电子工程温度测量热电偶大学毕业论文外文文献翻译及原文Word格式文档下载.docx

电子工程温度测量热电偶大学毕业论文外文文献翻译及原文Word格式文档下载.docx

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温度测量-热电偶

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2017.02.14

UniversityofTasmania

SchoolofEngineering

KNE222ElectronicEngineering

TemperatureMeasurement–Thermocouples

Introduction.

Athermocoupleisformedwhentwodissimilarmetalsarejoined.Ifasecondjointismadefromthesamematerials,andonejointisheatedwithrespecttotheother,thenacurrentwillflowroundtheloop.Alternatively,ifthecircuitisopened,thenasmallvoltagewillappearattheseterminals,proportionaltothedifferenceintemperaturebetweenthejunctions.ThisvoltageisknownastheSeebeckvoltage,afterThomasSeebeckwhodiscoveredthephenomenonin1821.

Figure1.ConceptualApplicationofaThermocouple

（Ref:

”OperationalAmplifierCircuitsTheoryandApplications”E.JKennedy.HRWInc.）

Inordertouseathermocoupletomeasuretemperaturewithonejunction,areferencetemperaturemustbeestablishedfortheother.ThiscanbedoneasillustratedinFigure1,wherea0º

Cicebathisprovidedforthereferencejunction.Howeveranicebathisnotreallypracticalforeverydaymeasurements.FortunatelyitispossibletointroduceacorrectionvoltageintothecircuitsothatthereferencejunctionappearstobeatzerodegreesC.Anexampleofthiswillbepresentedlater.

ThermocoupleCharacteristics.

Table1providesalistofcommonlyusedthermocouplematerials,togetherwiththeusefulrangeoftemperaturesforeach.WhiletheSeebeckvoltageincreaseswiththejunctiontemperaturedifference,therelationshipisunfortunatelynotentirelylinear.TheproportionalityconstantisknownastheSeebeckcoefficient,α,andhastheunitsμV/º

C.Table1providestypicalαvaluesat25º

C;

italsogivessomeideaofhowαvariesovertheuseabletemperaturerangeforeachthermocouple.

Table1.CharacteristicsofcommonThermocouples

“OperationalAmplifierCircuitsTheoryandApplications”E.JKennedy.HRWInc.）

Figure2showsthevariationofαasafunctionoftemperatureforKandTTypethermocouples.TheKtypedeviceoffersalmostconstantαvaluesovermuchofitsusefulrange,andthereforeisuseabletherewithoutlinearization.Incontrast,theSeebeckcoefficientoftheTtypethermocouplevariesconsiderablywithtemperatureandasaresultthesedevicesmustbecorrectedusingalinearizationalgorithminordertoobtainaccuratetemperaturemeasurements.

Figure2.VariationoftheSeebeckCoefficientwithTemperature,TandKTypeThermocouples.

”OperationalAmplifierCircuitsTheoryandApplications”E.JKennedy.HRWInc.）

Finally,figure3showstheSeebeckVoltageasafunctionoftemperaturedifferenceforthethermocoupleslistedinTable1.Ineachcasethereferencejunction（sometimescalledthecoldjunction）,isheldatzeroº

C,thusallthesecurvespassthroughtheorigin.Becausetheyareslightlynon-linear,itiscommontoapplyalinearizationalgorithmtotheSeebeckVoltage（E）,inordertoobtainthecorrecttemperature（T）.InitssimplestformthisalgorithmassumesthattherelationshipbetweenSeebeckVoltageandtemperatureislinearandanequationoftheform

isused.Thisassumptionresultsinerrors,particularlyoverawiderangeoftemperatures.Formoreaccurateresults,higherorderpolynomialsareused.

Figure3.SeebeckVoltageasafunctionofTemperatureDifference.

（Note:

TheReferenceJunctiontemperatureis0º

C.）

（Ref:

APracticalThermocoupleCircuit.

TheschematicshowninFigure5isthatofanelectronicallycompensatedthermocoupleamplifier.InthiscaseaJtypethermocouple（Iron-Constantan）hasbeenused.Thetwometallicjunctionsthatmakeupthisthermocouple,J1andJ2,areshownonthediagram.J1isthesensejunction,whileJ2isthereferencejunction.IdeallyJ2shouldbeheldinanicebathat0º

C,asshowninFigure1,howeverthisisgenerallyinconvenient.Insteaditismoreusualtoprovideelectronicreferencejunctioncompensation.Thisinvolvesmeasuringthetemperatureofthereferencejunctionandaddingacompensatingvoltagetothethermocouple,sothattheresidualSeebeckvoltagematchesthatofasimilarthermocouplewithitsreferencejunctionheldat0º

C.

GenerationofaCompensationVoltage

Table2showstheSeebeckvoltageasfunctionoftemperatureforJtypethermocouples,（referencedto0º

C）.Fromthisweseethatthethermocouplevoltagewillbe1.277mVwhenthesensejunctionisatroomtemperature,（25º

Cor298º

K）.Thereforethisvoltagemustbesubtractedfromthethermocouplepotential,whenthereferencejunctionisat25º

C,inordertoleaveapotentialproportionalonlytothesensejunctiontemperature.

Ifwecankeepth