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.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.docx

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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

毕业设计（论文）

外文文献翻译

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温度测量-热电偶

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翻译日期：

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,wherea0ºCicebathisprovidedforthereferencejunction.Howeveranicebathisnotreallypracticalforeverydaymeasurements.FortunatelyitispossibletointroduceacorrectionvoltageintothecircuitsothatthereferencejunctionappearstobeatzerodegreesC.Anexampleofthiswillbepresentedlater.

ThermocoupleCharacteristics.

Table1providesalistofcommonlyusedthermocouplematerials,togetherwiththeusefulrangeoftemperaturesforeach.WhiletheSeebeckvoltageincreaseswiththejunctiontemperaturedifference,therelationshipisunfortunatelynotentirelylinear.TheproportionalityconstantisknownastheSeebeckcoefficient,α,andhastheunitsμV/ºC.Table1providestypicalαvaluesat25ºC;italsogivessomeideaofhowαvariesovertheuseabletemperaturerangeforeachthermocouple.

Table1.CharacteristicsofcommonThermocouples

（Ref:

“OperationalAmplifierCircuitsTheoryandApplications”E.JKennedy.HRWInc.）

Figure2showsthevariationofαasafunctionoftemperatureforKandTTypethermocouples.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

isused.Thisassumptionresultsinerrors,particularlyoverawiderangeoftemperatures.Formoreaccurateresults,higherorderpolynomialsareused.

Figure3.SeebeckVoltageasafunctionofTemperatureDifference.

（Note:

TheReferenceJunctiontemperatureis0ºC.）

（Ref:

”OperationalAmplifierCircuitsTheoryandApplications”E.JKennedy.HRWInc.）

APracticalThermocoupleCircuit.

TheschematicshowninFigure5isthatofanelectronicallycompensatedthermocoupleamplifier.InthiscaseaJtypethermocouple（Iron-Constantan）hasbeenused.Thetwometallicjunctionsthatmakeupthisthermocouple,J1andJ2,areshownonthediagram.J1isthesensejunction,whileJ2isthereferencejunction.IdeallyJ2shouldbeheldinanicebathat0ºC,asshowninFigure1,howeverthisisgenerallyinconvenient.Insteaditismoreusualtoprovideelectronicreferencejunctioncompensation.Thisinvolvesmeasuringthetemperatureofthereferencejunctionandaddingacompensatingvoltagetothethermocouple,sothattheresidualSeebeckvoltagematchesthatofasimilarthermocouplewithitsreferencejunctionheldat0ºC.

GenerationofaCompensationVoltage

Table2showstheSeebeckvoltageasfunctionoftemperatureforJtypethermocouples,（referencedto0ºC）.Fromthisweseethatthethermocouplevoltagewillbe1.277mVwhenthesensejunctionisatroomtemperature,（25ºCor298ºK）.Thereforethisvoltagemustbesubtractedfromthethermocouplepotential,whenthereferencejunctionisat25ºC,inordertoleaveapotentialproportionalonlytothesensejunctiontemperature.

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