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AutomationBasics:TakingTemperatures:OptionsforMeasuringISA
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HomeISAPublicationsInTechMagazine2006/FebAutomationBasics:TakingTemperatures:OptionsforMeasuring
1 February 2006
Taking Temperatures: Options for MeasuringBy Vernon Trevathan
P.E.TemperaturemeasuresinunitsofKelvin,Celsius,Fahrenheit,orRankin.
Unliketheother"bigfour"measurements,inmanytemperatureapplications,thesensorisseparatefromthetransmitter.Whilethetransmitteroramplifiercanbeinthehousingofthesensor,itcanalsoberemote,whetherinafieldboxcontaininganumberoftemperaturetransmitters,orinthecontrolroom,ortheoutputofthesensorcanconnectdirectlytoatemperatureloggerordirectlytoadistributedcontrolsystemoraprogrammablelogiccontroller.
Thethermocoupleisthemostpopulartypeofsensor.(Seeaccompanyingdiagram.)Thermocouplesoperateontheprinciplethattwowiresmadeofdissimilarmaterialsconnectedateitherendwillgenerateapotential(adifferenceinvoltage)betweenthetwoendsthatisafunctionofthematerialsandthetemperaturedifferencebetweenthetwoends.
Anumberofmaterialchoicesareincommonuse.Basemetalthermocouplesareusefulformeasuringtemperaturesunder1000C.Thisclassincludesiron/constantan(TypeJ),Chromel/Alumel(TypeK),andanumberofothers.
Nobelmetalthermocouplesareusefultoabout2000C.Thisclassincludestungstenrheniumalloythermocouplesandothers.
Thepotentialgeneratedisinmillivoltsandisanonlinearfunctionoftemperature.Inpractice,oneendgoesnearthematerialwhosetemperatureyouwanttomeasure,andtheotherendconnectstotheinstrument.
Becausethethermocouplematerialsarenottypicallygoodmaterialsfortransmission,wireswithsimilarcharacteristicsarethechoicewhenthetransmittinginstrumentisremote.
Thethermocoupleisthemostpopulartypeofsensor.Thermocouplesarebasedontheprinciplethattwowiresmadeofdissimilarmaterialsconnectedateitherendwillgenerateapotentialbetweenthetwoendsthatisafunctionofthematerialsandtemperaturedifferencebetweenthetwoends.
Resistive
temperatureResistanceTemperatureDetectors(RTDs)consistofametalwireorfiberorofsemiconductormaterialthatrespondstotemperaturechangebychangingitselectricalresistance.
Platinum,nickel,tungsten,andothermetalsthathavehighresistivity,goodtemperaturecoefficientofresistance,goodductileortensilestrength,andchemicalinertnesswithpackagingandinsulationmaterialsworkbest.
Whenthematerialisasemiconductor,thesensorisathermistor.
Thechangeinresistancecanbedeterminedusingabridgecircuit.Sinceresistancechangesintheconnectionwireduetoambienttemperaturechangescanalsoaffecttheresistancereading,athirdwirefromanotherleginthebridgetobalancethatchangeisalsoapartofthescheme.
RTDsaregenerallymoreaccuratethanthermocouplesbutlessruggedanddonotworkatashightemperatures.
Alltypesoftemperaturemeasuringdevicessufferfromslowresponse,sinceitisnecessaryfortheheattoconductthroughtheprotectivesheath,andthroughanyinstalledwell.Locatingthewell(orunprotectedsensor)soitseesashighavelocityofprocessmaterialaspossiblehelpsreducethislag,asdoeshavingthesensorcontactthewell.Abarethermocoupletouchingthesheathand/orwell,however,generatesagroundandrequiresanisolatedamplifier.
About the
AuthorVernonTrevathanP.E.isanISAFellowandchairoftheISACAP(CertifiedAutomationProfessional)SteeringTeam.Thisarticleisanexcerptfromhisrecentbook,AGuidetotheAutomationBodyofKnowledge,ISAPress,2006.
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