lecture 2 daq
TRANSCRIPT
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Mid Term 35%
Assignments 5%
Quizzes 10%
Final 50%
Assignment # 1 - P 1.14 till P 1.24 (Curtis)
Due Date: 19 July 2011
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A typical process transducer.
A measurement transducer typically consists of a sensingelement combined with a driving element (transmitter).
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Transducers for process measurements convert the
magnitude of a process variable (e.g., flow rate,
pressure, temperature, level, or concentration) into a
signal that can be sent directly to the controller. The sensing element is required to convert the
measured quantity, that is, the process variable, into
some quantity more appropriate for mechanical or
electrical processing within the transducer.
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Mechanical transducers are those in whichthe output signals are mechanical by nature.For example, a bimetallic element reacts tochanges in temperature and its response is in
the form of mechanical displacement. Electrical transducers respond to
nonelectrical quantities but develop outputsignals, which are electrical by nature.
Example: In resistance thermometer element,a change in temperature is recognized as achange in the resistance of the element.
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Passive electrical transducers are based on the
variation of resistance, inductance or capacitance due
to application of any external stimulus.
Active electrical transducers provide electrical signals
in the form of voltages or currents, dc or ac. They
function as sources of energy, though very small, and
do not need excitation from an external source of
electrical power. Examples are thermocouples andpiezoelectric crystals.
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The degree of hotness of a body, substance, or medium;
a physical property related to the average kinetic
energy of the atoms or molecules of a substance.
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Resistivity of metals is a function of temperature. Platinum often used since it can be used for a wide
temperature range and has excellent stability. Nickel ornickel alloys are used as well, but they arent as accurate.
In several common configurations, the platinum wire is
exposed directly to air (called a bird-cage element),wound around a bobbin and then sealed in molten glass,or threaded through a ceramic cylinder.
Metal film RTDs are new. To make these, a platinum ormetal-glass slurry film is deposited onto a ceramicsubstrate. The substrate is then etched with a laser.
These RTDs are very small but arent as stable (andhence accurate).
RTDs are more accurate but also larger and moreexpensive than thermocouples.
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How it works: Utilizes the fact that
resistance of a metalchanges withtemperature.
Make up: Traditionally made up of
platinum, nickel, iron orcopper wound around aninsulator.
Temperature range: From about -196C to
482C.
Thin Film RTD
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Wire wound and thin filmdevices.
Nearly linear over a wide
range of temperatures.
Can be made small enoughto have response times of a
fraction of a second.
Require an electrical current
to produce a voltage drop
across the sensor
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Advantages:
Stable
Very accurate
Change in resistanceis linear
Disadvantages:
Expensive
Current source
required Small change in
resistance
Self heating
Less rugged than
thermocouples.
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Air conditioning and
refrigeration servicing
Furnace servicing
Foodservice processingMedical research
Textile production
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Several different bridge circuits are used to determine
the resistance. Bridge circuits help improve the
accuracy of the measurements significantly. Bridge
output voltage is a function of the RTD resistance.
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Published equations relating bridge voltage totemperature can be used.
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RTDs are more fragile than thermocouples. An external current must be supplied to the RTD. This current
can heat the RTD, altering the results. For situations with high
heat transfer coefficients, this error is small since the heat is
dissipated to air. For small diameter thermocouples and stillair this error is the largest. Use the largest RTD possible and
smallest external current possible to minimize this error.
Be careful about the way you set up your measurement
device. Attaching it can change the voltage.
When the platinum is connected to copper connectors, a
voltage difference will occur (as in thermocouples). This
voltage must be subtracted off.
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How it works: Like the RTD a thermistor
uses the fact thatresistance of asemiconductor changes
with temperature.
Make up: Generally made up of
semiconductor materials
Temperature Range: About -45C - 150C
Thermistor
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Thermistors measure the change in resistance withtemperature.
Thermistors are very sensitive (up to 100 times more than
RTDs and 1000 times more than thermocouples) and can
detect very small changes in temperature. They are also
very fast. Due to their speed, they are used for precision
temperature control and any time very small temperature
differences must be detected.
They are made of ceramic semiconductor material (metaloxides).
The change in thermistor resistance with temperature is
very non-linear.
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A semiconductor used as a temperature sensor.
Mixture of metal oxides pressed into a bead, wafer or othershape.
Beads can be very small, less than 1 mm in some cases.
The resistance decreases as temperature increases, negativetemperature coefficient (NTC) thermistor.
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Most are seen in medicalequipment markets.
Thermistors are also
used are for engine
coolant, oil, and air
temperature
measurement in the
transportation industry.
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Advantages:
Very sensitive (has
the largest output
change from inputtemperature)
Quick response
More accurate than
RTD and
Thermocouples
Disadvantages:
Output is a non-linear
function
Limited temperaturerange.
Require a current
source
Self heating
Fragile
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How it works: Made up of two different
metals joined at one endto produce a smallvoltage at a given
temperature. Make up:
Made of up two differentmetals. Ex: A type J ismade up of Iron and
Constantan. Temperature Range
Type J: - 190C to 750C
A few Thermocouples
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Thermocouples
Two dissimilar metals are joined together
Change in temperature at junction generatesvoltage
Smaller size compared to thermistors Fast response time (time constant as small as 1 ms)
Ease of fabrication, long term stability
Low sensitivity, small output voltage
Need reference temperature
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Seebeck EffectAn electromotive force (EMF) exists in a loop
made oftwo dissimilar metals when the two
junctions are different temperatures.
T1 T2
A
B
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T1 T2
A
B
+
_+
_
pAB pBA
Peltier EMF Voltage at the junction of two dissimilar
metals
Denoted aspAB, pBA
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Seebeck effect
If two wires of dissimilar metals are joined at both ends
and one end is heated, current will flow.
If the circuit is broken, there will be an open circuit
voltage across the wires. Voltage is a function of temperature and metal types.
For small Ts, the relationship with temperature is
linear
For larger Ts, non-linearities may occur.
2 1whereT T T T
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Type B very poor below 50C; reference junction temperature notimportant since voltage output is about the same from 0 to 42 C
Type E good for low temperatures since dV/dT (a) is high for low
temperatures
Type J cheap because one wire is iron; high sensitivity but also
high uncertainty (iron impurities cause inaccuracy)
Type T good accuracy but low max temperature (400 C); one
lead is copper, making connections easier; watch for heat being
conducted along the copper wire, changing your surface temp
Type K popular type since it has decent accuracy and a wide
temperature range; some instability (drift) over time
Type N most stable over time when exposed to elevated
temperatures for long periods
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Appendix 3 of book gives the output voltage ofseveral thermocouples (TC) types over a rangeof temperature in 5C increments. In each case,the TC reference temperature is 0C. The
temperature is in C and the output is in mV.Each column is in 5C increments from thetemperature of that row.
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Plastic injection moldingmachinery
Food processing
equipment
Deicing
Semiconductor processing
Heat treating
Medical equipment
Industrial heat treating
Packaging equipment
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Simple, Rugged High temperature operation
Low cost
No resistance lead wire
problems
Point temperature sensing
Fastest response to
temperature changes
Least stable, leastrepeatable
Low sensitivity to small
temperature changes
Extension wire must be ofthe same thermocouple
type
Wire may pick up radiated
electrical noise if notshielded
Lowest accuracy
Advantages Disadvantages
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Cost thermocouples are cheapest by far, followed by RTDs. Accuracy RTDs or thermistors.
Sensitivity thermistors.
Speed thermistors.
Stability at high temperatures not thermistors.
Size thermocouples and thermistors can be made quite
small.
Temperature range thermocouples have the highest range,
followed by RTDs.
Ruggedness thermocouples are best if your system will be
taking a lot of abuse.
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Bimetallic Strip
Application
Thermostat (makes or
breaks electrical
connection withdeflection)
Metal A
Metal B
)]T-(T1[ 00 LL
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Minco Pyrotek
Omega
Watlow Texas Instrument
National Semiconductor
Maxim
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I2C Interface -55 to 125C range
1 accuracy (3 max)
0.0625C resolution
2.7 to 5.5 operating voltage
45 to 75 A operating current,0.1 to 1A shutdown current
40ms/320ms conversionrate(9/12 bit)
25/3 conversions per second(9/12 bit)
Online sample request 6 pin SO23 package
Needs 400kHz clock for I2CInterface
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I2C Interface
-55 to 125C range
1 accuracy (2 max)
5/0.0625C resolution(625/626)
3.0 to 5.5 operating voltage 250A to 1mA operating current,
1A shutdown current
133ms conversion rate
Online sample request
6 pin SO23 package
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I2C Interface
-55 to 125C range
2/ 1 accuracy
9 bits/ 12 bits or 0.0625Cresolution
3/3.3 to 5.5 operating voltage 0.25 to 0.5 A operating current,
4/5A shutdown current
100ms/400ms conversionrate(9/12 bit)
Online sample request
8 pin SOP package
Needs 400kHz clock for I2CInterface
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Texas Instrument TMP100/101 Small package
Can operate on lower voltage
Low power consumption Accurate