with liquid from the tank or by means of a pointer directly connected to a floa t in the tank.

direct record In instrumentation tape, the mode in which tape magnetization is directly related to data voltage level.

direct storage access (DSA) See access, direct storage.

direct storage access channel (DSAC) A channel for providing direct access to stor­age. See access, direct storage, and channel [ISA-RP55.1-1975 (R1983)]

direct wave A wave that is propagated through space without relying on the proper­ties of any gas or other substance occupying the space.

direct-writing recorder A pen-and-ink recorder in which the position of the pen on the chart is controlled directly by a mechani­cal link to the coil of a galvanometer or indi­rectly by a motor controlled by the galvanometer.

DIS Draft international standard; ISO stan­dard stahlS that has been registered and numbered but not yet given final approval. DCE Independent Sublayer

disable 1. To remove or inhibit a computer hardware or software feature. 2. To disallow the processing of an established interrupt until interrupts are enabled. Contrast with enable. See also disarm.

disarm To cause an interrupt to be com­pletely ignored. Contrast with arm. See also disable.

disassemble To reduce an assembly to its component parts by loosening or removing threaded fasteners, pins, clips, snap rings, or other mechanical devices. In most instances, disassembly is done for such purposes as cleaning, inspections, maintenance, or repair and is followed by reassembly.

discharge head The pressure at which a .• pUIlJ.p'discharges freely to the atmosphere,

usually measured as feet of water above the intake level.

disconnect 1. To disengage the apparatus used in a connection and to restore it to its ready condition when not in use. 2. To disen­gage the linkage between an interrupt and a designated interrupt servicing program. See connect.

disconnect signal A signal transmitted from oue end of a transmission line that indicates to the device on the other end that the con­nection should be terminated.

disconnect switch An electrical switch for interrupting the power supplied to a machine. It is usually separate from the machine controls (often mounted nearby on the wall) and serves mainly to deenergize the equipment to ensure safety during setup or maintenance.

discontinuity Any feature within a bulk solid that acts as a free surface. A discontinuity may be a crack, lap, seam, pore, or other physical defect, or it may be a sharp bound­ary between the normal structure and an inclusion or other second phase. A disconti­nuity mayor may not impair the usefulness of a part.

discrete 1. Pertaining to distinct elements or to representation by means of distinct ele­ments, such as characters. 2. In data process­ing, data that are organized into specific parts. 3. Referring to an individual bit from a selected word. 4. Discrete manufacturing refers to the manufacture of distinct products or parts.

discrete component circuit A circuit that is implemented by use of individual transis­tors, resistors, diodes, capacitors, and so on. Contrast with integrated cirwit.

discrete control (control, discrete) Onloff control. One of the two output values is equal to zero.

discrete increment An increment that pro­vides an output that represents the magni­tude of the measurand in the form of discrete or quantized values. [ISA-37.1-1975 (R1982)] I

I 150

t ....c

I

discrete input I DISOSS

discrete input 1. Inputs that have a separate disk clutch A device for engaging or disen­and distinct identity. 2. A digital input that is gaging a connection between two shafts. The either on or off. chief clutch element is a pair of disks, one

discrete manufacturing The production of coupled to each shaft, that when engaged individual (discrete) items (e.g., automobiles, electronic devices).

discrete output 1. Outputs that have a sepa­rate and distinct identity. 2. A digital output tha t is either on or off. 3. On/off control.

discrete part manufacturing A manufactur­ing process that produces discrete parts in comparatively small lots or batches of one to perhaps fifty thousand.

discrete programming See integer program­ming.

. i, discrete value A variable that has only two

states: "1" (true, on) or "0" (false, off) . discriminator A circuit that selects signals

with a particular range of amplitude or fre­quency and rejects all others. Also, a circuit that converts a frequency-modulated or phase-modulated signal into an amplitude­modulated signal.

disdrometer An apparatus that is capable of measuring and recording the size distribu­tion of raindrops in the atmosphere.

disengage To intentionally pull apart two normally meshing or interlocking parts, such as gears or splines, especially for the purpose of interrupting the transmission of mechani­cal power.

disengaging surface The surface of the boiler water from which steam is released.

dish antenna An antenna in which a parab­ola-shaped "dish" serves as the reflector to increase antenna gain.

dishing A metal-forming operation that forms a shallow concave surface.

disinfectant A chemical agent that destroys microorganisms, bacteria, and viruses or ren­ders them inactive.

disk [Eng] 1. In engineering, an essentially , {

flat, circular-shaped part that modifies the flow rate through either linear or rotary motion. [ANSI/ISA-7S.0S.01-2000] [Comp]2. In computing, a high-speed rotating mag­netic platter for storing computer data.

disk brake A mechanical brake in which the friction elements, normally called "pads," press against opposite sides of a spinning disk, which is attached to the rotating ele­ment so as to slow or stop its motion.

disk cam A flat cam with a contoured edge that rotates about an axis perpendicular to the plane of the cam. It communicates radial linear motion to a follower that rides on the edge of the cam.

transmit power by means of disk-face linings made of friction materials.

disk coupling A flexible coupling in which power and motion is transmitted by means of a disk that is made of elastomeric or other flexible material.

disk directory .A table for storing the location of files held on· the'disk.'

disk drive 1. The mechanism that moves the disk in a disk storage unit, usually including the spindle, drive motor, read-record heads, and head-actuating mechanism. The term disk drive is sometimes used to include the logic control unit and other electronic circuits that are part of the drive unit. 2. A device that reads and writes computer data on disks.

diskette A round, flat, flexible platter that is coated with magnetic material and used for storing software or data.

disk formatting See format. disk map The organization of information

stored on disks. . disk meter A flow-measurement device that

contains a nutating disk mounted in such a way that each time the disk nutates, a known volume of fluid passes through the meter.

disk operating system (DOS) A set of pro­grams that instruct a disk-based computing system to operate equipment through appli­cations programs. The DOS also manages the computing system's resources, such as track­ing, saving, and retrieving files; allocating storage space; and so on.

disk pack A large disk with very high stor­age capacity.

disk server A mass storage device that can be accessed by several computers, thus making possible the sharing of stored data.

disk spring A mechanical spring that con­sists of a dished circular plate and washer. The spring is supported in such a way that one opposing force is distributed uniformly around the periphery, and the second acts at the center. Washer-type disk springs are sometimes known as "Belleville washers."

disk valve A valve that has a closure member consisting of a disk that moves with a rotary or linear motion against a stationary disk. Each disk has flow passages through it. [ANSl/ISA-7S.05.01-2000]

OISOSS Distributed office support system; server portion of client/server facility for handling e-mail.

151

dispatching priority / display

dispatching priority A number that is assigned to tasks to deterrnine the prece­dence in which the central processing unit may be used in a multitask situation.

dispersing prism A prism tha t is designed to spread out the wavelengths of light so as to form a spectrum.

dispersion 1. Any process that breaks up an inhomogeneous, lumpy mixture and con­verts it into a smooth paste or suspension in which the particles of the solid component are more uniform and small in size. 2. Breaking up globs of oil and mixing them into water to make an emulsion. 3. Intentionally breaking up concentrations of objects or substances and scattering them over a wide area. 4. The pro­cess by which an electromagnetic signal is dis­torted because the various frequency components of that signal have different propagation characteristics. 5. The relation­ship between refractive index and frequency (or wavelength). 6. In wave rnechanics, linear dispersion is the rate of change of distance along a spectrum with frequency, whereas reciprocal linear dispersion is the rate of change of frequency with distance along a spectrum. 7. Dispersion is a general term for those phenomena that cause light to broaden or spread as it propagates through optical fiber. The three types of dispersion are modal, material, and waveguide.

dispersion-limited operation An operation in which the dispersion of the pulse, rather than its amplitude, limits the distance between repeaters. In this regime of opera­tion, waveguide and material dispersion pre­clude an intelligent decision on the presence or absence of a pulse.

displacement 1. The change in position of a body or point with respect to a reference point. Note: Position is the spatial location of a body or point with respect to a reference point. [ISA-37.1-1975 (R1982)] 2. The volume that is swept out by a piston as it moves inside a cylinder from one extreme of its stroke to the other. 3. For a reciprocating engine, pump, or compressor, the volume that is swept out by one piston as it moves from top dead center to bottom dead center, multiplied by the number of cylinders. 4. The act of forcing a fluid or granular substance to move out of a cavity or tube by forcing more of the substance in or by means of a piston or inflatable bladder that moves or expands into the space.

displacement anti resonance A condi tion of antiresonance in which the external sinusoi­dal excitation is a force, and the specified

response is displacement at the point where the force is applied.

displacement meter A meter that measures the amount of a material flowing through a system by recording the number of times a vessel or cavity of known volume is filled and emptied.

displacement resonance A condition of reso­nance in which the external sinusoidal exci­tation is a force, and the speCified response is d;iSplacement at the point where the force is ~pplied.·::~'.· .

displacement transducer, noncontact Usually a probe-type configu­ration that operates on eddy current princi­ples. A noncontact displacement transducer is a small system made up of a probe, exten­sion cable, and oscillator demodulator. The oscillator / demodulator excites the probe, which is a co~l of wire, through the extension cable, at a radio frequency of approximately 1.5 MHz. This produces a magnetic field radiating from the tip of the probe. When the probe tip is brought near to a conductive material, eddy currents are induced at the surf?ce of the material, which extracts energy from the probe's excitation and decreases its amplitude. Thus, as the distance (probe tip to conductive surface) changes, the probe's excitation amplitude changes, and a DC volt­age from the oscillator demodulator changes proportionally.

displacement-type density meter A device that measures liquid density by means of a float and balance beam and is used in con­junction with a pneumatic sensing system. The float is confined within a small chamber through which the test liquid continually flows, so that density variations with time can be determined.

displacer-type liquid-level detector A device for determining liquid level by means of force measurements on a cylindrical ele­ment that is partly submerged in the liquid in a vessel. As the level in the vessel rises and falls, the displacement (buoyant) force on the cylinder varies and is measured by the lever system, torque tube, or other force measure­ment device.

displacer-type meter An apparatus for detecting liquid level or determining gas density by measuring the effect of the fluid on the buoyancy of a displacer unit immersed in it.

display 1. A visual presentation of data. 2. In data processing, the visible representation of data on a screen.

152

I

display station I distortion analyzer

display station The location of the video ISS-51.1-1979 (R1993)] Also known as "trans­screen, keyboard, and related equipment, portation lag" and "transport lag." from which the plant or process can be moni­ distillate 1. The distilled product from a frac­tored and perhaps operated. tionating column. 2. The overhead product

display tube A cathode ray tube that is used to display information.

display unit A device that provides a tempo­rary visual representation of data. Compare hard copy. See also cathode ray tube.

dissector tube A camera tube that produces an output signal by moving the elec­tron-optical image, formed by photoelectric emission on a continuous photocathode sur­face, past an aperture.

dissipation constant A ratio for a thermistor that relates change in internal power dissipa­tion to the resultant change of body tempera­ture.

dissociation The process by which a chemical compound breaks down into simpler constit ­uents, as the CO2 and H 20, at high tempera­ture.

dissolve In video development, any of sev­eral scene transitions that involve a gradual decrease or increase in opacity of a film clip relative to another clip, graphic, or matte.

dissolved gases Gases that are "in solution" in water.

dissolved solids Those solids in water that are in solution.

dissymmetrical transducer A transducer in which interchanging at least one pair of spec­ified terminals will change the output signal that is delivered when the input signal remains the same.

distance through casting compound The shortest distance through a casting com­pound (for example, epoxy resin) between two conductive parts. [ISA-12.02.01-1999(IEC 60079-11 Mod)]

distance through filling material The short­est distance through a filling material between two conductive parts. [ANSI/lSA­12.25.01-1998(lEC 79-05 Mod)]

distance through solid insulation The short­est distance through solid insulation (for example, insulation that is extruded or mold!~d, not poured) between two conduc­tive parts. Note 1: For the purposes of ISA standard ISA-12.02.01-1999(IEC 60079-11 Mod), solid insulation is considered to be prefabricated (for example, sheeting or sleev­ing). Note 2: Varnish and similar coatings are not considered to be solid insulation. [ISA­12.02.01-1999(IEC 60079-11 Mod)]

distance/velocity lag A delay that is attribut­able to the transportation of material or to the finite rate of propagation of a signal. [ANSI/

from a distillation column. When a partial condenser is used, both a liquid and a vapor distillate stream may be present. 3. In the oil and gas industry, the term distillate refers to a specific product withdrawn from the col­umn, usually near t~~ bottom.

distillate fuel Any of the fuel hydrocarbons obtained during the di.s.tillation of petroleum that have boiling points higher than that of gasoline.

distillation 1. A unit operation that is used to separate a mixture into its individual chemi­cal components. 2·, Th~ vaporization of a sub­stance in which the vapor is subsequently recovered by condensation. 3. Distillation is often used in a less precise sense to refer to the vaporization of the volatile constituents of a fuel without subsequent condensation.

X AT o

Binary Distillation Tower

distilled water Water that is produced by vaporization and condensation with a result ­ing higher purity.

distortion 1. The deformation of signal shape by the device or system to which it is applied. [ISA-26-1968] 2. The ex~ent to which a system or component fails to reproduce accurately at its output the characteristics of the input. 3. An undesired change in the waveform of a given signal. 4. A lens defect that causes the images of straight lines to appear deformed or non-straight. See harmonic content.

distortion analyzer An instrument that rejects the selected fundamental frequency and measures the remaining energy of the spectrum.

153

I

distortion meter I diverse

distortion meter An instrument that visually puting Environment (DCE) that unifies the indicates the harmonic content of an audio system and network management of stand­frequency signal. alone and distributed systems in a het­

distributed In a control system, distributed erogeneous computing environment. A DME refers to control that is achieved by intelli­gence that is distributed about the process to be controlled, rather than by a centrally located single unit.

distributed communications network A communications link among the various (hardware) components in distributed con­

.- trol system. See also data highway. Distributed Computing Environment

(DCE) A software technology licensed by the Open Software Foundation (OSF). It pro­vides services for distributed computing and includes technologies for threads, remote procedure calls, directory service, security, time service, distributed file system, personal computer integration, and management.

distributed control The use of multiple microprocessors to distribute the functions of direct digital control (DOC) (central or host computer) so as to perform process control, thereby distributing the risk from component failure. Later techniques to minimize ground loops permitted the physical distribution of

,I control around a plant, placing micro­processors at various points in the process.

distributed control system (DCS) 1. That class of instrumentation (input/output devices, control devices, and operator interface devices) that in addition to executing the stated control functions also permits the trans­mission of control, measurement, and operat­ing information to and from one or many user-specifiable locations, all connected by a communication link. [ISA-5.3-1983] 2. A sys­tem that, while being functionally integrated, consists of subsystems that may be physically separate and remotely located from one another. [ANSl/ISA-5.1-1984 (R1992)J 3. A system of dividing plant or process control into several areas of responSibility, each man­aged by its own controller (processor). The whole is interconnected to form a single entity, usually by using communication buses of various kinds. DCS evolved from central computer control of the 1960s and was devel­oped initially for continuous flow processes that required loop, analog, and limited dis­crete control.

distributed database Relational computer data that can be stored in more than one net­worked computer but can be accessed entirely by one computer.

Distributed Management Environment (DME) The part of the Distributed Com­

consists of a graphical user interface (GUI) and application services for software instal­lation, distribution, and licensing; printer services; and user group administration.

distributed processing The interconnection of two or more computers so they can work together on the sam~ problem, not necessar­ily under the direction of a single control pro­gram. See also computer networking.

distributed system A group of connected computers that share software, information, and/or load.

'distributor . 1. Any device for apportioning current or flow among various output paths. 2. In an automotive engine, a device for send­ing an ignition spark to the individual cylin­der in a fixed order at a rate determined by engine speed.

disturbance An undesired change in a vari­able being applied to a system that tends to affect adversely the value of a controlled variable. [ANSl/ISA-51.1-1979 (R1993)]

disturbance resolution The minimum change caused by a disturbance in a mea­sured variable that will induce a net change of the ultimately controlled variable.

disturbance variable A measured variable that is uncontrolled and that affects the oper­ations of the process.

dither A useful oscillation of small magni­tude, which is introduced to overcome the effect of friction, hysteresis, or recorder pen clogging. See also hunting. [ANSl/ISA-51.1­1979 (R1993)]

dithering 1. An electronic graphics technique in which the gap between two pixels is filled with another pixel that has an average value of the two surrounding pixels so as to mini­mize the difference between their two values or to add detail that smoothes the resulting line. Also, the intermingling of dots of vari­ous colors to create a color not in the palette that supports the display being presented. 2. The application of intermittent or oscillatory forces that are just sufficient to minimize static friction within the transducer. [ISA-37.1-1975 (R1982)]

divergence The spreading out of a laser beam over distance, measured as an angle.

divergence loss The portion of energy in a radiated beam that is lost due to nonparallel transmission or spreading.

diverse The use of different technologies, equipment, or design methods to perform a

154

I

diversion valve I dominant wavelength

common function in order to minimize com­ DME Distributed Management Environ­mon cause. [ANSI/ISA-84.01-1996] ment; part of the Distributed Computing

diversion valve A type of fluidic control Environment (DCE) that unifies the system device that uses the COal1da effect to either and network management of stand-alone switch flow from one outlet port to another or to proportion flow between two divergent outlet ports.

diversity combiner A device that accepts two radio signals from a single source, which have been received with polarization, fre­quency, or space diversity. It then combines them to yield an output that is better than either original signal.

diversity reception The use of two or more radio receivers, each connected to different antennas, in order to improve the signal level. The antennas have diversity in space, phasing, and polarity.

divider A layout tool that resembles a drafts­man's compass and is used in toolmaking or sheet-metal work to draw cirdes or arcs or to scribe hole spacings or other linear dimen­sions.

dividing network See crossover network. .Division 1 The classification that is assigned

to a location that has either a high probability of a frequently or regularly occurring dust hazardous atmosphere or a location where the dust is electrically conductive.

Division 2 The classification assigned to a location that has where a low probability of a dust hazardous atmosphere occurring and/ or a high probability that a hazardous dust layer will be present

DLC Data link control; rules (protocol) used by two devices on a communication network to perform the orderly exchange of informa­tion. Also, distributed line carrier.

'I

DLL 1. Data link layer; - Defined in ISO 7498 2.dynamic link library; unlike static libraries in DOS, code for all functions of a program is not copied into an executable file at link time, but rather linking occurs at run time when library code is joined with program code. This allows a single DDL file to be shared by multiple applications without increasing memory or hard disk, so there is a standard interface independent of languages, compil­ers, and applications software, and so updat­ing DDL does not require all applications to be relinked or rebuilt.

OLT Digital linear technology; contains com­pressed digital data, generally for the cre­ation of commercial CDs.

OMA Direct memory access; method of mov­ing data from a storage device to RAM with­out the need for CPU intervention.

and distributed systems in a heterogeneous computing environment; consists of graphi­cal user interface (GUI) and application ser­vices for software installation, software distribution, software licensing, printer ser­vices, and user group administration.

DMT (BvS) Deutsche Montan Technologie); German certification an<;i testing laQoratory for testing the equipment of different ven­dors to some common standard.

DNA Digital Network Architecture; Digital Equipment Corpor!ition'S layered data com­munication prot8t~t.·

DNP Distributed Network Protocol; from Westronics, now Harris Controls Division; user's group formed in 1993.

document A medium and the data recorded on it for human use, for example, a report sheet or a book. By extension, a document may also be any record that has permanence and can be read by man or machine.

documentation 1. The creating, collecting, organizing, storing, citing, and disseminat­ing of documents or the information recorded in them. 2. A collection of docu­ments or information on a given subject. 3. A term often used in specific references to explanations of computer programs.

document management system A system that provides for the storage, retrieval, and manipulation of (computer) documents in a compact space.

DOE Department of Energy (United States); US. agency responsible for research and development of energy technology.

dog Any of several simple devices for fasten­ing, gripping, or holding.

DO loop A FORTRAN-based iterative tech­nique that enables any number of instruc­tions to be executed repeatedly.

domain 1. A part of computer memory that is associated with the software component of a resource. It may contain programs or data. [ISA-TR50.02, Part 9-2000] 2. An organiza­tional unit that has responsibility for naming hosts or networks.

domain name A unique name that defines an Internet site, such as "company.com". A domain name always has two or more parts that are separated by periods. The portion of the name to the left is more specific, the por­tion to the right more general.

dominant wavelength The wavelength of monochromatic light that matches a given

155

I

dope / double sampling

color when it is combined in suitable propor­tions with a standard reference light.

dope A cellulose ester lacquer that is used as an adhesive or coating.

doped germanium A type of detector in which impurities are added to germanium to make the detector respond to infrared radia­tion at wavelengths that are much longer than those detectable by pure germanium.

doping. 1. The act of adding a small amount of a substance to a material or mixture to achieve a special effect. 2. The process of coating a mold or mandrel to prevent a molded part from sticking to it.

Doppler-effect flowmeter A type of device that uses ultrasonic techniques to determine flow rate. A continuous ultrasonic beam is projected across fluid flowing through the pipe, and the difference between the inci­dent-beam and transmitted-beam frequen­cies provide a measure of the fluid flow rate.

TRANSMlnER AND

RECEIVER

LIQUID FLOW --+

Doppler shift 1. A phenomenon that causes electromagnetic or compression waves ema­nating from an object to have a longer wave­length if the object moves away from an observer than would be the case if the object were stationary with respect to the observer. The Doppler shift also causes electromag­netic or compression waves to have a shorter wavelength if the object moves toward the observer. It is the physical phenomenon that forms the basis for analyzing certain sonar data and astronomical observations. 2. A change in the wavelength of light caused by the motion of an object emitting (or reflect­ing) the light. Motion toward the observer causes a shift toward shorter wavelengths, while motion away causes a shift toward longer wavelengths.

dose The amount of radiation received at a specific location per unit area or unit volume, or the amount received by the whole body.

dose rate Radiation dose per unit of time. dose-rate meter Any of several instruments

for directly indicating radiation dose rate.

1 I

dosimeter (d~semeter) Any of several instru­ments for directly measuring the total dos of radiation received in a given period. e

dot An individual element of a halftone. It can be as small as a pixel in screen displays.

dot generator A test generator that is used with a video receiver to adjust the conver­gence of a picture tube.

dot matrix Refers to characters that are formed by varied. groupings or matrices of p-ripted dots. .

dot matrix'printer . Aprinter that produces letters, numbers, and symbols from a two-dimensional group of dot patterns.

double-action forming A metal-forming pro­cess in whicJq one stroke of the press per­forms two die operations.

double-acting 1. An actuator in which the power supply acts both to extend and retract the actuator stem. [ANSI/ISA-75.05.01-2000J 2. Acting in two directions. Examples include a reciprocating compressor in which each piston has a working chamber at both ends of the cylinder, a pawl that drives in both directions, or a forging hammer that is raised and driven down by air or steam pressure.

double-acting actuator A device in which power is supplied in either direction. [ANSI! ISA-75.05.01-2000]

double-acting positioner See positioner, dOl/­

ble-acting. double amplitude Tne peak-to-peak value. double-buffered 110 An input or output

operation that uses two buffers to transfer data. While one buffer is being used by the program, the other buffer is being read from or written to by an I/O device.

double-density A type of computer diskette that has twice the storage capacity of a sin­gle-density diskette.

double groove weld A weldment in which the joint is beveled or grooved from both sides to prepare the joint for welding.

double insulation Insulation that is com­prised of both basic insulation and supple­mentary insulation. [ANSI/ISA-82.02.01­1999(IEC 1010-1 Mod)]

double pole A device such as a switch, relay, or circuit breaker that is capable of either closing or opening two electrical paths.

double precision 1. Pertaining to the use of two computer words to represent a number. 2. In floating-point arithmetic, the use of additional bytes or words representing the number to double the number of bits in the mantissa.

double sampling A type of sampling inspec­tion in which the lot can be accepted or

156

I

double-sided / draft gauge

rejected based on results from a single sam­ple, or the decision can be deferred until the results from a second sample are known.

double-sided Of a computer diskette that stores data on both sides.

doublet lens A lens that has two components of different refractive index. It is generally designed to be achroma tic.

double-welded joint A weldment in which the joint is wel"ded from both sides.

double ~indow fibers Optical fibers that are designed to be transmitted at two wave­length regions, 0.8 to 0.9 micrometer and around 1.3 micrometers.

dowel 1. A headless, cylindrical pin that is used to locate parts in an assembly or to hold them together. 2. A round wood stick or metal rod that is used to make dowel pins.

dowel screw A dowel that is threaded at both ends.

down 1. Said of any machinery or equipment that is not operating. 2. In data processing, computer hardware that is not running.

downhand welding See flat-position welding. download 1. The process of data or program

transfer, usually from a larger computer to a Pc. 2. A service that is used to load data from the client into the server's domain. [ISA­TRSO.02, Part 9-2000]

downloading The process of sending config­uration parameters, operating software, or related data to remote stations or devices from a (usually central) configuration sta­tion.

downstream The outlet side of an instru­ment.

downstream seating A seating that is assisted by the pressure differential across the closure member in the closed position. The closure member is moved slightly down­stream into tighter contact with the seat ring seal that is supported by the body. [ANSI! ISA-7S.0S.01-2000]

downtimeThe time during which a piece of equipment, system, or the like is unavailable for various reasons, such as maintenance, setup, power failure, or equipment malfunc­tion.

dowtherm A constant boiling mixture of phe­nyl oxide and diphenyl oxide that is used in high-temperature heat transfer systems (boil­ing point 494

c

F, or 2siC). dp cell A pressure transducer that responds to

the difference in pressure between two pres­sure sources, frequently, a diaphragm capsule and an integral part of a dp (differential-pres­sure) transmitter. Dp cells are often used to measure flow by observing the pressure dif­

ference across a restriction in the flow line. They are also used to measure level by mea­suring the pressure difference between the head pressure produced by the height of a liq­uid in a vessel or tank and a reference pres­sure.

DPDT Double pole double throw; electrical switch action on a pair of wires that can be used to select one of two paths for the pair or used to reverse the direction of a single polarized pair; relay output contact form C.

dpi Dots per inch; mea·~u:reo(screen image or printed page.

DPMI DOS~protectedmode interface; multitasking DOS extender, replaces VCPI. See VCPI.

DPS Distributed processor system; term cre­ated by Sam Her-Din 1994 to identify the newer architectures being developed for dis­tributed control systems in light of connec­tivity to MES/ERP systems, emerging fieldbus capabilities, "smart" transmitters, "smart" valves, etc., as well as the blurring of the distinction between process controllers and programmable logic controllers; see DCS.

DPSK Differential phase-shift keying; modu­lation technique used in Bell 201 modem.

DPST Double pole single throw; electrical switch action used to interrupt flow through a pair of wires.

DIP transducer A transducer that measures differential pressure and converts it into another signal.

DQDB Distributed Queue Dual Bus; implementation of reservation strategy in a Metropolitan Area Network (MAN) in which each station on a 150 Mbs twin bus records and maintains reservations in local queue. SeeMAN.

draft Also spelled "draught." 1. The side taper on molds and dies that makes it easier to remove finished parts from the cavity. 2. The depth to which a boat or other vessel is submerged in a body of water. The value var­ies with vessel weight and water density. 3. Drawing a product in a die. 4. The small, positive pressure that propels exhaust gas out of a furnace and up the stack. 5. The dif­ference between atmospheric pressure and some lower pressure that exists in the fur­nace or gas passages of a steam-generating unit. [ISA-77.41-1992] 6. A preliminary docu­ment.

draft differential The difference in static pressure between two points in a system.

draft gauge 1. A type of manometer that is used to measure small gas heads, such as the

157

I

Draft International Standard (DIS) / drift, point

draft pressure in a furnace. 2. A hydrostatic drawing back 1. A shop term for tempering. indicator that is used to determine the depth 2. The process of reheating hardened steel at which a ship is submerged. below the critical temperature so as to reduce

Draft International Standard (DIS) The sec­ its hardness. ond stage of the ISO standard promulgation process.

draft loss A decrease in the static pressllre in a boiler or furnace as a result of flow resis­tance.

draft proposal (DP) The first stage of the ISO standard promulgation process.

drag 1. The resistance of a vehicle body to motion through the air as a result of the total force acting parallel to and opposite to the direction of motion. 2. Generally, any resis­tance to the motion of a solid shape through a body of fl1..1id. 3. In data processing, the movement of an object on a screen by using a mouse.

drag,-body flowmeter A device that nlea­sures the net force on a submerged solid body in a direction parallel to the direction of flow. It converts this value into an indication of flow or flow rate.

drain 1. A pipe that carries away waste solu­tions or effluent. 2. To empty a tank or vessel by means of gravity flow into a vvaste system or auxiliary holding vessel. 3. A valved con­nection at the lowest point for the purpose of removing all water from the pressure parts.

DRAM [pronounced dee-ram] Dynamic ran­dom access memory is high speed but must be refreshed regularIy.

draw 1. To pull a load. 2. To form cup-shaped parts out of sheet metal. 3. To reduce the size of wire or bar stock by pulling it tl1rough a die. 4. To remove a pattern from a sand-mold cavity. 5. A fissure or pocket in a casting that is caused by inadeqllate feeding of molten metal during solidification.

draw bead 1. A bead or offset that is used to control metal flow during sheet-metal form­ing. 2. A contoured rib or projection on a draw ring or holddown for the purpose of controlling metal flow in deep drawing.

drawbench The stand that holds a die and draw head, which are used for reducing the size of wire, rod, bar stock, or tubing.

drawdown The curvature of the liquid sur­face upstream of the weir plate.

drawdown ratio The ratio of a die opening to the product's thickness in a deep-dravving operation.

drawhead 1. The die holder on a dra"vbench. 2. A group of rollers through whicll strip, tubing, or solid stock is pulled so as to form angle stock.

drawing compound A lubricating substance such as soap or oil that is applied to prevent draw marks, scoring, or other defects caused by metal-to-metal contact during a stan1ping, vviredrawing, or similar metal-forming oper­ation. • ".

drawj~gs .°r:'Graphicrepresentations of the ° contiol cerrter, w·bicl1 may also include bills-.

of material, hard copies ~f video display tube (VDT) displays, photographs, and tables (e.g., wire and cable lists). [ISA-RP60.4-1990] 2. A general term that refers to any drawing used for the design, construction, or mainte­nance of instrumentation. Examples inclllde P&IDs, installation detail drawings, loop drawings, electrical drawings, emergency shutdown system logic drawings,.PCB ~che­matic drawings, and so on.

dress 1. To shape a tool such as a grinding wheel. 2. To restore a tool to its original con­tour and sharpness.

drawing tower A type of eqllipment for mak­ing optical fibers, in which optical fibers are drawn from hea ted glass preforms.

draw mark Any surface flaw or blemish that occurs during drawing, incillding scoring, galling, pickup, or die lines.

draw radius The Cllrvat"ure at the edge of the cavity in a deep-drawing die.

draw ring A ring-shaped die part. The punch pulls the dravv blank over the dravv ring dur­ing a drawing operation.

drift 1. An undesired change in output over a period of time, where the challge is llnrelated to the input, environment, or load. See ANSI/ISA-S51.1-1979 (RI993). [ISA­RP67.04.01-2000.ISA-RP67.04.02-2000] 2. An undesired cllange in the output/input rela­tionship over time. [ANSI/ISA-75.05.01­2000] 3. An undesired change in output over time that is not a function of the measurand. [ISA-37.1-1975 (1992)] 4. Drift is usually expressed as the change in output over a specified time "vith fixed input and operation conditions. It is usually used in t11e context of analog transducers, analyzers, and so on.

drift, point The change in output over a spec­ified period of time for a constant input under specified reference operating condi­tions. Note: Point drift is frequently deter­mined at more than one input, as for example: at 0%

, 50%, and 1000/0 of range.

Thus, any drift of zero or span may be calcu­lated. Poirlt drift is typically expressed as fol­

158

I

drift pin / drum�

lows: "the drift at mid-scale for ambient drive shaft A shaft that transmits power and temperature (70 ± 2 2'F) for a period of 48 motion from a motor or engine to the other hours was within 0.1% of output span. elements of a machine. [ANSI/ISA-51.1-1979 (R1993)] driving pinion The gear in a gear train that

drift pin A round, tapered metal rod that is driven into matching holes in mating parts in order to stretch them and bring them into alignment, such as for riveting or bolting.

drift plug A tapered rod that can be driven into a pipe to straighten it or flare its end.

drill A cylindrical tool with one or more cut­ting edges on one end. It makes or enlarges holes in solid material by being rotating about its longitudinal axis with the applica­tion of axial force. "drill drift A flat, tapered piece of steel that is� used to remove taper shank drills and other� tools from their tool holders.�

drill gauge A flat, thin steel plate withIl numerous holes of accurate sizes that can be� ~ used to check the size of drills.� , drill jig A tool that is constructed to guide a�f drill during repeated drilling of the same size�

holes. The drill jig can be used either at many�t .locations in a given piece or at the same loca­�l tion in many identical pieces, especially�

where exceptional straightness or accuracy ~ ,.,t of location is desired. I drill press A vertical drilling machine that is

constructed to hold a work piece stationary and to apply vertical force in order to press a rotating drill into the work.

drill sleeve A hollow, tapered cylinder that is used as an adapter in order to fit the shank of a taper-shank drill or other tool into the spin­dle of a drill press or similar machine tool.

drip tight A nonstandard term that is used to refer to control valve leakage. ANSI/FCI FeI 70-2 specifies the leakage classifications. [ANSI/ISA-75.05.01-2000]

drive In data processing, a device that manipulates a diskette, disk, or magnetic tape so the computer can read or write data to it. See disk drive and tape drive.

drive fit A type of interference fit that requires light to moderate force to assemble.

driven gear The member(s) of a gear train that receive power and motion from another gear.

'. driver 1. A software element that converts , operator instructions into the appropriate lan­guage to drive a hardware device (unit or stream drivers, for example). 2. A small pro­gram or routine that handles the control of an external peripheral device or executes other programs. 3. A circuit or device that provides input for another circuit or controls the oper­ation of that circuit.

159

receives power and motion by means of a� shaft. The shaft is connected to the source of� power and transmits the power and motion� through its teeth to the next gear in the train.�

driving-point impedance The complex ratio of applied s}nusoid,al.voltage, force, or pres­sure at the dri'vipg.p'oint of a transducer to the current, velocity, or volume velocity, respectively, that result at the same point (assuming that all inputs and outputs termi­nate in some specified manner).

driving-point reactance The imaginary com­ponent of driving-point impedance. See driv­ing-point impedance.

driving-point resistance The real compo­nent of driving-point impedance. See driv­ing-point impedance.

drone A remotely controlled, self-powered aircraft or missile.

droop See offset. droop rate The rate at which the voltage out­

put of a storage device decays. [ISA-RP55.1­1975]

drop cap In the typographical composition of screen displays and printing, the enlarged, initial capital letter set into the first few lines of that paragraph for stylistic or design rea­sons.

drop-in A character that appears erroneously (on a display screen, printer, file, etc.) because the disk or tape drive misstored or misread one or more bits.

drop leg The section of measurement piping that is below the process tap location to the instrument.

drop-out 1. A character that vanishes (from a display, printout, or file) because the disk or tape drive misstored or misread one or more bits. In data transmission, drop-out refers to a momentary loss in signal, usually because of noise or a system malfunction. 2. Any dis­crete variation in signal level during the reproduction of recorded data that results in a data-reduction error.

drop tight A nonstandard term used to refer to control valve leakage. ANSI/FCI FCI 70-2 specifies leakage classifications. [ANSI/ISA­75.05.01-2000]

drosometer An instrument for measuring the amount of dew that condenses on a given surface.

drum 1. Any machine element that consists essentially of a thin-walled, hollow cylinder. 2. A thin-walled, cylindrical container, espe­

I

drum baffle / DSP�

cially a flat-ended shipping container, that holds liquids or bulk solids and has a capacity of 12 to 110 gallons (50 to 400 liters). 3. The cylindrical member around which a hoisting rope is wound. 4. A high-capacity computer storage device. 5. A steam drum is a closed vessel that is designed to withstand internal pressure. It collects and separates the steam/ water mixtllre that is circul?ted through the boiler. [ANSI/ISA-77.42.01-1999]

drum baffle A plate or series of plates or screens that is placed within a drum to divert or change the direction of the flow of water or water and steam.

drum brake A mechanical brake in which the friction elements, normally called "shoes," press against the inside surface of a cylindri­cal member (the drum), which is attached to the rotating element to slow or stop its motion.

drum course A cylindrical section of a drum. drum head A plate that closes the end of a

boiler drum or shell. drum internals All apparatus within a drum. drum operating pressure The pressure of the

steam that is maintained in the steam drum or steam-and-water drum of a boiler in oper­ation.

dry air Air with which no water vapor is mixed. This term is used comparatively, since in nature there is always some "vater vapor in air, and such water vapor, being a gas, is dry.

dry ash Industrial refuse in the solid state, usually in a granular or dust form.

dry assay The act of determining the amOllnt of a metal or compound in an alloy, ore, or metallurgical residue by means that do not involve using liquid to separate or analyze the constittlents.

dry back The baffle that is provided in a fire­tube boiler that joins the furnace to the sec­ond pass in order to direct the products of combustion. The dry back is constructed of heat-resistant material (generally refractory and insulating material) and is designed to be separate from the pressure vessel.

dry basis A method for expressing moisture content in which the amount of moisture present is calculated as a percentage of the weight of bone-dry material. Dry basis is used extensively in the textile industry.

dry-blast cleaning The use of a dry abrasive medium StIch as grit, sand, or shot to clean metal slirfaces by driving the medium against the surface with a blast of air or by centrifugal force.

dry-bulb temperature The temperature of the air that is indicated by a thermometer not affected by the water vapor content of the air.

dry corrosion The atmospheric corrosion that takes place at temperatures above the dew point.

dry gas Gas that contains no water vapor. dry-gas loss The loss that represents the dif­

ference between the heat content of the dry exhaust gases and their heat content at the temperatllre of ambient air.

drying oven .A. ·<;lo.~ed. chamber for driving moisture from surfaces or bulk materials by heating them at relatively low temperatlires.

dry pipe A perforated pipe in the steam space above th~water level in a boiler that helps keep entrained liquid from entering steam outlet lines.

dry steam Steam that contains no moisture. Commercially dry steam that contains not more than 1/2 of 1 percent of moisture.

dry steam drum A pressure chamber, usually serving as the steam offtake drum, that is located above and in communication vvith the steam space of a boiler's steam-and-water drum.

dry test meter A type of meter that is used extensively to determine gas flow rates for billing purposes and to calibrate other flow-measuring instruments. It has two chambers that are separated by a flexible dia­phragm, vvhich is connected to a dial by means of a gear train. In operation, the cham­bers are filled alternately as a flow control valve switches from one chamber to the other as the first becomes completely filled. The flow rate is indicated indirectly from movement of the diaphragm.

OS Data structure [ISA-TR50.02, Part 9-2000]; Dansk Standardiseringsrad (Denn1ark); stan­dards group in Denmark.

DSA Directory Service Agent, for directory services.

DSL Digital subscriber line; technology that uses special modems to send digital signals, rather than analog waveforms, over existing twisted-pair copper phone lines; see also ADSL.

OSL (delta slope) algorithm See compressor. OSM Demand-side management; electric

power industry term. DSP Digital signal processor (or processing);

a RISe microprocessor optimized to execute digital signal processing algorithms; an algo­rithm process by which a sampled and digi­tized data stream is modified to extract relevant information. See RISC.

160�

DSR I Dumet wire�

DSR Data set ready; modem interface control voltage is input into a ramp generator and signal (typically ErA RS-232/422) that indi­ integrated for a specified time. When that cates that the terminal is ready for transmis­ time has passed, a counter is started and a

I�h

sion from DCE. reference voltage applied to cause a con­�DSS Decision support system; a form of com­ trolled ramp-down. The counter is stopped�

puter- based data warehousing and manage­ when the voltage becomes zero. The count� ment using DBMS techniques to focus the gives the digital number output.� retrieval of information for specific needs. dual-sealing valve A valve that uses a resil­ IDSSS Direct sequence spread spectrum; used ient seating material for the primary seal and� for wireless communication systems. a metal-to-metal seat for a secondary seal.�

DSVD Digital simultaneous voice and data; [ANSI/ISA-75.05.01-2000]�

II telephone connections that permit the simul­ dual-slope coriverterl. Arhntegrating aml­�

taneous transfer of voice and data similar to log-to-digital converter in which the� ISDN. unknown signals are converted into a pro­�

DTE Data terminal equipment; end point of portional time interval. 2. An AID converter� communication link such as a device acting that integrates the si&nal for a specific time,� as data source, data sink, or both (worksta­ then counts time intervals for a reference�

,I .. .tion, repeater, file server, etc.); usually pro­ voltage so as to bring the integrated signal ~..

ducing data in human-readable form, as with back to zero.� a printer or video screen. dual system Special configurations that use�

DTMF Dial tone multiple frequency. Also see two computers to receive identical input and� dual tone multiple frequency; audio sig-' execute the same routines. The results of� I naling frequency on Touch-tone, pushbutton such parallel processing are then compared.�

1 telephones. Exceptionally high reliability requirements� f DTR Data terminal ready; modem interface usually are involved.�

1:control signal (typically, EIA RS-232/422) dub In video development, a copy of a mas­�

; that indicates that the terminal is ready for ter tape, usually at lower resolution, for� j transmission to DCE. offline editing.

I DTV Desktop video; combines animation, duct An enclosed fluid-flow passage, which� t

image metamorphosis, photography, etc., may be any size up to several feet in cross� within common data manager. section. A duct is usually constructed of gal­�

DVA Directory User Agent; for directory ser­ vanized sheet metal and is not intended to� vices. sustain internal pressures of more than a few�

dual-axis tracking antenna A tracking psi. The term duct is most often applied to� antenna that is steered automatically in both passages for ventilating air and to intakes� azimuth and elevation. and exhausts for engines, boilers, and fur­�

dual-beam analyzer A type of radia­ naces.� tion-absorption analyzer that compares the ductile iron The term preferred in the United� intensity of a transmitted beam with the States for cast iron that contains spheroidal� intensity of a reference beam of the same nodules of graphite in the as-cast condition.� wavelength. Also known as "nodular cast iron," "nodular�

dual in-line package (DIP) A standardized iron," and "spherulitic-graphite cast iron."� component package that is fabricated from ductility The property of a metal that indi­�two parallel rows of leads on O.l-inch cen­ cates its relative ability to deform without� ters. It is intended to allow the printed-circuit fracturing. It is usually measured as the per­�mounting of integrated circuits, switches, centage elongation or reduction of an area in� and components. a uniaxial tensile test.�

dual-mode control A type of control law that dumb terminal An expression used to� consists of two distinct types of operation. In describe most ASCII asynchronous termi­�linear systems, these modes usually consist nals, both hard-copy and VDT-type devices.� of a linear feedback mode and a bang-bang­ Dumb terminals do not use a data transmis­�type (two-position) mode. The latter mode is sion protocol and usually send data one� often used for startup. character at a time.�

dual output Providing two separate and non­ Dumet wire Wire made of Fe-42Ni covered� interacting outputs that are functions of the with a layer of copper. It is used to replace� applied measurand. [ISA-37.1-1975 (RI982)] expensive platinum as the seal-in wire in�

dual-ramp ADC A technique for converting incandescent lamps and vacuum tul:,es. The� analog data into digital form. The unknown copper coating prevents gassing at the seal.�

161�

dummy / dust, combustible�

dummy 1. A device that is constructed to resemble another device physically, but with­out the original's operating characteristics. 2. In engineering, a cathode, usually corru­gated so to provide varying current densi­ties, that is plated at low-current densities to preferentially remove impurities from an electroplating solution. 3. A substitute cath­ode that is used during the adjustment of the operating conditions in electroplating. 4. In computing, an artificial address, instruction, or record of computer information that is inserted solely to fulfill prescribed condi­tions, such as to achieve a fixed word length or block length, but that does not itself affect machine operations except to permit the machine to perform desired operations.

dummy argument A variable such as the one that appears in the argument list of a func­tion definition but is replaced by the actual argument when the function is used.

dummy instruction An artificial instruction or address that is inserted in a list to serve an instructional purpose rather than for execu­tion.

dump 1. A printout of computer memory or a file in hexadecimal and character form. 2. The transfer of data without regard for its significance. Same as "storage dump."

dump valve A large valve in the bottom of a tank or container that can quickly empty the tank in an emergency.

dunnage Loose material that is used around an item to prevent damage during shipment. [ISA-RP60.11-1991]

duodecimal number A number, of successive characters, that represents a sum in which the individual quantity represented by each character is based on a radix of twelve. The characters used are 0, 1,2,3,4,5,6, 7, 8, 9, T (for ten) and E (for eleven). Related to number system.

duplex 1. Pertaining to a twin, pair, or a two-in-one situation. An example would be a channel that provides simultaneous trans­mission in both directions or a second set of equipment to be used in event of the failure of the primary device. 2. Referring to any item or process that consists of two parts working in conjunction with each other.

duplex, full The method of operation of a communication circuit in which each end can simultaneously transmit and receive. [ISA­RP55.1-1975 (RI983)]

duplex, half Permitting one-directional elec­trical communication between stations. Tech­nical arrangements may permit operation in

either direction but not simultaneously. [ISA­RP55.1-1975 (RI983)]

duplex cable 1. In fiber optics, two fiber cables that are suitable for duplex transmis­sion. With copper wire, a pair of wires that are insulated from each other and in which there is an outer jacket of insulation around the inner insulated pair. 2. A cable that con­tains two optical fibers in a single cable struc­ture. Light is not coupled between the two fibers. Typically OI~e c~ble is used to transmit signals in one direction and the other to trans­mit in the opposite direction.

duplex connector A connector that simulta­neously makes two connections, joining one pair of optical fibers with another.

duplex control A control in which two inde­pendent control elements share a common input signal to operate separate final control elements, both of which influence the value of the controlled condition.

duplexed system A system that has two dis­tinct and separate sets of facilities, each of which is capable of assuming the system function while the other assumes a standby status. Usually, both sets are identical in nature.

duplex mode The communication link that allows simultaneous the transmission and receipt of data.

duplex operation The operation of an associ­ated transmitter and receiver that are designed for concurrent transmission and reception.

duplex process Any integrated process in which a manufacturing operation is carried out by two procedures in series. An example is refining steel by the Bessemer process and then producing ingots or continuously cast slabs by the basic-oxygen or electric-furnace process.

duplex pump A reciprocating or diaphragm pump that has two parallel flow paths through the same housing, with a common inlet and a common outlet.

duplex transmission Transmission in both directions, either one direction at a time (half duplex) or both directions simultaneously (full duplex).

dust 1. Any finely divided solid material that is 420 ~m or smaller in diameter (i.e., mate­rial that passes a U.s. No. 40 Standard Sieve). [ISA-12.1O-1988] 2. Particles of gas-borne solid matter that are larger than one micron in diameter.

dust, combustible Any finely divided solid material that is 420 microns or less in diame­ter (i.e., material passing a U.s. No. 40 sieve)

162

I I

dust counter / DVCAM

dust-tight Of an enclosure constructed so SENDER f-.------------I RECEIVER I that dust will not enter the enclosing case

(9) SIMPLEX under specified test conditions. [ISA­

SENDER 1------------1 RECEIVER II OR

SENDER I-1------------1 RECEIVER I (b) HALF DUPLEX

SENDER 1-----------1 RECEIVER

SENDER I---------------j RECEIVER

(c) FULL DUPLEX

Types of Transmission - Duplex

and that presents a fire or explosion hazard when dispersed. [ISA-12.01.01-1999]

dust counter A photoelectric instrument that measures the number and size of dust parti­cles in a known volume of air. Also known as "Kern counter."

dust-ignition proof Used to describe an , enclosure that will exclude dust and that,

when installed according to original design I intent, will not permit arcs, sparks, or heat i·� generated or liberated inside the enclosure to

ignite exterior accumulations or atmosphere suspensions of a specified dust anywhere near the enclosure. [ISA-12.01.01-1999]

, dust ignition-proof enclosure An enclosure that excludes ignitable amounts of dusts or amounts that might affect the performance or rating. Moreover, an enclosure that, when installation and protection conform with the NEC, will not permit arcs, sparks, or heat generated or liberated inside the enclosure to ignite exterior accumulations .o.r atmo- . spheric suspensions of a speClfled dust ill or near the enclosure. [ANSI/ISA-12.01.01­1999]

dust layer, combustible Any surface accu­mulation of combustible dust that is thick enough to propagate flame or that will degrade and ignite. me or will degrade and ignite. [ANSI/ISA-12.01.01-1999].

I� dust loading The amount of dust In a gas,�

usually expressed in grains per cubic feet or� pounds per thousand pounds of gas.�

dust-protected enclosure An international term for an enclosure in which dust is not totally prevented from entering, but in whichI it does not enter in quantities sufficient to interfere with the safe operation of the equip­ment or accumulate where it can cause an ignition hazard.e to cause an ignition hazard. [ISA-12.01.01-1999]

12.01.01-1999] dust-tight enclosure An enclosure of sub­

stantial mechanical construction that is pro­vided with gaskets or otherwise designed to exclude dust. It has no open through holes and no knockouts. The conduit entrance is by tapped threads that have a minimum of 3 1/2 threads engaged or by a gasket, bonded conduit hub. The dust-tight enclosure has a substantial door or cover that is made dust tight by a securely fastened gasket or by the width and closeness of fit of the mating flanges. Its door or cover fasteners are of sub­stantial construction and are permanently captive. The door or cover itself is also per­manently captive to the enclosure. Threaded-hub conduit connections are made dust tight by welding or gasketing. Threaded hub conduit connections are solidly bonded to the enclosure by welding or bonding through proper fittings. Such enclosures are NEMA 3, 3X, 4, 4X, 6, 12 or 13 enclosures. [ANSI/ISA-12.01.01-1999]

dutch oven A furnace that. extends forward of the wall of a boiler setting. It usually is constructed entirely of refractories although in some cases it is water cooled.

duty 1. The statement of operating conditions and their durations to which a device or equipment is subjected, including rest and deenergized periods. 2. The specification of service conditions that defines the type, dura­tion, and constancy of applied load or driving power.

duty cycle 1. For a device that operates repeat­edly, but not continuously, the time intervals that are involved in starting, running, and stopping plus any idling or warm-up time. 2. For a device that operates intermittently, the ratio of working time to total time, usually expressed as a percentage. Also known as "duty factor." 3. The ratio of the amount of time a valve spends performing one particu­lar function to the valve's total installed time period. It may be expressed as a percentage of total time (service time versus installed time). [ISA-RP75.23-1995] 4. In digital transmission, the ratio of high levels to low levels. In elec­trical apparatus, the pulse width times the frequency.

duty cyclometer A meter for directly indicat­ing duty cycle.

DVCAM Digital videocassette camera video format; potential emerging video produc­tion standard promoted by SONY. DVCPRO

163�

DVD / dynamic dispatching

is a version by Panasonic; Digital-S is a ver­ dynamically relocatable coding Coding for a sion by JVe. computer that has special hardware for per­

DVD Digital video disk (or digital versatile forming the derelativization. With an appro­disk); a concept to bring the worldwide (con­ priately designed computer system, coding

I sumer) electronics industry into one stan­dard for both computer and entertainment industries (large CD- ROM, movie playback platform, game platform, large computer data drive and erasable memory). Five-inch disk fits 3.7 GB of data on a single side using the size of pits and spacing between pit tracks and read the da ta using a red laser beam rather than the current infrared beam. A variation provides up to 7.4 GB on a single side by using two layers, a semireflective layer above a fully reflective one; another variation provides 5 GB on each side of a two-sided disk. A later format for an agreed­upon standard among audio, video, and computer industries exceeds 17 GB. See MMCo, SO, and HoCo.

DVD-ROM Digital video (versatile) disk read-only memory; provides 4.7 GB of mem­ory, roughly seven times more than the 650 megabytes held by CD-ROMs.

DVI Digital video interactive; brand name of ,, Intel for a variety of product families involv­

i ing digital video and audio. DVR Dynamic voltage restorer; turns a dis­

torted waveform, including voltage dips, into a required waveform by interjecting a precise amount of voltage using a series-con­nected transformer into a distribution feeder between the power supply side and the load side.

dwell 1. A contour on a cam that causes the follower to remain at maximum lift for an extended portion of the cycle. 2. In a hydrau­lic or pneumatic operating cycle, a pause during which pressure is neither increased nor decreased.

dwell period The time a commutator spends at a given channel position.

dwell time In any variable cycle, the portion of the cycle when all controlled variables are held constant. Examples of using dwell time include allowing a parameter such as tem­perature or pressure to stabilize or allowing a chemical reaction to go to completion.

DXF Document Exchange Format; a drawing file format developed by AutoCAD and adopted by others.

dye penetrant A low-viscosity liquid that contains a dye used in nondestructive exami­nation to detect surface discontinuities such as cracks and laps in both magnetic and non­magnetic materials.

can be loaded into various sections of core, appropriate addresses changed, and the pro­gram executed.

dynamic analysis The study of data under� changing internal and lor externa1cond i­�tions. Such analysis can include static analy­�sis, linear, or nonlinear,�

dynamic burn-in: The;application of patterns� so as to exercise devices during burn-in and� to produce higher power dissipation, current� density, and chip temperature than static� burn-in provides.�

dynamic calibration 1. Calibration in which� the input varies over a specific length of time� and the output is recorded against time. 2. A� calibration procedure in which the quantity of� liquid is measured while liquid is flowing into� or out of the measuring vessel.�

dynamic characteristics Those characteris­�tics of a transducer that relate to its response� to variations of the measurand with time.� [ISA-37.1-1975 (R1982)]�

dynamic compensation A technique used in� control to compensate for dynamic response� differences to the different input streams to a� process. A combination of lead and lag algo­�rithms will handle most situations.�

DISTURBANCESET POINT

FEEDFOqyyARD 1'--.,.--'� CONTROLLER I.----,-~...,

CONTROLLED

VARiABLE

Dynamic Compensation

Dynamic Data Exchange (DOE) A Microsoft­�developed interapplication communications� protocol in which the data from one program� (application) automatically updates another.� DOE was originally designed to move data� from a spreadsheet to word processor. Is the� baseline protocol for OLE 1.0, but not for� OLE 2.0 (it is supported there, however, to� maintain upward compatibility). DDE has� become more complex with the advent of� Windows and Windows NT in industrial� applications.�

dynamic dispatching Dispatching that� occurs in real time (as it occurs). Dynamic� dispatching reports to an MES system all sta­�tus changes, such as work completed, opera-� j

164 _.J; I

dynamic tunctional board tester / dynamometer�

tionaI problems, and priority changes; dynamic sensitivity In leak testing, the mini­updates the entire plant floor; and auto­ mum leak rate that a particular device is matically reschedules all operations for all capable of detecting. released jobs to reflect those changes. dynamic stability The property that permits

dynamic functional board tester A device the response of a positively damped physical that accesses a circuit board under test system to asymptotically approach a con­through a connector that applies predeter­ stant value when the level of excitation is mined inputs and then examines the outputs constant. Compare with static stability. for correct board response. dynamic stiffness The apparent stiffness of a

dynamic gain See gain, dynamic. spring member under vibration or shock dynamic load That portion of a service load loading. This apparent stiffness is frequency

that varies over time and cannot be charac­ dependent. . terized as a series of different, unvarying dynamic' stop Aloor stop that consists of a (static) loads successively applied and single jump instruction. removed. dynamic storage The storage of data on a

dynamic memory Same as dynamic storage. device or in a manner that permits the data dynamic model A model in which the vari­ to move or vary with time. Thus, the data is

ables are functions of time. Contrast with not always available instantly for recovery. steady-state model. Examples of dynamic storage include acous­

dynamic optimization A type of control, fre­ tic delay line, magnetic drum, or the circulat­quently multivariable and adaptive in ing or recirculating of information in a nature, that optimizes some criterion func­ medium. Synonymous with dynamic //lemory. tion in bringing the system to the set points dynamic storage allocation A storage alloca­of the controlled variables. The sum of the tion technique in which the location of pro­

. weighted, time-absolute errors is an example grams and data is determined by criteria that of a typical criterion function to be mini- are applied at the ~oment of need.

i mized. Contrast with steady-state optimization. dynamic subroutine A subroutine that ~ynamic pressure The increase in pressure involves parameters, such as decimal point

above the static pressure that results from the position or item size, from which a relatively complete transformation of the kinetic coded subroutine is deri ved. The computer energy of the fluid into potential energy. itself is expected to adjust or generate the

dynamic programming In operations subroutine according to the parametric val­research, a procedure for optimizing a multi­ ues chosen. Contrast with static subroutine. stage problem wherein a number of deci­ dynamic test A test of a device or mechanism sions are available at each stage of the that is conducted under variable loading or process. Contrast with convex programming, stimulation. integer programming, linear programming (LP), dynamic unbalance The net force or torque mathematical programming, nonlinear program­ produced on the valve stem or shaft by fluid millg, and quadratic programming. pressure acting on the closure member and

dynamic RAM Random access memory that stem or shaft at stated travel and flowing needs to be refreshed at regular time inter­ conditions. [ANSI/ISA-75.05.01-2000] vals. It involves the extra complexity of dynamic variables Process variables that can refresh circuits, but higher densities can be change from moment to moment because of achieved. unspecified or unknown sources.

dynamic range 1. The range of signals that is dynamometer 1. An electrical instrument in accepted by a device without manual adjust­ which current, voltage, or power is mea­ment. 2. The difference between the highest sured by determining the force between a voltage level that will overload the instru­ fixed coil and a moving coil. 2. A special type ment and the lowest voltage level that is of rotating machine that is used to measure detectable. Dynamic range is usually the output or driving torque of rotating expressed in decibels, typically 60 to 90 dB equipment. for modern instruments.

dynamic response The behavior of the out­put of a device as a function of the input, both with respect to time. See response, dynamic. [ANSIjISA-67.04.01-2000] 2. The behavior of an output in response to a chang­ing input.

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