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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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
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