Thermocouples热电偶英文讲义

KNE222-ThermocouplesUTasSchoolofEngineeringG.VertiganPage12009UniversityofTasmaniaSchoolofEngineeringKNE222ElectronicEngineeringTemperatureMeasurement–ThermocouplesIntroduction.Athermocoupleisformedwhentwodissimilarmetalsarejoined.Ifasecondjointismadefromthesamematerials,andonejointisheatedwithrespecttotheother,thenacurrentwillflowroundtheloop.Alternatively,ifthecircuitisopened,thenasmallvoltagewillappearattheseterminals,proportionaltothedifferenceintemperaturebetweenthejunctions.ThisvoltageisknownastheSeebeckvoltage,afterThomasSeebeckwhodiscoveredthephenomenonin1821.Figure1.ConceptualApplicationofaThermocouple(Ref:”OperationalAmplifierCircuitsTheoryandApplications”E.JKennedy.HRWInc.)Inordertouseathermocoupletomeasuretemperaturewithonejunction,areferencetemperaturemustbeestablishedfortheother.ThiscanbedoneasillustratedinFigure1,wherea0ºCicebathisprovidedforthereferencejunction.Howeveranicebathisnotreallypracticalforeverydaymeasurements.FortunatelyitispossibletointroduceacorrectionvoltageintothecircuitsothatthereferencejunctionappearstobeatzerodegreesC.Anexampleofthiswillbepresentedlater.ThermocoupleCharacteristics.Table1providesalistofcommonlyusedthermocouplematerials,togetherwiththeusefulrangeoftemperaturesforeach.WhiletheSeebeckvoltageincreaseswiththejunctiontemperaturedifference,therelationshipisunfortunatelynotentirelylinear.TheproportionalityconstantisknownastheSeebeckcoefficient,α,andhastheunitsμV/ºC.Table1providestypicalαvaluesat25ºC;italsogivessomeideaofhowαvariesovertheuseabletemperaturerangeforeachthermocouple.Table1.CharacteristicsofcommonThermocouples(Ref:“OperationalAmplifierCircuitsTheoryandApplications”E.JKennedy.HRWInc.)KNE222-ThermocouplesUTasSchoolofEngineeringG.VertiganPage22009Figure2showsthevariationofαasafunctionoftemperatureforKandTTypethermocouples.TheKtypedeviceoffersalmostconstantαvaluesovermuchofitsusefulrange,andthereforeisuseabletherewithoutlinearization.Incontrast,theSeebeckcoefficientoftheTtypethermocouplevariesconsiderablywithtemperatureandasaresultthesedevicesmustbecorrectedusingalinearizationalgorithminordertoobtainaccuratetemperaturemeasurements.Figure2.VariationoftheSeebeckCoefficientwithTemperature,TandKTypeThermocouples.(Ref:”OperationalAmplifierCircuitsTheoryandApplications”E.JKennedy.HRWInc.)Finally,figure3showstheSeebeckVoltageasafunctionoftemperaturedifferenceforthethermocoupleslistedinTable1.Ineachcasethereferencejunction(sometimescalledthecoldjunction),isheldatzeroºC,thusallthesecurvespassthroughtheorigin.Becausetheyareslightlynon-linear,itiscommontoapplyalinearizationalgorithmtotheSeebeckVoltage(E),inordertoobtainthecorrecttemperature(T).InitssimplestformthisalgorithmassumesthattherelationshipbetweenSeebeckVoltageandtemperatureislinearandanequationoftheformETisused.Thisassumptionresultsinerrors,particularlyoverawiderangeoftemperatures.Formoreaccurateresults,higherorderpolynomialsareused.Figure3.SeebeckVoltageasafunctionofTemperatureDifference.(Note:TheReferenceJunctiontemperatureis0ºC.)(Ref:”OperationalAmplifierCircuitsTheoryandApplications”E.JKennedy.HRWInc.)KNE222-ThermocouplesUTasSchoolofEngineeringG.VertiganPage32009APracticalThermocoupleCircuit.TheschematicshowninFigure5isthatofanelectronicallycompensatedthermocoupleamplifier.InthiscaseaJtypethermocouple(Iron-Constantan)hasbeenused.Thetwometallicjunctionsthatmakeupthisthermocouple,J1andJ2,areshownonthediagram.J1isthesensejunction,whileJ2isthereferencejunction.IdeallyJ2shouldbeheldinanicebathat0ºC,asshowninFigure1,howeverthisisgenerallyinconvenient.Insteaditismoreusualtoprovideelectronicreferencejunctioncompensation.Thisinvolvesmeasuringthetemperatureofthereferencejunctionandaddingacompensatingvoltagetothethermocouple,sothattheresidualSeebeckvoltagematchesthatofasimilarthermocouplewithitsreferencejunctionheldat0ºC.GenerationofaCompensationVoltageTable2showstheSeebeckvoltageasfunctionoftemperatureforJtypethermocouples,(referencedto0ºC).Fromthisweseethatthethermocouplevoltagewillbe1.277mVwhenthesensejunctionisatroomtemperature,(25ºCor298ºK).Thereforethisvoltagemustbesubtractedfromthethermocouplepotential,whenthereferencejunctionisat25ºC,inordertoleaveapotentialproportionalonlytothesensejunctiontemperature.Ifwecankeepthereferencejunctionfirmlyat25ºC,thenthisfixedcompensationvoltagewillsuffice.Howeverthisisgenerallyimpractical,andasambientconditionschangesowillthereferencejunctiontemperature,andwithittherequiredcompensationvoltageaswell.Ontheotherhand,ifthecompensationvoltageismadetovarywithtemperature,injusttherightway,thenchangesinambienttemperaturecanbecompensatedforautomatically.Specifically,ifthecompensationvoltagehasatemperaturecoefficientequaltotheSeebeckcoefficientforthethermocoupleinquestion,atsay25ºC,thenambientchangesaroundthistemperaturewillautomaticallybecompensatedfor.Lettheinputvoltagetothedifferentialamplifier,IC2inFigure5,beVi(Ts)whereTsisthetemperatureofthesensejunction(J1).(IC2isanInstrumentationAmplifier*providedsimplytoamplifythenetthermocouplevoltagetomoreuseablelevels.)Thisvoltagewillequa