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Rolta Academy : Engineering Design Services

CHAPTER 1

SENSORS AND PRIMAR TRANSD!CERS

1"# INTROD!CTIONA basic instrument system consists of three elements: SENSOR or INPUT DEVICE SINA! PROCESSOR RECEIVER or OUTPUT DEVICE

This cha"ter is #e$ote# to in"ut #e$ices but you can ne$er se"arate it fromthe rest of the system as in many cases they are all inte%ral &e'%' a mechanical"ressure %au%e incor"orates all of these elements(' A bloc) #ia%ram of a basicsystem is sho*n but they are usually more com"le+ .

$ig 1"1: %ASIC INSTR!MENT SSTEM

,ost mo#ern analo%ue e-ui"ment *or)s on the follo*in% stan#ar# si%nal ran%es'

Electric . to /0 mA Pneumatic 0'/ to 1'0 bar

Ol#er electrical e-ui"ment use 0 to 10 V' Increasin%ly the instruments are #i%ital*ith a binary #i%ital enco#er built in to %i$e a binary #i%ital out"ut' Pneumatic si%nalsare commonly use# in "rocess in#ustries for safety es"ecially *hen there is a ris)of fire or e+"losion'

The a#$anta%e of ha$in% a stan#ar# ran%e or usin% #i%ital si%nals is that alle-ui"ment may be "urchase# rea#y calibrate#' 2or analo%ue systems the minimumsi%nal &Tem"erature3 s"ee#3 force3 "ressure an# so on( is re"resente# by . mA or0'/ bar an# the ma+imum si%nal is re"resente# by /0 mA or 1'0 bar'

This cha"ter is an attem"t to familiari4e you *ith the many ty"es of in"ut sensorson the mar)et to#ay' Usually such sensors are calle# PRI,AR5 TRANSDUCERS'

Thin%s that *e commonly measure are:

Tem&erat're$lo( rate

Movement) *elocity and Acceleration+evel or De&t,

Density

Instr'mentation Engineering 1

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Rolta Academy : Engineering Design ServicesAcidity-Al.alinity

S&eed$orce

Press're

Stress and StrainMass or /eig,tSi0e or *ol'me

Sensors may o"erate sim"le on6off s*itches to #etect the follo*in%:

O2ects 3Pro4imity s(itc,5Hot or cold 3t,ermostat5

Em&ty or 6'll 3level s(itc,5Press're ,ig, or lo( 3&ress're s(itc,5

The bloc) #ia%ram of a sensor is sho*n belo*'

$ig 1"7: SENSOR %+OC8 DIA9RAM

1"1 TEMPERAT!RE TRANSD!CERS1"1"1 THERMOCO!P+ES

7hen t*o *ires *ith #issimilar electrical "ro"erties are 8oine# at both en#s an# one

8unction is ma#e hot an# the other col#3 a small electric current is "ro#uce#"ro"ortional to the #ifference in the tem"erature' See bec) #isco$ere# this effect' Itis true no matter ho* the en#s are 8oine# so the col# en# may be 8oine# at a sensiti$emilli$olt meter' The hot 8unction forms the sensor en#'

$ig 1": THERMOCO!P+EThe "icture sho*s a ty"ical in#ustrial "robe *ith a fle+ible e+tension an# stan#ar#

"lu%'


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$ig 1";: IND!STRIA+ THERMOCO!P+E

Peltier sho*e# that heat is absorbe# at the hot en# an# re8ecte# at the col# en#'Thom"son sho*e# that "art of the e'm'f' is #ue to the tem"erature %ra#ient in the*ire as *ell as the tem"erature #ifference bet*een the 8unctions' ,ostthermocou"le metals "ro#uce a relationshi" bet*een th e t*o tem"eratures an# thee'm'f as follo*s:

E91&1/(;&1/1/(An# are constants for the ty"e of thermocou"le' The relationshi" is nearly

linear o$er the o"eratin% ran%e' The actual characteristic an# suitable o"eratin%tem"eratures #e"en#s u"on the metals use# in the *ires' The $arious ty"es are#esi%nate# in international an# national stan#ar#s' Ty"ical linear o"eratin% ran%es aresho*n for stan#ar# ty"es'

It is im"ortant that thermocou"les are stan#ar# so that the same e'm'f *illal*ays re"resent the same tem"erature'

T5PE < 0 TO =00CT5PE > 0 TO 1/00CT5PE T 1?? TO /@0CT5PE E 0 TO 00CT5PE R6S 0 TO 100CT5PE B @00 TO 1=00CT5PE N 0 TO 1/00CT5PE ! 0 TO =00C

Thermocou"les come in se$eral forms' They may be *ires insulate# from eachother *ith "lastic or %lass fiber materials' 2or hi%h tem"erature *or)3 the *ire "airs are"ut insi#e a tube *ith mineral insulation' 2or in#ustrial uses the sensor comes in ametal enclosure such as stainless steel'

1"1"7 RESISTANCE TPE SENSORS


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Rolta Academy : Engineering Design Servicesflui# such as use# in refri%erators' In both cases the insi#e of the sensor hea# an# theconnectin% tube are com"letely full' Any rise in tem"erature "ro#uces e+"ansion ore$a"oration of the li-ui# so the sensor becomes "ressuri4e#' The "ressure is relate#to the tem"erature an# it may be in#icate# on a sim"le "ressure %au%e'

7ays an# means e+ist to con$ert the "ressure into an electrical si%nal' Themo$ement may also #irectly o"erate a thermostat' These instruments are robust an#use# o$er a *i#e ran%e' They can be fitte# *ith electric s*itches to set off alarms'

$ig 1"@: *APOR PRESS!RE SENSOR

1" %IMETA++IC TPES

It is a *ell)no*n "rinci"le that if t*o metals are ri%i#ly 8oine# to%ether as a t*olayer stri" an# heate#3 the #ifference in the e+"ansion rate causes the stri" toben#'

$ig 1": %IMETA++IC TPESIn the in#ustrial ty"e3 the stri" is t*iste# into a lon% thin coil insi#e a tube' One en# isfi+e# at the bottom of the tube an# the other turns an# mo$es a "ointer on a #ial' Theout*ar# a""earance is $ery similar to the "ressure ty"e' They can be ma#e too"erate limit s*itches an# set off alarms or act as a thermostat' &E'%' on a boiler('


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1"; 9+ASS THERMOMETERThe or#inary %lass thermometer is also a com"lete system' A%ain the bulb is the

sensor but the column of li-ui# an# the scale on the %lass is the "rocessor an#

in#icator' ,ercury is use# for hot tem"eratures an# colore# alcohol for col#tem"eratures'

$ig 1"B: THERMOMETER

The "roblems *ith %lass thermometers are that they are

Brittle ,ercury soli#ifies at .0C' Alcohol boils at aroun# 1/0 C' Accurate manufacture is nee#e# an# this ma)es accurate ones e+"ensi$e' It is easy for "eo"le to ma)e mista)es rea#in% them'

lass thermometers are not use# much no* in in#ustry but if they are3 they areusually "rotecte# by a shiel# from acci#ental brea)a%e' In or#er to measure thetem"erature of somethin% insi#e a "i"e they are "lace# in thermometer "oc)ets'

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$ig 1"1#: %O!RDON T!%EThe Bour#on tube is a hollo* tube *ith an elli"tical cross section' 7hen a "ressure

#ifference e+ists bet*een the insi#e an# outsi#e3 the tube ten#s to strai%hten out an#the en# mo$es' The mo$ement is usually cou"le# to a nee#le on a #ial to ma)e acom"lete %au%e' It can also be connecte# to a secon#ary #e$ice such as an airno44le to control air "ressure or to a suitable trans#ucer to con$ert it into an electricsi%nal' This ty"e can be use# for measurin% "ressure #ifference'

1"

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Rolta Academy : Engineering Design Servicesof the "iston is #irectly relate# to the "ressure' A *in#o* in the outer case allo*s the"ressure to be in#icate#' This ty"e is usually use# in hy#raulics *here the ability to*ithstan# shoc)3 $ibration an# su##en "ressure chan%es is nee#e# &shoc) "roof%au%e(' The "iston mo$ement may be connecte# to a secon#ary #e$ice to con$ert

mo$ement into an electrical si%nal'

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There are $arious *ays of con$ertin% the mechanical mo$ement of the "rece#in%ty"es into an electric si%nal' The follo*in% are ty"es that #irectly "ro#uce an electricsi%nal'

Strain au%e ty"es' Pie4o electric ty"es' Other electric effects'

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Rolta Academy : Engineering Design Servicesob8ect' There are many ty"es usin% #ifferent "rinci"les an# most of them "ro#ucean electrical out"ut'

1"="1 OPTICA+ TPES

$ig 1"1< :OPTICA+ TPES

These use a li%ht beam an# a li%ht sensiti$e cell' The beam is either reflecte# orinterru"te# so that "ulses are "ro#uce# for each re$olution' The "ulses are thencounte# o$er a fi+e# time an# the s"ee# obtaine#' Electronic "rocessin% is re-uire# totime the "ulses an# turn the result into an analo%ue or #i%ital si%nal'

1"="7 MA9NETIC PIC8 !PS

$ig 1"1= MA9NETIC PIC8 !PSThese use an in#ucti$e coil "lace# near to the rotatin% bo#y' A small ma%net on the

bo#y %enerates a "ulse e$ery time it "asses the coil' If the bo#y is ma#e of ferrousmaterial3 it *ill *or) *ithout a ma%net' A #iscontinuity in the surface such as a notch *illcause a chan%e in the ma%netic fiel# an# %enerate a "ulse' The "ulses must be"rocesse# to "ro#uce an analo%ue or #i%ital out"ut'

1"=" TACHOMETERS


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$ig 1"1@: TACHOMETERS

There are t*o ty"es3 A'C' an# D'C' The A'C' ty"e %enerates a sinusoi#alout"ut' The fre-uency of the $olta%e re"resents the s"ee# of rotation' Thefre-uency must be counte# an# "rocesse#' The D'C' ty"e %enerates a $olta%e#irectly "ro"ortional to the s"ee#' Both ty"es must be cou"le# to the rotatin% bo#y'Very often the tachometer is built into electric motors to measure their s"ee#.

1"@ $+O/ METERSThere are many hun#re#s of ty"es of flo* meters #e"en#in% on the ma)e an#

a""lication' They may be classifie# rou%hly as follo*s'

POSITI*E DISP+ACEMENT TPES IN$ERENTIA+ TPES *ARIA%+E AREA TPES DI$$ERENTIA+ PRESS!RE TPES

1"@"1 POSITI*E DISP+ACEMENT TPES

These ty"es ha$e a mechanical element that ma)es the shaft of the meter rotateonce for an e+act )no*n -uantity of flui#' The -uantity of flui# hence #e"en#s on thenumber of re$olutions of the meter shaft an# the flo* rate #e"en#s u"on the s"ee# ofrotation' Both the re$olutions an# s"ee# may be measure# *ith mechanical orelectronic #e$ices' Some of the most common liste# belo*'

1' Rotary "iston ty"e'/' Vane ty"e'' !obe ty"e or meshin% rotor'.' Reci"rocatin% "iston ty"e@' 2lute# s"iral %ear'

' MESHIN9 ROTOR


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$ig 1"7# :IND!STRIA+ $+O/ METERSThe turbine ty"e sho*n has an a+ial rotor3 *hich is ma#e to s"in by the flui#3 an#

the s"ee# re"resents the flo* rate' This may be sense# electrically by cou"lin% the

shaft to a small electric tachometer' Often this consists of a ma%netic slu% on the rotor3*hich %enerates a "ulse of electricity each time it "asses the sensor'

ROTATIN9 *ANE TPE

$ig 1"71: ROTATIN9 *ANE TPEThe 8et of flui# s"ins aroun# the rotatin% $ane an# the s"ee# of the rotor is

measure# mechanically or electronically'

1"@" *ARIA%+E AREA TPESThere are t*o main ty"es of this meter1' 2loat ty"e &Rotameter(

/' Ta"ere# "lu% ty"e' $+OAT TPE


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$ig 1"77: $+OAT TPEThe float is insi#e a ta"ere# tube' The flui# flo*s throu%h the annular %a" aroun#

the e#%e of the float' The restriction causes a "ressure #ro" o$er the float an# the"ressure forces the float u"*ar#s' Because the tube is ta"ere#3 the restriction is

#ecrease# as the float mo$es u"' E$entually a le$el is reache# *here the restrictionis 8ust ri%ht to "ro#uce a "ressure force that counteracts the *ei%ht of the float' Thele$el of the float in#icates the flo* rate' If the flo* chan%es the float mo$es u" or #o*nto fin# a ne* balance "osition'

7hen #an%erous flui#s are use#3 "rotection is nee#e# a%ainst the tube fracturin%'The tube may be ma#e of a nonma%netic metal' The float has a ma%net on it' As itmo$es u" an# #o*n3 the ma%net mo$es a follo*er an# "ointer on the outsi#e' The"osition of the float may be measure# electrically by buil#in% a mo$ement trans#ucerinto the float' TAPERED P+!9 TPE

$ig

1"7: TAPPERED P+!9 TPE

In this meter3 a ta"ere# "lu% is ali%ne# insi#e a hole or orifice' A s"rin% hol#s it in

"lace' The flo* is restricte# as it "asses throu%h the %a" an# a force is "ro#uce#*hich mo$es the "lu%' Because it is ta"ere# the restriction chan%es an# the "lu%ta)es u" a "osition *here the "ressure force 8ust balances the s"rin% force' Themo$ement of the "lu% is transmitte# *ith a ma%net to an in#icator on the outsi#e'

1"@"; DI$$ERENTIA+ PRESS!RE $+O/ METERSThese are a ran%e of meters that con$ert flo* rate into a #ifferential "ressure' The

im"ortant ty"es are


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Rolta Academy : Engineering Design Services1' ORI2ICE ,ETERS'/' VENTURI ,ETERS' NOFF!E ,ETERS.' PITOT TUBES'

The #ia%ram sho*s a cross section throu%h the four ty"es of D'P' ,eters


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$ig 1"7;: DI$$ERENTIA+ PRESS!RE $+O/ METERSThe *or)in% "rinci"le for all these is that somethin% ma)es the $elocity of the flui#

chan%e an# this "ro#uces a chan%e in the "ressure so that a #ifference " 9 "/ "1is

create#' It can be sho*n for all these meters that the $olume flo* rate G is relate#to " by the follo*in% formula'> 8 3

&5#" is the meter constant' A full e+"lanation of these meters is co$ere# in the tutorials onflui# mechanics' The "icture sho*s an in#ustrial D'P''meter' E+tra instrumentationhea#s can be fitte# to "ro#uce an electrical out"ut &. /0 mA( or a "neumaticout"ut &0'/ 1 bar('

1"$ORCE SENSORSThe main ty"es of force sensors are1' ,echanical ty"es'

/' y#raulic ty"es'' Electrical strain %au%e ty"es'

1""1 MECHANICA+ TPES

,echanical ty"es are usually com"lete measurin% systems in$ol$in% some form ofs"rin% such as in a sim"le s"rin% balance or bathroom scale' It is a basic mechanical"rinci"le that the #eflection of a s"rin% is #irectly "ro"ortional to the a""lie# force so if


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Rolta Academy : Engineering Design Servicesthe mo$ement is sho*n on a scale3 the scale re"resents force'

$ig 1"7

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$ig 1"7@:STRAIN 9!A9E TPE

1"B POSITION SENSORSPosition sensors are essential elements in the control of actuators' The "osition of

both linear an# rotary actuators is nee#e# in robotic ty"e mechanisms' There arethree "rinci"le ty"es'1' RESISTIVE/' OPTICA!' INDUCTIVE

1"B"1 RESISTI*E TPES

$ig 1"7: RESISTI*E TPEA "otentiometer is a $ariable electrical resistance' A len%th of resistance material

has a $olta%e a""lie# o$er its en#s' A sli#er mo$es alon% it &either linear or rotary( an#"ic)s off the $olta%e at its "osition or an%le' The trac)s may be ma#e from carbon3resistance *ire or "ie4o resisti$e material' The latter is the best because it %i$es a%oo# analo%ue out"ut' The *ire *oun# ty"e "ro#uces small ste" chan%es in theout"ut #e"en#in% on ho* fine the *ire is an# ho* closely it is coile# on the trac)'

1"B"7 OPTICA+ TPES


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$ig 1"7B :OPTICA+ TPEO"tical ty"es are mainly use# for "ro#ucin% #i%ital out"uts' A common e+am"le is

foun# on machine tools *here they measure the "osition of the *or)table an# #is"lay itin #i%its on the %au%e hea#' Di%ital micrometers an# $erniers also use this i#ea' Thebasic "rinci"le is as follo*s' !i%ht is emitte# throu%h a trans"arent stri" or #isc onto a

"hotoelectric cell' Often reflecte# li%ht is use# as sho*n' The stri" or #isc has $ery finelines en%ra$e# on it3 *hich interru"t the beam' The numbers of interru"tions arecounte# electronically an# this re"resents the "osition or an%le' This is $ery much o$ersim"lifie# an# you shoul# refer to more a#$ance# te+t to fin# out ho* $ery accuratemeasurements are obtaine# an# also the #irection of mo$ement'

1"B" IND!CTI*E TPESThe most common of these is the !inear Variable Differential transformer or !VDT'

The transformer is ma#e *ith one "rimary coil an# t*o secon#ary coils3 one "lace#abo$e an# the other belo* the "rimary' The coils are forme# into a lon% narro* hollo*tube' A ma%netic core sli#es in the tube an# is attache# to the mechanism bein%monitore# *ith a nonma%netic stem &e'%' brass(' A constant alternatin% $olta%e isa""lie# to the "rimary coil' This in#uces a $olta%e in both secon#ary coils' 7hen thecore is e+actly in the mi##le3 e-ual $olta%es are in#uce# an# *hen connecte# as


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$ig 1"# Ind'ctive ty&essho*n3 they cancel each other out' 7hen the core mo$es3 the $olta%e in onesecon#ary coil %ro*s but re#uces in the other' The result is an out"ut $olta%e3 *hich

re"resents the "osition of the core3 an# hence the mechanism to *hich it is attache#'The out"ut $olta%e is usually con$erte# into D'C' 7ith suitable electronic e-ui"ment for"hase #etection3 it is "ossible to #etect *hich #irection the core mo$es an# to s*itchthe DC $olta%e from "lus to minus as the core "asses the center "osition' These canbe $ery accurate an# are *i#ely use# for %au%in% the #imensions of machine#com"onents'

1"1# DEPTH 9A!9ESDe"th %au%es measure the #e"th of li-ui#s an# "o*#er in tan)s' They use a

$ariety of "rinci"les an# "ro#uce out"uts in electrical an# "neumatic forms' The ty"eto use #e"en#s on the substance in the tan)' ere are a fe*'

$ig 1"1: DEPTH 9!A9ES

The ultrasonic system reflects soun# *a$es from the surface an# #etermines the#e"th from the time ta)en to recei$e the reflecte# soun#' The electronic $ersion uses a$ariety of electrical affects inclu#in% con#uction of the flui# an# ca"acitance' The"neumatic $ersion bubbles air throu%h the li-ui# an# the "ressure of the air is relate# to

the #e"th' A sim"le "ressure %au%e attache# to a tan) is also in#icates the #e"thsince #e"th is "ro"ortional to "ressure'

1"11 STRAIN 9A!9ESStrain %au%es are use# in many instruments that "ro#uce mechanical strain

because of the affect bein% measure#' In their o*n ri%ht3 they are use# to measure thestrain in a structure bein% stretche# or com"resse#'The strain %au%e element is a $ery thin *ire that is forme# into the sha"e sho*n' This


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Rolta Academy : Engineering Design Services"ro#uces a lon% *ire all in one #irection but on a small surface area' The element isoften forme# by etchin% a thin foil on a "lastic bac)in%' The com"lete# element is then%lue# to the surface of the material or com"onent that *ill be straine#' The a+is of thestrain %au%e is ali%ne# *ith the #irection of the strain' 7hen the com"onent is

stretche# or com"resse#3 the len%th of the resistance *ire is chan%e#' This "ro#uces acorres"on#in% chan%e in the electrical resistance'

!et the len%th of the %au%e be ! an# the chan%e in len%th be !'The mechanical strain 9 !6!!et the resistance of the %au%e be R &ty"ically 1/0 ( an# the chan%e in resistancebe R'The electrical strain 9 R6R'The electrical an# mechanical strains are #irectly "ro"ortional an# the constantrelatin% them is calle# the %au%e factor &ty"ically /('

au%e 2actor 9 Electrical Strain6,echanical strain 9 69 !R6R!

STRAIN 9A!9E ARRAN9EMENTSA strain %au%e is of little use unless *e can con$ert the chan%e in resistance into a$olta%e' This is best #one *ith a 7heat stone bri#%e'

$ig 1"7 Strain 9'age arrangementIf only one acti$e %au%e is use#3 this *oul# be R 1or R/' R1an# R/must be

e-ual3 so must Ran# R.' In this case3 the $olta%e at "oints 1 an# / are e-ual toVs6/ an# so the out"ut Vois 4ero' In or#er to ensure this3 the balancin% resistor RBisa#8uste# to ma)e the out"ut 4ero *ith no strain a""lie# to the %au%e' Su""ose that

R1is the acti$e %au%e' If the bri#%e is balance# then the $olta%e at "oints 1 an# / ishalf theSu""ly $olta%e' V19 V/9 Vs6/


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TEM PERA T!

RE

E$$ECTSOne of the "roblems *ith strain %au%es is that the resistance also chan%es *ith

tem"erature an# so it is $ital that each "air of resistors is maintaine# at the same

tem"erature'

$ig 1": TEMPERAT!RE E$$ECTSIf one acti$e %au%e is use# say R1then the other resistor R/must be "lace#

near to it an# this is best #one by usin% a DU,,5 UAE fi+e# close to theacti$e %au%e but in a "osition *here it is unstraine#' Better stillH ma)e R /anotheracti$e %au%e an# so #ouble the out"ut from the bri#%e' 2or e+am"le if a beam is

use# to "ro#uce the strain3 one %au%e is "lace# on the to" an# the other on thebottom as sho*n'

!et R1increase an# R/#ecrease by R the $olta%e at "oint 1 becomes:The out"ut becomes: VS&RR(6/R &usin% ratio of resistances(VO9V/V19VS6/VS&RR(6/RVO9VSR6/&/R;R(JVO9VSR6/R


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VO9VS6/ *hich is almost #ouble the out"utIf the loa# cell only "ro#uces tension or com"ression3 the acti$e %au%es are R 1an# R/ *ith R an# R.bein% #ummy %au%es' All . %au%es are then at the sametem"erature' This is sho*n in the #ia%ram'

$ig 1"; Strain g'ages

The $olta%e at "oint 1 becomes VSR/(2R+ R)

An# at "oint / becomes VS(R+ R)/ (2R+ R)

The out"ut becomes VO=V2-V1=VS R/(2R+ R)

Di$i#in% to" an# bottom *e %et VO=VS( R/R){2+ R/R}

VO=VSG/(2+G)

This is #ouble the out"ut of a sin%le out"ut %au%e an# fully tem"erature stable'

$ig 1"< 9a'ges in Tension and Com&ressionIf a beam is use# in the loa# cell all . %au%es may be ma#e acti$e as sho*n'The out"ut at 1 becomes

V1=VS(R-

R)/2RAn# at "oint / becomes

V2=VS(R+ R)/2R

The out"ut becomes VO=V2-V1=VS R/R VO=VSG

This is . times the out"ut of a sin%le acti$e %ua%e an# fully tem"erature stable


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CHAPTER 7

SI9NA+ PROCESSORS AND RECEI*ERS

7"1"INTROD!CTIONA basic instrument system consists of a sensor a "rocessor an# a recei$er' This

tutorial is about the "rocessor an# the recei$er' The technolo%y use# in si%nal"rocessin% is also im"ortant for automatic control systems'

$ig 7"1: %+OC8 DIA9RAM

7"7 SI9NA+ PROCESSIN9 AND CONDITIONIN95ou shoul# no* be familiar *ith trans#ucers an# sensors' These *ere PRI,AR5

TRANSDUCERS' 7e must no* e+amine ho* to "rocess the out"ut of thetrans#ucers into the form re-uire# by the rest of the instrument system' These mayalso be trans#ucers but in this case3 SECONDAR5 TRANSDUCERS' ,ost mo#erne-ui"ment *or)s on the follo*in% stan#ar# si%nal ran%es'

Electric . to /0 mA Pneumatic 0'/ to 1'0 bars Di%ital stan#ar#sOl#er electrical e-ui"ment uses 0 to 10 V'

The a#$anta%e of ha$in% a stan#ar# ran%e is that all e-ui"ment is sol# rea#y


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Rolta Academy : Engineering Design Servicescalibrate#' This means that the minimum si%nal &Tem"erature3 s"ee#3 force3 "ressurean# so on( is re"resente# by . mA or 0'/ bar an# the ma+imum si%nal is re"resente#by /0 mA or 1'0 bar' The "rimary trans#ucer *ill not "ro#uce these stan#ar# ran%esso the "ur"ose of "rocessin% an# con#itionin% is usually to con$ert the out"ut into the

stan#ar# ran%e'The $ast array of instrumentation an# control e-ui"ment a$ailable uses many forms ofsi%nals' ere is a summary'

E!ECTRICA! Volta%e3 current3 #i%ital' ,ECANICA! 2orce an# mo$ement' PNEU,ATIC AND 5DRAU!IC Pressure an# flo*' OPTICA! i%hs"ee# #i%ital si%nal transmission' RADIO Analo%ue an# #i%ital transmission' U!TRA VIO!ET Similar a""lication to ra#io o$er short ran%es'Processin% may #o the follo*in% thin%s'

Chan%e the le$el or $alue of the si%nal &e'%' $olta%e le$el( Chan%e the si%nal from one form to another' &e'%' current to "neumatic( Chan%e the o"eratin% characteristic *ith res"ect to time' Con$ert analo%ue an# #i%ital si%nals from one to the other'2irst letKs e+amine those "rocessors *hich chan%e the le$el or $alue of the si%nal'7"7"1 AMP+I$IERS

$ig 7"7 Am&li6iers SymolAm"lifiers may am"lify VO!TAE3 CURRENT or BOT in *hich case it is a PO7ERA,P!I2IER'Am"lifier %ain may be e+"resse# as a ratio or in #ecibels' The letter 7 in#icates it refersto "o*er %ain' The %ain in #b7 is %i$en by

In "ractice3 an am"lifier %enerates some noise an# the in"ut an# out"ut terminalsha$e a resistance that %o$erns the ratio of current to $olta%e' A mo#el is sho*n in *hicha noise %enerator is in#icate# an# in"ut an# out"ut resistors'

$ig 7" Am&li6ier model


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Rolta Academy : Engineering Design ServicesSince electric "o*er into a resisti$e loa# is %i$en as

7"7"7 DI$$ERENTIA+ AMP+I$IERSThese ha$e t*o in"uts an# the #ifference bet*een them is am"lifie#' The electronic

symbol is sho*n'

$ig 7"; Di66erential Am&li6iers

7"7" ATTEN!ATORS

Sometimes a si%nal is too bi% an# must be re#uce# by attenuatin% it' Electricalsi%nals are attenuate# *ith resistors3 *hich #issi"ate the electric "o*er as heat' Ste"#o*n transformers an# %earbo+es for e+am"le3 are not strictly attenuators becausethey re#uce the le$el3 not the "o*er' The %ain of an attenuator in #b is ne%ati$e as thene+t e+am"le sho*s'

/OR8ED E?AMP+ECalculate $olta%e the %ain of an attenuator *ith an in"ut $olta%e of 1/ V an# out"ut$olta%e of / V'SO+!TION 9 /0 lo%10&/61/( 9 1@'@ #bV

The term am"lification is often use# *hen the le$el of a si%nal is increase# but notthe "o*er' Strictly s"ea)in%3 such #e$ices shoul# be calle# TRANS2OR,ERS' 2ore+am"le an A'C' electric transformer may increase the $olta%e but not the "o*er' 7eha$e $olta%e am"lifiers an# current am"lifiers3 *hich #o not necessarily chan%e the

"o*er le$el'

7"7"; TRANS$ORMERS

E+ECTRICA+,any #e$ices only chan%e the le$el of a si%nal *ithout chan%in% the "o*er' A

$olta%e am"lifier is one e+am"le' An electrical transformer for alternatin% $olta%esbasically consists of t*o *in#in%s3 a "rimary an# a secon#ary' The coils are *oun#on a ma%netic core'


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$ig 7"< Trans6ormer

The "rimary coil has the in"ut a'c' $olta%e a""lie# an# a'c' current flo*s

Accor#in% to the reactance' The flu+ "ro#uce# is concentrate# in the core an#"asses aroun# the core' It follo*s that the same flu+ cuts the turns on the secon#arycoil an# so an e'm'f *ill be %enerate# in the secon#ary coil' The flu+ #e"en#s u"onthe number of turns T1 an# the same flu+ cuts the secon#ary' The e'm'f in thesecon#ary *ill #e"en# on the number of turns T/' It follo*s that

V1/V2=T1/T2'In an i#eal transformer there is no ener%y loss an# so the "o*er in an# "o*er out

are e-ual' V1 i19 V/ i/' It follo*s that if the $olta%e is ste""e# #o*n3 the current isste""e# u" an# $ice $ersa'

MECHANICA+,echanical transformers are le$ers an# %earbo+es3 *hich chan%e mo$ement3

force3 s"ee# an# tor-ue but not the "o*er' There are use# in many instruments &e'%'a mechanical "ressure %au%e an# the no44le fla""er system #escribe# later('

$ig 7"= Mec,anical trans6ormersThe %ear ratio is in #irect "ro"ortion to the "itch circle #iameters &mean

#iameters( or number of teeth on each *heel'


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Rolta Academy : Engineering Design Services7""1 NO+E $+APPER and DI$$ERENTIA+ PRESS!RE CE++S

The no44le fla""er system is *i#ely use# in D'P' cells' The form sho*n belo*con$erts #ifferential "ressure &e'%' from a #ifferential "ressure flo* meter( into astan#ar# "neumatic si%nal' This is *i#ely use# in the control of air o"erate# "i"eline

$al$es'

$ig 7"B $la&&er No00le SystemThe bello*s res"on# to the #ifferential "ressure an# mo$es the le$er' This mo$es

the fla""er to*ar#s or a*ay from the no44le' The air su""ly "asses throu%h a restrictor

an# lea)s out of the no44le' The out"ut "ressure hence #e"en#s on ho* close thefla""er is to the en# of the no44le' The ran%e of the instrument is a#8uste# by mo$in%the "i$ot an# the 4ero "osition is a#8uste# by mo$in% the relati$e "osition of the fla""eran# no44le'

This system is use# in a $ariety of forms' Instea# of bello*s3 a bour#on tubemi%ht be use# an# this is o"erate# by an e+"ansion ty"e tem"erature sensor to"ro#uce a tem"erature "neumatic si%nal con$erter'


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$ig 7"11 I-P Converter

7""; ANA+O9!E DI9ITA+ CON*ERSIONSo many mo#ern systems no* use #i%ital si%nals that it is im"ortant to a""reciate

ho* analo%ue si%nals are "rocesse# into #i%ital form an# $ice $ersa' Analo%ue to#i%ital con$ersion is a "rocess of turnin% an analo%ue $olta%e or current into a #i%ital

"attern *hich can be rea# by a com"uter an# "rocesse#'Di%ital to analo%ue con$ersion is a "rocess of turnin% a #i%ital "attern from a com"uterinto an analo%ue $olta%e or current'

%INAR N!M%ERS

A number may be re"resente# in #i%ital form by sim"ly settin% a "attern of $olta%es ona line hi%h or lo*' It is normal to use .3=3 1 or / lines' An = bit binary "attern issho*n belo*'

The total "attern is calle# a *or# an# the one sho*n is an =bit *or#' The "atternmay be store# in an =bit re%ister' A re%ister is a tem"orary store *here the *or#may be mani"ulate#'Bit 4ero is calle# the least si%nificant bit &!SB( an# the bit *ith hi%hest $alue is calle#the most si%nificant bit &,SB('Each bit has a $alue of 4ero *hen off &lo*( or the $alue sho*n *hen on &hi%h(' The

ma+imum $alue for an = bit *or# is hence /@@'

PRINCIP+ES O$ A-D CON*ERSION

DI9ITA+ TO ANA+O9!E CON*ERTERSThese are #e$ices for con$ertin% a binary number into an analo%ue $olta%e' The

chan%e in the binary $alue from 4ero to a ma+imum corres"on#s *ith a chan%e in theanalo%ue $alue from 0 to a ma+imum'


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Rolta Academy : Engineering Design ServicesRESO+!TION

7hen a #i%ital number is con$erte# into a $olta%e3 each increment of the binary$alue corres"on#s *ith an increment in the $olta%e out"ut' The $alue of thisincrement is the resolution'

ADDER TPE CON*ERTEROne *ay to con$ert a #i%ital "attern into an analo%ue $olta%e is *ith the a##er

ty"e sho*n belo*' The #ia%ram sho*s a @bit "attern'

$ig 7"17 Adder Ty&e ConverterThe out"ut $olta%e is controlle# by the state of the I6O s*itches &these mi%ht be

a re%ister(' The out"ut of the summin% am"lifier isV 9 &Vref 6/(L161 ; 16= ; 16. ; 16/ ; 1M161 is the least si%nificant bit an# 1 is the most si%nificant bit'

The ma+imum number re"resente# by the #i%ital number is 1 for the @bitsystem sho*n' An increase of one "ro#uces a $olta%e increment of 16/V' Thema+imum $olta%e out"ut is 1V6/'

One "roblem *ith this system is that the theoretical resistance $alues can becomeri#iculously lar%e'+ADDER TPE

Another *ay to con$ert a #i%ital si%nal into an analo%ue si%nal is *ith the la##er ty"eillustrate# belo*'


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Rolta Academy : Engineering Design ServicesINTE9RATIN9 TPE

$ig 7"1< Integrating ty&e

This time no fee#bac) is use#' The reference $olta%e is inte%rate# so the out"ut ofthe inte%rator is a ram" *hich %ro*s *ith time' The time ta)en for the ram" to reachthe same $alue as the in"ut is con$erte# into a #i%ital form by a counter'

7"; SI9NA+ CONDITIONERS"No* letKs loo) at "rocessors *hich chan%e the characteristic of the si%nal'

These are often referre# to as SINA! CONDITIONERS' One e+am"le is thes-uare root e+tractor' 2or e+am"le DP flo* meters "ro#uce an out"ut3 *hich is#irectly "ro"ortional to the s-uare of the flo*' A "rocessor mi%ht be use# alon% *iththe DP cell to e+tract the s-uare root so that the resultin% si%nal is #irectly "ro"ortionalto flo*'

2i% /'1 Bloc) Dia%ram of D6P flo* meterOther con#itioners brin% in the time element' 2or e+am"le the flo* meter "ro#uces

a si%nal re"resentin% the flo* rate' If the si%nal is inte%rate# *ith time3 the out"ut is-uantity' Such a unit is calle# an INTERATOR an# there are mechanical3"neumatic an# electric metho#s'

Some meters "ro#uce a -uantity' 2or e+am"le a mo$ement trans#ucer "ro#uces

#istance' If the si%nal is #ifferentiate# *ith res"ect to time3 *e %et the rate or$elocity' Such units are calle# DI22ERENTIATORS.


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$ig 7"1@ Integrators And Di66erentiators

$I+TERSAnother form of si%nal con#itionin% is the SINA! 2I!TER' This is use# *ith

electric si%nals to remo$e un*ante# "arts of the si%nal' 2or e+am"le a si%nal maycontain AC an# DC si%nals' A sim"le ca"acitor *oul# filter out the DC si%nal' Thesi%nal may contain AC si%nals of #ifferent fre-uencies' A more so"histicate# filter*oul# remo$e un*ante# fre-uencies'

M!+TIP+E? SSTEMS"A mo#ern in#ustrial information transmission system re-uires that many channels

be sent from one "lace to another usin% either a sin%le fre-uency of transmission or asin%le "air of *ires such as a tele"hone line' A system that #oes this is calle#multi"le+in%'

,o#ems are units that recei$e an# transmit the information to each other' Theyre-uire a mar)er si%nal to let each other )no* *hen to recei$e or transmit' Themo#em ac)no*le#%es a re-uest to recei$e an# this is calle# han#sha)in%' 7henac)no*le#%ement is recei$e# by the transmittin% mo#em3 it sen#s the #ata to therecei$in% mo#em'

The channels are sam"le# $ery ra"i#ly one after another for a fi+e# "erio# oftime' This is calle# Time Di$ision ,ulti"le+in% &TD,(' Each sam"le is tac)e# in a-ueue &a buffer( an# sent one after another' The "rocess is continually re"eate#' Therecei$in% en# re$erses the o"eration' The incomin% #ata is "lace# in a buffer3 e+tracte#in se-uence an# #i$erte# to the correct out"ut .

$ig 7"1 TDM SystemIf the information is #i%ital3 it sen#s the information for each channel as a "ac)a%e of


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Rolta Academy : Engineering Design Services#i%its one channel at a time'

%!$$ERS and STORESVery often a si%nal $alue nee#s to be store#' This is usually #one by con$ertin%

them into a #i%ital number an# storin% them in a memory chi"' If a system is recei$in%

or sen#in% serial #ata but the incomin% #ata is arri$in% -uic)er than it can be"rocesse#3 then the incomin% si%nals are "ut into a tem"orary store calle# a buffer*here they *ait to be "rocesse#'

7"< SI9NA+ RECEI*ERSThe in"ut of an instrument system is sense# *ith a "rimary trans#ucer an# then

"rocesse# or con#itione# before bein% sent on to the last item in the chain3 *hich is thesi%nal recei$er' The si%nal recei$er may be

An in#icator A chart recor#er A "lotter A "rinter An alarm A com"uter

Each of these is a com"lete system in its o*n ri%ht an# may itself contain furthercon#itionin%' They are usually #esi%ne# to o"erate from stan#ar# si%nals' In controlsystems3 the si%nal recei$er mi%ht be an actuator' This is co$ere# in later tutorials oncontrol'INDICATORS

In#icators may be analo%ue or #i%ital' Analo%ue in#icators ha$e a scale an# a"ointer mo$es alon% the scale to in#icate the $alue' Di%ital in#icators may be a #is"lay of

numerals *hich chan%e in fi+e# ste"s' Sometimes a #i%ital in#icator is ma#e to simulatean analo%ue scale &e'%' a #i%ital *atch *ith han#s(' The "ointer ho*e$er mo$es infi+e# ste"s' Di%ital in#icators ha$e a minimum resolution an# usually this is smaller thanthe resolution a human eye can ma)e on an analo%ue scale so #i%ital in#icators areoften more accurate to rea#'It is easier to ma)e a mista)e rea#in% an analo%ue scale than a #i%ital in#icator'o*e$er *hen the $alue in#icate# is chan%in% ra"i#ly3 an analo%ue in#ication is bestbecause you can see the *ay the "ointer is mo$in%' It *oul# be #ifficult to rea# ara"i#ly chan%in% set of numerals' 2or e+am"le3 s"ee#ometer an# altimeters are best#is"laye# on an analo%ue scale'


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Rolta Academy : Engineering Design Services$ig 7"1B Indicators

CHART RECORDERSChart recor#ers )ee" a recor# of the measurements o$er a "erio# of time' There

are electrical an# "neumatic $ersions' The recor#er may use "a"er stri"s or "olar

charts' The "a"er mo$es one *ay an# the mar)er "en mo$es at ri%ht an%les to this'Often they use more than one "en to recor# se$eral channels' The recor#er is often"art of a controlle# system an# contains other e-ui"ment to #o *ith the control'Pa"er recor#ers are bein% re"lace# by com"uter systems *hich store theinformation on a ma%netic me#ium an# #is"lays either the in#icate# rea#in% or thechart recor# or both on a $isual #is"lay unit &com"uter screen or VDU('

$ig 7"7# C,art RecordersP+OTTERS AND PRINTERS

Plotters #ra* %ra"hs or charts' The "en recor#ers #escribe# abo$e are ty"es of"lotters s"ecifically for recor#in% a si%nal o$er a lon% time "erio#' The term "lotterusually refers to a system *here the "a"er is stationary an# the "en is able to mo$e inthe + an# y #irections' Often the + #irection is time an# so the "lotter become arecor#er' Plotters are more often use# to "lot one si%nal a%ainst another' They are $erycommonly use# for "ro#ucin% en%ineerin% #ra*in%s create# on a com"uter'Printers refer to a system *here the "a"er is mar)e# by a "rintin% hea#' There are

many ty"es such as #ot matri+3 in) 8ets3 laser an# other electrostatic #e$ices' Printers*ill "rint te+t or they may be use# to "rint "ictures an# #ra*in%s re"ro#ucin% on"a"er *hat can be seen on a VDU'A+ARMS

The "ur"ose of an instrument may be to set off an alarm such as a bell3 siren3flashin% li%ht an# so on' They are s*itche# on by the si%nal "rocessor' An e+am"leis a smo)e #etector'COMP!TERS

Com"uter technolo%y is ta)in% o$er many rolls "erforme# by tra#itional e-ui"ment'The si%nal is "rocesse# into #i%ital form3 in other *or#s into numbers' The com"uter"rocesses the numbers an# stores the #ata3 sen#s it on to a VDU or to other control

e-ui"ment'


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CHAPTER

INSTR!MENT SSTEMS MODE+ F CA+I%RATION"1 MODE+S O$ INSTR!MENT SSTEMS

A mathematical mo#el relates the in"ut an# out"ut of a system or subsystem'In other *or#s it is a formula relatin% the in"ut an# out"ut' The instrument isusually #ra*n as a bloc) *ith the in"ut an# out"ut sho*n' The mathematicalmo#el is *ritten insi#e the bloc)' The %eneral symbol for si%nals is - but s"ecificsymbols may be use#' The suffi+ i #enotes the in"ut an# o the out"ut'7hen the in"ut an# out"ut is a sim"le ratio3 the mo#el is 8ust a number

re"resentin% the ratio of out"ut to in"ut' It is often #enote# by 3 es"ecially if it isa %ain' In such case 9 o6i' If the in"ut an# out"ut ha$e #ifferent units3 then has units also'Some sensors ha$e non linear e-uations an# *e cannot re"resent therelationshi" *ith a sim"le ratio so must use the full e-uation' 2or e+am"le a#ifferential "ressure flo* meter has an e-uation

2lo* rate 9 C &P(16/

7here C is a constant an# P is the #ifferential "ressure'

"7 MODE+S $OR COMP+ETE SSTEMSA com"lete instrument system is ma#e u" from se$eral subsystems

connecte# in series' The best *ay to #e#uce the in"ut or out"ut of a com"letesystem is a ste"byste" analysis of the information "assin% throu%h' Consi#erthe case of a D'P' flo* meter' The meter con$erts flo* rate into #ifferential"ressure' The D'P' is then con$erte# into current an# the current is in#icate# on ameter'


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Rolta Academy : Engineering Design Servicesre"eatability' This is often cause# by friction or some other erratic fault in thesystem'"";STA%I+IT

Instability is most li)ely to occur in instruments in$ol$in% electronic "rocessin%

*ith a hi%h #e%ree of am"lification' A common cause of this is a#$erseen$ironment factors such as tem"erature an# $ibration' 2or e+am"le3 a rise intem"erature may cause a transistor to increase the flo* of current *hich in turnma)es it hotter an# so the effect %ro*s an# the #is"laye# rea#in% DRI2TS' Ine+treme cases the #is"laye# $alue may 8um" about' This3 for e+am"le3 may because# by a "oor electrical connection affecte# by $ibration""< TIME +A9 ERROR

In any instrument system3 it must ta)e time for a chan%e in the in"ut to sho*u" on the in#icate# out"ut' This time may be $ery small or $ery lar%e #e"en#in%u"on the system' This is )no*n as the res"onse time of the system' If thein#icate# out"ut is incorrect because it has not yet res"on#e# to the chan%e3

then *e ha$e time la% error'A %oo# e+am"le of time la% error is an or#inary %lass thermometer' If you "lun%e itinto hot *ater3 it *ill ta)e some time before the mercury reaches the correct le$el'If you rea# the thermometer before it settle# #o*n3 then you *oul# ha$e time la%error' A thermocou"le can res"on# much more -uic)ly than a %lass thermometerbut e$en this may be too slo* for some a""lications'7hen a si%nal chan%es a lot an# -uite -uic)ly3 &s"ee#ometer for e+am"le(3 the"erson rea#in% the #ial *oul# ha$e %reat #ifficulty #eterminin% the correct $alueas the #ial may be still %oin% u" *hen in reality the si%nal is %oin% #o*n a%ain'""= RE+IA%I+IT

,ost forms of e-ui"ment ha$e a "re#icte# life s"an' The more reliable it is3 the

less chance it has of %oin% *ron% #urin% its e+"ecte# life s"an' The reliability ishence a "robability ran%in% from 4ero &it *ill #efinitely fail( to 1'0 &it *ill #efinitelynot fail('""@ DRI$T

This occurs *hen the in"ut to the system is constant but the out"ut ten#s tochan%e slo*ly' 2or e+am"le *hen s*itche# on3 the system may #rift #ue to thetem"erature chan%e as it *arms u"'"; INSTR!MENT CA+I%RATION

,ost instruments contain a facility for ma)in% t*o a#8ustments' These are

The RANE a#8ustment' The FERO a#8ustment'

In or#er to calibrate th e instrument an accurate %au%e is re-uire#' This isli)ely to be a SECONDAR5 STANDARD' Instruments calibrate# as a secon#arystan#ar# ha$e themsel$es been calibrate# a%ainst a PRI,AR5 STANDARD'";"1 PROCED!RE

An in"ut re"resentin% the minimum %au%e settin% shoul# be a""lie#' Theout"ut shoul# be a#8uste# to be correct' Ne+t the ma+imum si%nal is a""lie#' Theran%e is then a#8uste# to %i$e the re-uire# out"ut' This shoul# be re"eate# untilthe %au%e is correct at the minimum an# ma+imum $alues'


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Rolta Academy : Engineering Design Services";"7 CA+I%RATION ERRORS

RANE AND FERO ERROAfter obtainin% correct 4ero an# ran%e for the instrument3 a calibration

%ra"h shoul# be "ro#uce#' This in$ol$es "lottin% the in#icate# rea#in%

a%ainst the correct rea#in% from the stan#ar# %au%e' This shoul# be #onein about ten ste"s *ith increasin% si%nals an# then *ith re#ucin% si%nals'Se$eral forms of error coul# sho* u"' If the 4ero or ran%e is still incorrectthe error *ill a""ear as sho*n'

$ig " Range And ero error

5STERESIS an# NON!INEAR ERRORSysteresis is "ro#uce# *hen the #is"laye# $alues are too small for

increasin% si%nals an# too lar%e for #ecreasin% si%nals' This is commonlycause# in mechanical instruments by loose %ears an# lin)a%es an#friction' It occurs *i#ely *ith thin%s in$ol$in% ma%neti4ation an#

#ema%neti4ation'The calibration may be correct at the ma+imum an# minimum $alues of the

ran%e but the %ra"h 8oinin% them may not be a strai%ht line &*hen it ou%ht to be('This is a non linear error' The instrument may ha$e some a#8ustments for this an#it may be "ossible to ma)e it correct at mi# ran%e as sho*n'

$ig "; Hysterisis and nonGlinear error


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CHAPTER ;%ASIC INSTR!MENT MEAS!RIN9 DE*ICES

O%ECTI*ES

This mo#ule co$ers the follo*in% areas "ertainin% to instrumentation an# control' Pressure 2lo* !e$el Tem"erature

;"# INSTR!MENTATION E>!IPMENT

INTROD!CTION

Instrumentation is the art of measurin% the $alue of some "lant "arameter3"ressure3 flo*3 le$el or tem"erature to name a fe* an# su""lyin% a si%nal that is"ro"ortional to the measure# "arameter' The out"ut si%nals are stan#ar# si%nalan# can then be "rocesse# by other e-ui"ment to "ro$i#e in#ication3 alarms orautomatic control' There are a number of stan#ar# si%nalsH ho*e$er3 those mostcommon in a CANDU "lant are the ./0 mA electronic si%nal an# the /0100 )Pa"neumatic si%nal'

This section of the course is %oin% to #eal *ith the instrumentation e-ui"mentnormal use# to measure an# "ro$i#e si%nals' 7e *ill loo) at the measurement offi$e "arameters: "ressure3 flo*3 le$el3 tem"erature3 an# force'

;"1 PRESS!RE MEAS!REMENT

This mo#ule *ill e+amine the theory an# o"eration of "ressure #etectors&bour#on tubes3 #ia"hra%ms3 bello*s3 force# balance an# $ariable ca"acitance('It also co$ers the $ariables of an o"eratin% en$ironment &"ressure3 tem"erature(an# the "ossible mo#es of failure'

;"1"1 9eneral T,eory

Pressure is "robably one of the most commonly measure# $ariables in the "o*er"lant' It inclu#es the measurement of steam "ressureH fee# *ater "ressure3con#enser "ressure3 lubricatin% oil "ressure an# many more' Pressure is actuallythe measurement of force actin% on area of surface'7e coul# re"resent this as:


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The units of measurement are either in "oun#s "er s-uare inch &PSI( in British

units or Pascals &Pa( in metric' As one PSI is a""ro+imately 000 Pa3 *e oftenuse )Pa an# ,Pa as units of "ressure'

;"1"7 Press're Scales

Before *e %o into ho* "ressure is sense# an# measure#3 *e ha$e toestablish a set of %roun# rules' Pressure $aries #e"en#in% on altitu#e abo$e seale$el3 *eather "ressure fronts an# other con#itions'

The measure of "ressure is3 therefore3 relati$e an# "ressure measurementsare state# as either %au%e or absolute'

au%e "ressure is the unit *e encounter in e$ery#ay *or) &e'%'3 tyre ratin%sare in %au%e "ressure('

A %au%e "ressure #e$ice *ill in#icate 4ero "ressure *hen ble# #o*n toatmos"heric "ressure &i'e'3 %au%e "ressure is reference# to atmos"heric"ressure(' au%e "ressure is #enote# by a &%( at the en# of the "ressure unitLe'%'3 )Pa &%(M'

Absolute "ressure inclu#es the effect of atmos"heric "ressure *ith the %au%e"ressure' It is #enote# by an &a( at the en# of the "ressure unit Le'%'3 )Pa &a(M' An

absolute "ressure in#icator *oul# in#icate atmos"heric "ressure *hencom"letely $ente# #o*n to atmos"here it *oul# not in#icate scale 4ero'Absolute Pressure 9 au%e Pressure ; Atmos"heric Pressure

2i%ure 1 illustrates the relationshi" bet*een absolute an# %au%e' Note thatthe base "oint for %au%e scale is L0 )Pa &%(M or stan#ar# atmos"heric "ressure101' )Pa &a('

The ma8ority of "ressure measurements in a "lant are %au%e' Absolutemeasurements ten# to be use# *here "ressures are belo* atmos"here' Ty"icallythis is aroun# the con#enser an# $acuum buil#in%'


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$ig ;"1 Relations,i& %et(een Asol'te and 9'age Press'res

;"1" Press're Meas'rement

The ob8ect of "ressure sensin% is to "ro#uce a #ial in#ication3 controlo"eration or a stan#ar# &. /0 mA( electronic si%nal that re"resents the "ressurein a "rocess'

To accom"lish this3 most "ressure sensors translate "ressure into "hysicalmotion that is in "ro"ortion to the a""lie# "ressure' The most common "ressuresensors or "rimary "ressure elements are #escribe# belo*'They inclu#e #ia"hra%ms3 "ressure bello*s3 bour#on tubes an# "ressureca"sules' 7ith these "ressure sensors3 "hysical motion is "ro"ortional to thea""lie# "ressure *ithin the o"eratin% ran%e'

5ou *ill notice that the term #ifferential "ressure is often use#' This term refers tothe #ifference in "ressure bet*een t*o -uantities3 systems or #e$ices

;"1"; Common Press're Detectors

%o'rdon T'es

Bour#on tubes are circularsha"e# tubes *ith o$al cross sections &refer to 2i%ure/(' The "ressure of the me#ium acts on the insi#e of the tube' The out*ar#"ressure on the o$al cross section forces it to become roun#e#' Because of thecur$ature of the tube rin%3 the bour#on tube then ben#s as in#icate# in the

#irection of the arro*'


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Pressure$ig ;"7 %o'rdon T'e

Due to their robust construction3 bour#on are often use# in harsh en$ironmentsan# hi%h "ressures3 but can also be use# for $ery lo* "ressuresH the res"onsetime ho*e$er3 is slo*er than the bello*s or #ia"hra%m'

%ello(

Bello*s ty"e elements are constructe# of tubular membranes that are con$olute#aroun# the circumference &see 2i%ure (' The membrane is attache# at one en#to the source an# at the other en# to an in#icatin% #e$ice or instrument' Thebello*s element can "ro$i#e a lon% ran%e of motion &stro)e( in the #irection of thearro* *hen in"ut "ressure is a""lie#'

$ig ;" %ello(sDia&,ragms

A #ia"hra%m is a circularsha"e# con$olute# membrane that is attache# to the"ressure fi+ture aroun# the circumference &refer to 2i%ure .(' The "ressureme#ium is on one si#e an# the in#ication me#ium is on the other' The #eflectionthat is create# by "ressure in the $essel *oul# be in the #irection of the arro*in#icate#


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$ig ;"; Dia&,ragmDia"hra%ms "ro$i#e fast actin% an# accurate "ressure in#ication' o*e$er3 the

mo$ement or stro)e is not as lar%e as the bello*s

$ig ;"< Ca&s'lesCa&s'les

There are t*o #ifferent #e$ices that are referre# to as ca"sule' The first is sho*nin fi% .'@'The "ressure is a""lie# to the insi#e of the ca"sule an#


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Rolta Academy : Engineering Design Servicesif it is fi+e# only at the air inlet it can e+"an# li)e a balloon' This arran%ement isnot much #ifferent from the #ia"hra%m e+ce"t that it e+"an#s both *ays'The ca"sule consists of t*o circular sha"e#3 con$olute# membranes &usuallystainless steel( seale# ti%ht aroun# the circumference' The "ressure acts on the

insi#e of the ca"sule an# the %enerate# stro)e mo$ement is sho*n by the#irection of the arro*'The secon# ty"e of ca"sule is li)e the one sho*n in the #ifferential "ressuretransmitter &DP transmitter( in fi%ure .'' The ca"sule in the bottom is constructe#*ith t*o #ia"hra%ms formin% an outer case an# the inters "ace is fille# *ith$iscous oil' Pressure is a""lie# to both si#e of the #ia"hra%m an# it *ill #eflectto*ar#s the lo*er "ressure'To "ro$i#e o$er"ressuri4e# "rotection3 a soli# "late *ith #ia"hra%mmatchin%con$olutions is usually mounte# in the center of the ca"sule' Silicone oil is thenuse# to fill the ca$ity bet*een the #ia"hra%ms for e$en "ressure transmission',ost DP ca"sules can *ithstan# hi%h static "ressure of u" to 1. ,Pa &/000 "si(

on both si#es of the ca"sule *ithout any #ama%in% effect' o*e$er3 the sensiti$eran%e for most DP ca"sules is -uite lo*' Ty"ically3 they are sensiti$e u" to only afe* hun#re# )Pa of #ifferential "ressure'Differential "ressure that is si%nificantly hi%her than the ca"sule ran%e may#ama%e the ca"sule "ermanently'

;"1"< Di66erential Press're Transmitters

,ost "ressure transmitters are built aroun# the "ressure ca"sule conce"t' Theyare usually ca"able of measurin% #ifferential "ressure &that is3 the #ifferencebet*een a hi%h "ressure in"ut an# a lo* "ressure in"ut( an# therefore3 are

usually calle# DP transmitters or DP cells'

2i%ure illustrates a ty"ical DP transmitter' A #ifferential "ressure ca"sule ismounte# insi#e a housin%' One en# of a force bar is connecte# to the ca"suleassembly so that the motion of the ca"sule can be transmitte# to outsi#e thehousin%' A sealin% mechanism is use# *here the force bar "enetrates thehousin% an# also acts as the "i$ot "oint for the force bar' Pro$ision is ma#e in thehousin% for hi%h "ressure flui# to be a""lie# on one si#e of the ca"sule an# lo*"ressure flui# on the other' Any #ifference in "ressure *ill cause the ca"sule to#eflect an# create motion in the force bar' The to" en# of the force bar is thenconnecte# to a "osition #etector3 *hich $ia an electronic system *ill "ro#uce a .

/0 ma si%nal that is "ro"ortional to the force bar mo$ement'


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$ig ;"= Ty&ical DP transmitter Constr'ctionThis DP transmitter *oul# be use# in an installation as sho*n in 2i%ure '

$ig ;"@ DP Transmitter A&&lications

A DP transmitter is use# to measure the %as "ressure &in %au%e scale( insi#e a$essel' In this case3 the lo*"ressure si#e of the transmitter is $ente# toatmos"here an# the hi%h"ressure si#e is connecte# to the $essel throu%h anisolatin% $al$e' The isolatin% $al$e facilitates the remo$al of the transmitter'The out"ut of the DP transmitter is "ro"ortional to the %au%e "ressure of the %as3i'e'3 . mA *hen "ressure is /0 )Pa an# /0 mA *hen "ressure is 0 )Pa'

;"1"= Strain 9a'ges

Instr'mentation Engineering

48

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$ig ;" Strain 9'age

The strain %au%e is a #e$ice that can be affi+e# to the surface of an ob8ect to#etect the force a""lie# to the ob8ect' One form of the strain %au%e is a metal *ireof $ery small #iameter that is attache# to the surface of a #e$ice bein%monitore#'2or a metal3 the electrical resistance *ill increase as the len%th of the metalincreases or as the cross sectional #iameter #ecreases'

7hen force is a""lie# as in#icate# in 2i%ure =3 the o$erall len%th of the *ire ten#sto increase *hile the crosssectional area #ecreases'The amount of increase in resistance is "ro"ortional to the force that "ro#uce#the chan%e in len%th an# area' The out"ut of the strain %au%e is a chan%e inresistance that can be measure# by the in"ut circuit of an am"lifier'Strain %au%es can be bon#e# to the surface of a "ressure ca"sule or to a forcebar "ositione# by the measurin% element' Sho*n in 2i%ure ? &ne+t "a%e( is astrain %au%e that is bon#e# to a force beam insi#e the DP ca"sule' The chan%ein the "rocess "ressure *ill cause a resisti$e chan%e in the strain %au%es3 *hichis then use# to "ro#uce a ./0 mA si%nal'

2i% .'? Resisti$e Pressure transmitter

;"1"@ Ca&acitance Ca&s'le


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Similar to the strain %au%e3 a ca"acitance cell measures chan%es in electricalcharacteristic' As the name im"lies the ca"acitance cell measures chan%es inca"acitance' The ca"acitor is a #e$ice that stores electrical char%e' It consists of

metal "lates se"arate# by an electrical insulator' The metal "lates are connecte#to an e+ternal electrical circuit throu%h *hich electrical char%e can be transferre#from one metal "late to the other'The ca"acitance of a ca"acitor is a measure of its ability to store char%e' Theca"acitance of the ca"acitance of a ca"acitor is #irectly "ro"ortional to the areaof the metal "lates an# in$ersely "ro"ortional to the #istance bet*een them' Italso #e"en#s on a characteristic of the insulatin% material bet*een them' Thischaracteristic3 calle# "ermitti$ity is a measure of ho* *ell the insulatin% materialincreases the ability of the ca"acitor to store char%e'

C is the ca"acitance in 2ara#sA is the area of the "latesD is the #istance of the "latesQ is the "ermitti$ity of the insulator

By buil#in% a DP cell ca"sule so there are ca"acitors insi#e the cell ca"sule3#ifferential "ressures can be sense# by the chan%es in ca"acitance of theca"acitors as the "ressure across the cell is $arie#'

;"1" Im&act o6 O&erating Environment

All of the sensors #escribe# in this mo#ule are *i#ely use# in control an#instrumentation systems throu%hout the "o*er station'Their e+istence *ill not normally be e$i#ent because the "hysical construction *illbe enclose# insi#e manufacturers "ac)a%in%' o*e$er3 each is hi%hly accurate*hen use# to measure the ri%ht -uantity an# *ithin the ratin% of the #e$ice' Theconstraints are not limite# to o"eratin% "ressure' Other factors inclu#etem"erature3 $a"or content an# $ibration'

*iration

The effect of $ibration is ob$ious in the inconsistency of measurements3 but themore #an%erous result is the stress on the sensiti$e membranes3 #ia"hra%msan# lin)a%es that can cause the sensor to fail' Vibration can come from manysources'Some of the most common are the lo* le$el constant $ibration of an unbalance#"um" im"eller an# the lar%er effects of steam hammer' E+ternal $ibration &loosesu""ort brac)ets an# insecure mountin%( can ha$e the same effect'


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Rolta Academy : Engineering Design ServicesTem&erat're

The tem"erature effects on "ressure sensin% *ill occur in t*o main areas:

The $olumetric e+"ansion of $a"or is of course tem"erature #e"en#ent'De"en#in% on the system3 the increase# "ressure e+erte# is usually alrea#yfactore# in'The secon# effect of tem"erature is not so a""arent' An o"eratin% tem"eratureoutsi#e the ratin% of the sensor *ill create si%nificant error in the rea#in%s' Thebour#on tube *ill in#icate a hi%her rea#in% *hen e+"ose# to hi%her tem"eraturesan# lo*er rea#in%s *hen abnormally col# #ue to the stren%th an# elasticity ofthe metal tube' This same effect a""lies to the other forms of sensors liste#'

*a&or Content

The content of the %as or flui# is usually controlle# an# )no*n' o*e$er3 it ismentione# at this "oint because the "urity of the substance *hose "ressure isbein% monitore# is of im"ortance *hether %aseous or flui# es"ecially3 if the#e$ice is use# as a #ifferential "ressure #e$ice in measurin% flo* of a %as orflui#'i%her than normal #ensity can force a hi%her #ynamic rea#in% #e"en#in% on*here the sensors are locate# an# ho* they are use#' Also3 the $a"or #ensity orambient air #ensity can affect the static "ressure Sensor rea#in%s an# DP cellrea#in%s' Usually3 lo*er rea#in%s are a result of the lo*er a$ailable "ressure ofthe substance' o*e$er3 a DP sensor locate# in a hot an# $ery humi# room *illten# to rea# hi%h'

;"1"B $ail'res and AnormalitiesOverGPress're

All of the "ressure sensors *e ha$e analy4e# are #esi%ne# to o"erate o$er arate# "ressure ran%e' Plant o"eratin% systems rely on these "ressure sensors tomaintain hi%h accuracy o$er that %i$en ran%e' Instrument rea#in%s an# controlfunctions #eri$e# from these #e$ices coul# "lace "lant o"erations in 8eo"ar#y ifthe e-ui"ment is sub8ecte# to o$er "ressure &o$er ran%e( an# subse-uently#ama%e#' If a "ressure sensor is o$er ran%e#3 "ressure is a""lie# to the "oint*here it can no lon%er return to its ori%inal sha"e3 thus the in#ication *oul# returnto some $alue %reater than the ori%inal'

Dia"hra%ms an# bello*s are usually the most sensiti$e an# fast actin% of all"ressure sensors'

They are also ho*e$er3 the most "rone to fracture on o$er"ressurin%' E$en asmall fracture *ill cause them to rea# lo* an# be less res"onsi$e to "ressurechan%es' Also3 the lin)a%es an# internal mo$ements of the sensors often become#istorte# an# can lea$e a "ermanent offset in the measurement' Bour#on tubes


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Rolta Academy : Engineering Design Servicesare $ery robust an# can han#le e+tremely hi%h "ressures althou%h3 *hene+"ose# to o$er"ressure3 they become sli%htly #isten#e# an# *ill rea# hi%h'Very hi%h o$er"ressurin% *ill of course ru"ture the tube'$a'lty Sensing +ines

2aulty sensin% lines create inaccurate rea#in%s an# totally misre"resent theactual "ressure7hen the "ressure lines become "artially bloc)e#3 the #ynamic res"onse of thesensor is naturally re#uce# an# it *ill ha$e a slo* res"onse to chan%e in"ressure' De"en#in% on the se$erity of the bloc)a%e3 the sensor coul# e$en retainan incorrect 4ero or lo* rea#in%3 lon% after the chan%e in $essel "ressure'

A crac)e# or "uncture# sensin% line has the characteristic of consistently lo*rea#in%s' Sometimes3 there can be #etectable #o*ns*in%s of "ressure follo*e#by slo* increases'

+oss o6 +oo& Electrical Po(er

As *ith any instrument that relies on AC "o*er3 the out"ut of the D6P transmitters*ill #ro" to 4ero or become irrational *ith a loss of "o*er su""ly'

;"7 $+O/ MEAS!REMENT

There are $arious metho#s use# to measure the flo* rate of steam3 *ater3lubricants3 air3 etc'3 in a nuclear %eneratin% station' o*e$er3 in this mo#ule *illloo) at the most common3 namely the DP cell ty"e flo* #etector' Also in thissection *e *ill #iscuss the a""lication of a s-uare root e+tractor an# cutoff relay

"lus the "ossible sources of errors in flo* measurements an# #ifferent failuremo#es that can occur'

;"7"1 $lo( Detectors

To measure the rate of flo* by the #ifferential "ressure metho#3 some form ofrestriction is "lace# in the "i"eline to create a "ressure #ro"' Since flo* in the"i"e must "ass throu%h a re#uce# area3 the "ressure before the restriction ishi%her than after or #o*nstream' Such a re#uction in "ressure *ill cause anincrease in the flui# $elocity because the same amount of flo* must ta)e "lacebefore the restriction as after it' Velocity *ill $ary #irectly *ith the flo* an# as the

flo* increases a %reater "ressure #ifferential *ill occur across the restriction' Soby measurin% the #ifferential "ressure across a restriction3 one can measure therate of flo*'

Ori6ice Plate

The orifice "late is the most common form of restriction that is use# in flo*measurement' An orifice "late is basically a thin metal "late *ith a hole bore# in


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Rolta Academy : Engineering Design Servicesthe center' It has a tab on one si#e *here the s"ecification of the "late isstam"e#' The u"stream si#e of the orifice "late usually has a shar"3 e#%e' 2i%ure1 sho*s a re"resentati$e orifice "late'

$ig ;"1# A ty&ical Ori6ice Plate

7hen an orifice "late is installe# in a flo* line &usually clam"e# bet*een a "air offlan%es(3 increase of flui# flo* $elocity throu%h the re#uce# area at the orifice#e$elo"s a #ifferential "ressure across the orifice' This "ressure is a function offlo* rate'7ith an orifice "late in the "i"e *or)3 static "ressure increases sli%htly u"streamof the orifice ue to bac) "ressure effect( an# then #ecreases shar"ly as theflo* "asses throu%h the orifice3 reachin% a minimum at a "oint calle# the $enacontracta *here the $elocity of the flo* is at a ma+imum' Beyon# this "oint3 static"ressure starts to reco$er as the flo* slo*s #o*n' o*e$er3 *ith an orifice "late3static "ressure #o*nstream is al*ays consi#erably lo*er than the u"streamPressure' In a##ition some "ressure ener%y is con$erte# to soun# an# heat #ue

to friction an# turbulence at the orifice "late' 2i%ure / sho*s the "ressure "rofileof an orifice "late installation'

$ig ;"11 Ori6ice Plate Installation (it, &ress're &ro6ileOn obser$in% 2i%ure /3 one can see that the measure# #ifferential "ressure#e$elo"e# by an orifice "late also #e"en#s on the location of the "ressuresensin% "oints or "ressure ta"s'


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Rolta Academy : Engineering Design Services*ena Contracta Ta&s

$ig ;"1; Ori6ice &late (it, *ena Contracta ta&

Vena contracta ta"s are locate# one "i"e inner #iameter u"stream an# at the"oint of minimum "ressure3 usually one half "i"e inner #iameter #o*nstream &2i%

.'1.('

Pi&e Ta&s

Pi"e ta"s are locate# t*o an# a half "i"e inner #iameters u"stream an# ei%ht "i"einner #iameters #o*nstream'7hen an orifice "late is use# *ith one of the stan#ar#i4e# "ressure ta" locations3an onlocation calibration of the flo* transmitter is not necessary' Once the ratioan# the )in# of "ressure ta" to be use# are #eci#e#3 there are em"irically #eri$e#charts an# tables a$ailable to facilitate calibration'

Advantages and Disadvantages o6 Ori6ice Plates

A#$anta%es of orifice "lates inclu#e:

i%h #ifferential "ressure %enerate# E+hausti$e #ata a$ailable !o* "urchase "rice an# installation cost Easy re"lacement

Disa#$anta%es inclu#e:

i%h "ermanent "ressure loss im"lies hi%her "um"in% cost' Cannot be use# on #irty flui#s3 slurries or *et steam3 as erosion *ill alter

the #ifferential "ressure %enerate# by the orifice "late'

*ent'ri T'es

2or a""lications *here hi%h "ermanent "ressure loss is not tolerable3 a $enturitube &2i%ure ( can be use#' Because of its %ra#ually cur$e# inlet an# outletcones3 almost no "ermanent "ressure #ro" occurs' This #esi%n also minimi4es


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Rolta Academy : Engineering Design Services*ear an# "lu%%in% by allo*in% the flo* to s*ee" sus"en#e# soli#s throu%h*ithout obstruction'

$ig ;"1< *ent'ri t'e installationo*e$er a Venturi tube #oes ha$e #isa#$anta%es:

Calculate# calibration fi%ures are less accurate than for orifice "lates' 2or%reater accuracy3 each in#i$i#ual Venturi tube has to be flo* calibrate# by"assin% )no*n flo*s throu%h the Venturi an# recor#in% the resultin%#ifferential "ressures'

The #ifferential "ressure %enerate# by a $enturi tube is lo*er than for anorifice "late an#3 therefore3 a hi%h sensiti$ity flo* transmitter is nee#e#'

It is more bul)y an# more e+"ensi$e'As a si#e noteH one a""lication of theVenturi tube is the measurement of flo* in the "rimary heat trans"ortsystem' To%ether *ith the tem"erature chan%e across these fuel channels3thermal "o*er of the reactor can be calculate#

$lo( No00le

A flo* no44le is also calle# a half $enturi 2i% .'1 sho*s a ty"ical flo* no44leinstallation'

$ig ;"1= $lo( No00le InstallationThe flo* no44le has "ro"erties bet*een an orifice "late an# a $enturi' Because ofits streamline# contour3 the flo* no44le has a lo*er "ermanent "ressure loss thanan orifice "late &but hi%her than a $enturi(' The #ifferential it %enerates is alsolo*er than an orifice "late &but a%ain hi%her than the $enturi tube(' They are alsoless e+"ensi$e than the $enturi tubes'


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Rolta Academy : Engineering Design Services2lo* no44les are *i#ely use# for flo* measurements at hi%h $elocities' They aremore ru%%e# an# more resistant to erosion than the shar"e#%e# orifice "late' Ane+am"le use of flo* no44les are the measurement of flo* in the fee# an# blee#lines of the PT system'

Elo( Ta&s

Centrifu%al force %enerate# by a flui# flo*in% throu%h an elbo* can be use# tomeasure flui# flo*' As flui# %oes aroun# an elbo*3 a hi%h"ressure area a""earson the outer face of the elbo*' If a flo* transmitter is use# to sense this hi%h"ressure an# the lo*er "ressure at the inner face of the elbo*3 flo* rate can bemeasure#' 2i%ure = sho*s an e+am"le of an elbo* ta" installation'One use of elbo* ta"s is the measurement of steam flo* from the boilers3 *herethe lar%e $olume of saturate# steam at hi%h "ressure an# tem"erature coul#cause an erosion "roblem for other "rimary #e$ices'

Another a#$anta%e is that the elbo*s are often alrea#y in the re%ular "i"in%confi%uration so no a##itional "ressure loss is intro#uce#'

$ig ;"1@ Elo( Ta& Installation

Pitot T'es

Pitot tubes also utili4e the "rinci"les ca"ture# in Bernoullis e-uation3 to measureflo*' ,ost "itot tubes actually consist of t*o tubes' One3 the lo*"ressure tubemeasures the static "ressure in the "i"e' The secon#3 the hi%h"ressure tube isinserte# in the "i"e in such a *ay that the flo*in% flui# is sto""e# in the tube' The

"ressure in the hi%h"ressure tube *ill be the static "ressure in the system "lus a"ressure #e"en#ant on the force re-uire# sto""in% the flo*'


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$ig ;"1 Pitot T'esPitot tubes are more common measurin% %as flo*s that li-ui# flo*s' They sufferfrom a cou"le of "roblems'

The "ressure #ifferential is usually small an# har# to measure' The #ifferin% flo* $elocities across the "i"e ma)e the accuracy #e"en#ent on

the flo* "rofile of the flui# an# the "osition of the "itot in the "i"e'Ann'ar

An annubar is $ery similar to a "itot tube' The #ifference is that there is morethan one hole into the "ressure measurin% chambers' The "ressure in the hi%h"ressure chamber re"resents an a$era%e of the $elocity across the "i"e'

Annubars are more accurate than "itots as they are not as "osition sensiti$e oras sensiti$e to the $elocity "rofile of the flui#'

$ig ;"1B Ann'ar


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Rolta Academy : Engineering Design Services;"7"7 S'are Root E4tractor

U" to no*3 our flo* measurement loo" can be re"resente# by the installationsho*n in 2i%ure ?' The hi%h an# lo*"ressure ta"s of the "rimary #e$ice &orifice

ty"e sho*n( are fe# by sensin% lines to a #ifferential "ressure &D6P( cell' Theout"ut of the D6P cell acts on a "ressure to milliam" trans#ucer3 *hich transmitsa $ariable ./0 ma si%nal' The D6P cell an# transmitter are sho*n to%ether as aflo* transmitter &2T('

$ig";"7#"A $lo( +oo& (it, Ori6ice Plate

This sim"le system althou%h %i$in% an in#ication of the flo* rate &G(3 is actuallytransmittin% a si%nal "ro"ortional to the #ifferential "ressure &P(' o*e$er3 therelationshi" bet*een the $olume of flo* G an# P is not linear' Thus such asystem *oul# not be a""ro"riate in instrumentation or meterin% that re-uires a

linear relationshi" or scale'In actuality the #ifferential "ressure increases in "ro"ortion to the s-uare of theflo* rate'

7e can *rite this as: P Q2

In other *or#s the flo* rate &G( is "ro"ortionalH to the s-uare root of the#ifferential "ressure'

Volumetric 2lo* Rate GP

To con$ert the si%nal from the flo* transmitter3 &fi%ure ? abo$e( to one that is

#irectly "ro"ortional to the flo*rate3 one has to obtain or e+tract the s-uare rootof the si%nal from the flo* transmitter' 2i%ure 10 illustrates the in"ut out"utrelationshi" of a s-uare root e+tractor'


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$ig ;"71 S'are root e4tractor In&'t and O't&'tThe s-uare root e+tractor is an electronic &or "neumatic( #e$ice that ta)es thes-uare root of the si%nal from the flo* transmitter an# out"uts a corres"on#in%linear flo* si%nal' Se$eral metho#s are use# in the construction of s-uare roote+tractors' o*e$er3 it is beyon# the sco"e of this course to #iscuss the actualcircuitries'

A ty"ical s-uare root e+tractor installation is sho*n in 2i%ure 1' This system*oul# "ro#uce a ./0ma si%nal that is linear *ith the flo* rate'

$ig ;"77 A ty&ical s'are root e4tractor installationS-uare root e+tractors are usually current o"erate# #e$ices so they can beconnecte# #irectly in the ./0 mA current loo" of a flo* transmitter' The out"ut ofthe s-uare root e+tractor is a%ain a ./0 mA si%nal' This si%nal is #irectly"ro"ortional to the flo*rate in the "i"e*or)'

The si%nal from the s-uare root e+tractor usually %oes to a controller3 as sho*n in

2i%ure .'//'The controller &*hich can be re%ar#e# as an analo% com"uter( is use# to controlthe final control element3 usually a $al$e'

C'tGo66 relay

S-uare root e+tractors #o ha$e a #ra*bac)' At lo* $alues of in"ut3 $ery smallchan%es in the in"ut ifferential "ressure( to the e+tractor *ill cause a lar%e


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Rolta Academy : Engineering Design Serviceschan%e in the s-uare root out"ut &flo* in#ication(' This system is #escribe# asha$in% hi%h %ain at $alues close to 4ero in"ut' Obser$e fi%ure .'/ belo*3 *hichis an e+"an#e# $ersion of fi%ure 1/ at the lo*er en#' The amount of chan%e from4ero "ressure to A an# from A to B is i#entical' o*e$er3 for the same in"ut

chan%e &P(3 the %ain at lo* in"ut is %reater'

$ig ;"7 S'are root e4tractor gra&, e4&anded vie(

To illustrate the effect of the $ery hi%h %ain in the s-uare root e+tractor at lo*scale $alues consi#er a ty"ical situation' A "i"e $al$e is close# an# the 4ero flo*"ro#uces a . mA out"ut from the flo* transmitter' If #ue to noise3 tem"erature orother #isturbances3 the in"ut #rifte# from 0 to 1 &i'e'3 from . mA to .'1 mA(3the out"ut *oul# ha$e chan%e# from 0 to 10 &. mA to @' mA(' It is ob$iousthat this si%nificant error sent to the controller has to be eliminate#'2or this reason3 s-uare root e+tractors are e-ui""e# *ith cutoff relays' Thesettin% for the relay can be a#8uste# from to 10 of out"ut' Sho*n in 2i%ure1@ is a res"onse cur$e for a cutoff relay set at out"ut' In this case3 any in"ut

si%nal belo* &0'0(/

or 0'.? *oul# be i%nore# by the e+tractor' The out"ut of thee+tractor *oul# remain at 0 as lon% as in"ut is belo* 0'.?'7hen the in"ut e+cee#e# 0'.?3 the out"ut *oul# resume its normal cur$e3startin% at '

$ig ;"7; res&onse c'rve 6or e4tractor (it, @J c't o66 setting

;"7" Density Com&ensating $lo( Detectors


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Rolta Academy : Engineering Design ServicesIt must be remembere# that a DP transmitter use# for flo* measurement3measures #ifferential "ressure3 not the $olume or mass of flo*' 7e ha$e sho*nthat #ifferential "ressure instruments re-uire that the s-uare root #ifferential"ressure be ta)en to obtain $olumetric flo* G:

Volume of 2lo* =Q P/p

2or com"ressible $a"our such as steam3 it is more im"ortant to )no* the mass ofthe flo* W rather than the $olume' To #etermine the mass of a li-ui#6%as the#ensity &" 9 mass "er unit $olume( must also be obtaine#'

,ass of 2lo* =W = pQ pP7e also )no* that #ensity $aries #irectly *ith "ressure an# in$ersely *ithtem"erature:

p K pressure /temperature

$ig ;"7< Density Com&ensating 6lo( detectorThe coefficient K &*hich can be obtaine# from tables( #e"en#s on a number of$ariables inclu#in% the "i"e si4e an# the characteristics of the flui#6%as' It issufficient to say that if the "rocess tem"erature an# static "ressure is )no*n3then the #ensity can be obtaine#

The #ensity com"ensatin% flo* #etector &sho*n schematically in fi%ure 1( is anecessity for steam flo* bet*een the boilers3 reheaters an# the turbines3 *herethe mass &*ei%ht( of the steam is more im"ortant than the $olume'

Process ConditionsAs "re$iously state#3 the measurement of flo* usin% any of the #e$ices#escribe# abo$e is "urely inferential' It relies on the si%nal from a #ifferential"ressure &D6P( cell to obtain an inferre# flo* measurement' This flo*measurement coul# be either the $olume or mass of the li-ui#6%as' In either casethe instrumentation can be affecte# by the "rocess con#itions' The three main"arameters are:


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$l'id Tem&erat'reThe tem"erature of the flo* -uantity has a #ramatic effect on the flo*measurement' Un#er the ri%ht con#itions the li-ui# can either boil &"ro#ucin% %as

"oc)ets an# turbulence( or free4e &"ro#ucin% bloc)a%es an# #istorte# flo*"atterns( at the sensors'

At the onset of tem"erature relate# flo* instrumentation "roblems the meterrea#in%s *ill become unstable' as "oc)ets &causin% intermittent lo* "ressure(at the hi%h "ressure sensin% lines *ill cause a""arent lo* flo* fluctuations' Thisis more "re#ominant in orifice an# flo*no44le installations' Turbulence at thelo*"ressure sensor *ill usually increase as the tem"erature increases to causea more stable but incorrect hi%h flo* rea#in%'Tem"erature also affects the #ensity of the li-ui#6%as3 as "er the follo*in%relationshi" &*here > is a constant for the li-ui#6%as('

The mass flo* &i'e'3 "oun#s of steam "er minute( $aries in$ersely *ithtem"erature an# must be com"ensate# for usin% a #ensity com"ensatin% flo*#etector'The elbo* ta" sensor uses centrifu%al force to #etect flo* an# is most sensiti$e to#ensity chan%es' The flo* rea#in%s *ill increase as the tem"erature #ecreases'

$l'id Press'reAs *e ha$e 8ust seen3 "ressure also affects the #ensity of the flui#6%as' 2or theelbo* ta" "re$iously mentione#3 the flo* rea#in%s *ill increase as the "rocess

"ressure increases'

2or all ty"es of D6P flo* sensors3 mass flo* *ill of course increase as the"ressure increases' To obtain the correct measurement of mass flo*3 a #ensitycom"ensatin% flo* #etector must be use# as #escribe# "re$iously';"7"; $lo( Meas'rement Errors

7e ha$e alrea#y #iscusse# the "ros an# cons of each ty"e of flo* #etectorcommonly foun# in a %eneratin% station' Some3 such as the orifice3 are more"rone to #ama%e by "articulate or saturate# steam then others' o*e$er3 thereare common areas *here the flo* rea#in%s can be inaccurate or in$ali#'

Erosion

Particulate3 sus"en#e# soli#s or #ebris in the "i"in% *ill not only "lu% u" thesensin% lines3 it *ill ero#e the sensin% #e$ice' The orifice3 by its #esi%n *ith a thin3shar" e#%e is most affecte#3 but the flo* no44le an# e$en $enturi can also be#ama%e#' As the material *ears a*ay3 the #ifferential "ressure bet*een the hi%h


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Rolta Academy : Engineering Design Servicesan# lo* si#es of the sensor *ill #ro" an# the flo* rea#in% *ill #ecrease'

Over ranging Damage to t,e D-P CellA%ain3 as "re$iously #escribe#3 the system "ressures are usually much %reater

than the #ifferential "ressure an# three $al$e manifol#s must be correctly use#'

*a&or $ormation in t,e T,roatD6P flo* sensors o"erate on the relation bet*een $elocity an# "ressure' As %asre-uires less "ressure to com"ress3 there is a %reater "ressure #ifferential acrossthe D6P cell *hen the %as e+"an#s on the !P si#e of the sensor' The flo* sensor*ill in#icate a hi%her flo* rate than there actually is' The turbulence create# atthe !P si#e of the sensor *ill also ma)e the rea#in% some*hat unstable' A smallamount of %as or $a"or *ill ma)e a lar%e #ifference in the in#icate# flo* rate'The o""osite can occur if the $a"or forms in the P si#e of the sensor #ue toca$itations or %as "oc)ets *hen the flui# a""roaches the boilin% "oint' In such an

instance there *ill be a fluctuatin% "ressure #ro" across the D6P cell that *ill %i$ean erroneously lo* &or e$en ne%ati$e( D6P rea#in%'

Clogging o6 T,roat

Particulate or sus"en#e# soli#s can #ama%e the flo* sensor by the hi%h$elocities *earin% at the flo* sensor surfaces' Also3 the buil#u" of material in thethroat of the sensor increases the #ifferential "ressure across the cell' The error inflo* measurement *ill increase as the flo* increases'

Pl'gged or +ea.ing Sensing +ines

The effects of "lu%%e# or lea)in% D6P sensin% lines is the same as #escribe# in"re$ious mo#ules3 ho*e$er the effects are more "ronounce# *ith the "ossiblelo* #ifferential "ressures' Perio#ic maintenance an# blee#in% of the sensin%lines is a must' The instrument error *ill #e"en# on *here the "lu%6lea) is:On the P si#e a "lu%%e# or lea)in% sensin% line *ill cause a lo*er rea#in%' Therea#in% *ill become irrational if the !P "ressure e-uals or e+cee#s the Psensin% "ressure'On the !P si#e a "lu%%e# or lea)in% sensin% line *ill cause a hi%her rea#in%'

;" +E*E+ MEAS!REMENT

Accurate continuous measurement of $olume of flui# in containers has al*aysbeen a challen%e to in#ustry' This is e$en more so in the nuclear stationen$ironment *here the flui# coul# be aci#ic6caustic or un#er $ery hi%h"ressure6tem"erature' 7e *ill no* e+amine the measurement of flui# le$el in$essels an# the effect of tem"erature an# "ressure on this measurement' 7e *illalso consi#er the o"eratin% en$ironment on the measurement an# the "ossiblemo#es of #e$ice failure'


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Rolta Academy : Engineering Design Servicesa""lie# to only one si#e of the DP ca"sule #urin% installation or remo$al of the DPcell from ser$ice3 o$er ran%in% of the ca"sule *oul# occur an# the ca"sule coul#be #ama%e# causin% erroneous in#ications'

;""7 T,ree *alve Mani6old

A three$al$e manifol# is a #e$ice that is use# to ensure that the ca"sule *ill notbe o$erran%e#' It also allo*s isolation of the transmitter from the "rocess loo"' Itconsists of t*o bloc) $al$es hi%h "ressure an# lo*"ressure bloc) $al$e an#an e-uali4in% $al$e. 2i%ure 1 sho*s a three $al$e manifol# arran%ement'

$ig ;"7= A t,ree valve mani6oldDurin% normal o"eration3 the e-uali4in% $al$e is close# an# the t*o bloc) $al$esare o"en' 7hen the transmitter is "ut into or remo$e# from ser$ice3 the $al$esmust be o"erate# in such a manner that $ery hi%h "ressure is ne$er a""lie# toonly one si#e of the DP ca"sule'

O&erational Se'ences o6 T,reeG*alve Mani6old *alving Transmitter intoServiceTo $al$e a DP transmitter into ser$ice an o"erator *oul# "erform the follo*in%ste"s:

1' Chec) all $al$es close#'/' O"en the e-uali4in% $al$e this ensures that the same "ressure *ill bea""lie# to both si#es of the transmitter3 i'e'3 4ero #ifferential "ressure'

' O"en the i%h Pressure bloc) $al$e slo*ly3 chec) for lea)a%e from boththe hi%h "ressure an# lo*"ressure si#e of the transmitter'

.' Close the e-uali4in% $al$e this loc)s the "ressure on both si#es of thetransmitter'

@' O"en the lo*"ressure bloc) $al$e to a""ly "rocess "ressure to the lo*


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Rolta Academy : Engineering Design Services"ressure si#e of the transmitter an# establish the *or)in% #ifferential"ressure'

' The transmitter is no* in ser$ice'Note it may be necessary to blee# any tra""e# air from the ca"sule housin%'

Removing Transmitter 6rom Service

Re$ersal of the abo$e ste"s allo*s the DP transmitter to be remo$e# fromser$ice'

1' Close the lo*"ressure bloc) $al$e'/' O"en the e-uali4in% $al$e'' Close the hi%h"ressure bloc) $al$e'

The transmitter is no* out of ser$ice'Note the transmitter ca"sule housin% still contains "rocess "ressureH this *ill

re-uire blee#in%'

;"" O&en Tan. Meas'rement

$ig ;"7@ O&en Tan. +evel Meas'rement InstallationThe sim"lest a""lication is the flui# le$el in an o"en tan)' 2i%ure .'/ sho*s aty"ical o"en tan) le$el measurement installation usin% a "ressure ca"sule le$eltransmitter'If the tan) is o"en to atmos"here3 the hi%h"ressure si#e of the le$el transmitter*ill be connecte# to the base of the tan) *hile the lo*"ressure si#e *ill be$ente# to atmos"here' In this manner3 the le$el transmitter acts as a sim"le"ressure transmitter' 7e ha$e:

Phi%h 9 Patm ; S

Plo* 9 Patm

Differential "ressure P 9 Phi%h Plo* 9 S K

The le$el transmitter can be calibrate# to out"ut . mA *hen the tan) is at 0le$el an# /0 mA *hen the tan) is at 100 le$el'

;""; Closed Tan. Meas'rement


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Shoul# the tan) be close# an# a %as or $a"our e+ists on to" of the li-ui#3 the %as"ressure must be com"ensate# for' A chan%e in the %as "ressure *ill cause achan%e in transmitter out"ut' ,oreo$er3 the "ressure e+erte# by the %as "hase

may be so hi%h that the hy#rostatic "ressure of the li-ui# column becomesinsi%nificant' 2or e+am"le3 the measure# hy#rostatic hea# in a boiler may be onlythree meters &0 )Pa( or so3 *hereas the steam "ressure is ty"ically @ ,Pa'Com"ensation can be achie$e# by a""lyin% the %as "ressure to both the hi%h an#lo*"ressure si#es of the le$el transmitter' This co$er %as "ressure is thus use#as a bac)"ressure or reference "ressure on the !P si#e of the DP cell' One canalso imme#iately see the nee# for the three$al$e manifol# to "rotect the DP cella%ainst these "ressures'

The #ifferent arran%ement of the sensin% lines to the DP cell is in#icate# a ty"icalclose# tan) a""lication &fi%ure .'/=('

$ig ;"7 Ty&ical Closed Tan. +evel Meas'rement System7e ha$e:

Phi%h 9 P%as ; S'

Plo* 9 P%as

P 9 Phi%h Plo* 9 S'

The effect of the %as "ressure is cancelle# an# only the "ressure #ue to thehy#rostatic hea# of the li-ui# is sense#' 7hen the lo*"ressure im"ulse line isconnecte# #irectly to the %as "hase abo$e the li-ui# le$el3 it is calle# a #ry le%'


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$ig ;"# A (et leg installation

At the to" of the lo* "ressure im"ulse line is a small catch tan)' The %as"hase or $a"or *ill con#ense in the *et le% an# the catch tan)' The catch

tan)3 *ith the incline# interconnectin% line3 maintains a constant hy#rostatic"ressure on the lo*"ressure si#e of the le$el transmitter' This "ressure3bein% a constant3 can easily be com"ensate# for by calibration' &Note thato"eratin% the three$al$e manifol# in the "rescribe# manner hel"s to "reser$ethe *et le%'(

If the tan) is locate# out#oors3 trace heatin% of the *et le% mi%ht be necessaryto "re$ent it from free4in%' Steam lines or an electric heatin% element can be*oun# aroun# the *et le% to )ee" the tem"erature of the con#ensate abo$eits free4in% "oint'Note the t*o sets of #rain $al$es' The transmitter #rain $al$es *oul# be use#

to #rain &blee#( the transmitter only' The t*o #rain $al$es locate# imme#iatelyabo$e the three$al$e manifol# are use# for im"ulse an# *et le% #rainin% an#fillin%'In a##ition to the three$al$e manifol# most transmitter installations ha$e$al$es *here the im"ulse lines connect to the "rocess' These isolatin% $al$es3sometimes referre# to as the root $al$es3 are use# to isolate the transmitterfor maintenance'+evel Com&ensation

It *oul# be i#ealistic to say that the DP cell can al*ays be locate# at the e+actthe bottom of the $essel *e are measurin% flui# le$el in' ence3 themeasurin% system has to

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