毕业设计文献翻译热电偶温度传感器Word文档格式.docx

毕业设计文献翻译热电偶温度传感器Word文档格式.docx

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Thermocoupletheory

Athermocouplecircuithasatleasttwojunctions:

themeasurementjunctionandareferencejunction.Typically,thereferencejunctioniscreatedwherethetwowiresconnecttothemeasuringdevice.Thissecondjunctionitisreallytwojunctions:

oneforeachofthetwowires,butbecausetheyareassumedtobeatthesametemperature（isothermal）theyareconsideredasone（thermal）junction.Itisthepointwherethemetalschange-fromthethermocouplemetalstowhatevermetalsareusedinthemeasuringdevice-typicallycopper.

Theoutputvoltageisrelatedtothetemperaturedifferencebetweenthemeasurementandthereferencejunctions.ThisisphenomenaisknownastheSeebeckeffect.（SeetheThermocoupleCalculatortogetafeelforthemagnitudeoftheSeebeckvoltage）.TheSeebeckeffectgeneratesasmallvoltagealongthelengthofawire,andisgreatestwherethetemperaturegradientisgreatest.Ifthecircuitisofwireofidenticalmaterial,thentheywillgenerateidenticalbutoppositeSeebeckvoltageswhichwillcancel.However,ifthewiremetalsaredifferenttheSeebeckvoltageswillbedifferentandwillnotcancel.

InpracticetheSeebeckvoltageismadeupoftwocomponents:

thePeltiervoltagegeneratedatthejunctions,plustheThomsonvoltagegeneratedinthewiresbythetemperaturegradient.

ThePeltiervoltageisproportionaltothetemperatureofeachjunctionwhiletheThomsonvoltageisproportionaltothesquareofthetemperaturedifferencebetweenthetwojunctions.ItistheThomsonvoltagethataccountsformostoftheobservedvoltageandnon-linearityinthermocoupleresponse.

EachthermocoupletypehasitscharacteristicSeebeckvoltagecurve.Thecurveisdependentonthemetals,theirpurity,theirhomogeneityandtheircrystalstructure.Inthecaseofalloys,theratioofconstituentsandtheirdistributioninthewireisalsoimportant.Thesepotentialinhomogeneouscharacteristicsofmetalarewhythickwirethermocouplescanbemoreaccurateinhightemperatureapplications,whenthethermocouplemetalsandtheirimpuritiesbecomemoremobilebydiffusion.

Thepracticalconsiderationsofthermocouples

Theabovetheoryofthermocoupleoperationhasimportantpracticalimplicationsthatarewellworthunderstanding:

1.Athirdmetalmaybeintroducedintoathermocouplecircuitandhavenoimpact,providedthatbothendsareatthesametemperature.Thismeansthatthethermocouplemeasurementjunctionmaybesoldered,brazedorweldedwithoutaffectingthethermocouple'

scalibration,aslongasthereisnonettemperaturegradientalongthethirdmetal.

Further,ifthemeasuringcircuitmetal（usuallycopper）isdifferenttothatofthethermocouple,thenprovidedthetemperatureofthetwoconnectingterminalsisthesameandknown,thereadingwillnotbeaffectedbythepresenceofcopper.

2.Thethermocouple'

soutputisgeneratedbythetemperaturegradientalongthewiresandnotatthejunctionsasiscommonlybelieved.Thereforeitisimportantthatthequalityofthewirebemaintainedwheretemperaturegradientsexists.Wirequalitycanbecompromisedbycontaminationfromitsoperatingenvironmentandtheinsulatingmaterial.Fortemperaturesbelow400°

C,contaminationofinsulatedwiresisgenerallynotaproblem.Attemperaturesabove1000°

C,thechoiceofinsulationandsheathmaterials,aswellasthewirethickness,becomecriticaltothecalibrationstabilityofthethermocouple.

Thefactthatathermocouple'

soutputisnotgeneratedatthejunctionshouldredirectattentiontootherpotentialproblemareas.

3.Thevoltagegeneratedbyathermocoupleisafunctionofthetemperaturedifferencebetweenthemeasurementandreferencejunctions.Traditionallythereferencejunctionwasheldat0°

Cbyanicebath:

Theicebathisnowconsideredimpracticalandisreplacebyareferencejunctioncompensationarrangement.Thiscanbeaccomplishedbymeasuringthereferencejunctiontemperaturewithanalternatetemperaturesensor（typicallyanRTDorthermistor）andapplyingacorrectingvoltagetothemeasuredthermocouplevoltagebeforescalingtotemperature.

Thecorrectioncanbedoneelectricallyinhardwareormathematicallyinsoftware.Thesoftwaremethodispreferredasitisuniversaltoallthermocoupletypes（providedthecharacteristicsareknown）anditallowsforthecorrectionofthesmallnon-linearityoverthereferencetemperaturerange.

4.Thelow-leveloutputfromthermocouples（typically50mVfullscale）requiresthatcarebetakentoavoidelectricalinterferencefrommotors,powercable,transformersandradiosignalpickup.Twistingthethermocouplewirepair（say1twistper10cm）cangreatlyreducemagneticfieldpickup.Usingshieldedcableorrunningwiresinmetalconduitcanreduceelectricfieldpickup.Themeasuringdeviceshouldprovidesignalfiltering,eitherinhardwareorbysoftware,withstrongrejectionofthelinefrequency（50/60Hz）anditsharmonics.

5.Theoperatingenvironmentofthethermocoupleneedstobeconsidered.Exposuretooxidizingorreducingatmospheresathightemperaturecansignificantlydegradesomethermocouples.Thermocouplescontainingrhodium（B,RandStypes）arenotsuitableunderneutronradiation.

Theadvantagesanddisadvantagesofthermocouples

Becauseoftheirphysicalcharacteristics,thermocouplesarethepreferredmethodoftemperaturemeasurementinmanyapplications.Theycanbeveryrugged,areimmunetoshockandvibration,areusefuloverawidetemperaturerange,aresimpletomanufactured,requirenoexcitationpower,thereisnoselfheatingandtheycanbemadeverysmall.Noothertemperaturesensorprovidesthisdegreeofversatility.

Thermocouplesarewonderfulsensorstoexperimentwithbecauseoftheirrobustness,widetemperaturerangeanduniqueproperties.

Onthedownside,thethermocoupleproducesarelativelowoutputsignalthatisnon-linear.Thesecharacteristicsrequireasensitiveandstablemeasuringdevicethatisableprovidereferencejunctioncompensationandlinearization.

Alsothelowsignalleveldemandsthatahigherlevelofcarebetakenwheninstallingtominimisepotentialnoisesources.

Themeasuringhardwarerequiresgoodnoiserejectioncapability.Groundloopscanbeaproblemwithnon-isolatedsystems,unlessthecommonmoderangeandrejectionisadequate.

Typesofthermocouple

About13'

standard'

thermocoupletypesarecommonlyused.Eighthavebeengivenaninternationallyrecognisedlettertypedesignators.Thelettertypedesignatorreferstotheemftable,notthecompositionofthemetals-soanythermocouplethatmatchestheemftablewithinthedefinedtolerancesmayreceivethattable'

sletterdesignator.

Someofthenon-recognisedthermocouplesmayexcelinparticularnicheapplicationsandhavegainedadegreeofacceptanceforthisreason,aswellasduetoeffectivemarketingbythealloymanufacturer.Someofthesehavebeengivenlettertypedesignatorsbytheirmanufacturersthathavebeenpartiallyacceptedbyindustry.

Eachthermocoupletypehascharacteristicsthatcanbematchedtoapplications.IndustrygenerallyprefersKandNtypesbecauseoftheirsuitabilitytohightemperatures,whileothersoftenprefertheTtypeduetoitssensitivity,lowcostandeaseofuse.

Atableofstandardthermocoupletypesispresentedbelow.Thetablealsoshowsthetemperaturerangeforextensiongradewireinbrackets.

Accuracyofthermocouples

Thermocoupleswillfunctionoverawidetemperaturerange-fromnearabsolutezerototheirmeltingpoint,howevertheyarenormallyonlycharacterizedovertheirstablerange.Thermocoupleaccuracyisadifficultsubjectduetoarangeoffactors.Inprincipalandinpracticeathermocouplecanachieveexcellentresults（thatis,significantlybetterthantheabovetableindicates）ifcalibrated,usedwellbelowitsnominaluppertemperaturelimitandifprotectedfromharshatmospheres.Athighertemperaturesitisoftenbettertouseaheaviergaugeofwireinordertomaintainstability（WireGaugebelow）.

Asmentionedpreviously,thetemperatureandvoltagescaleswereredefinedin1990.Theeightmainthermocoupletypes-B,E,J,K,N,R,SandT-werere-characterisedin1993toreflectthescalechanges.（See:

NISTMonograph175fordetails）.Theremainingtypes:

C,D,G,L,M,PandUappeartohavebeeninformallyre-characterised.

Trythethermocouplecalculator.Itallowsyouthedeterminethetemperaturebyknowingthemeasuredvoltageandthereferencejunctiontemperature.

Thermocouplewiregrades

Therearedifferentgradesofthermocouplewire.Theprincipaldivisionsarebetweenmeasurementgradesandextensiongrades.Themeasurementgradehasthehighestpurityandshouldbeusedwherethetemperaturegradientissignificant.Thestandardmeasurementgrade（Class2）ismostcommonlyused.Specialmeasurementgrades（Class1）areavailablewithaccuracyabouttwicethestandardmeasurementgrades.

Theextensionthermocouplewiregradesaredesignedforconnectingthethermocoupletothemeasuringdevice.Theextensionwiremaybeofdifferentmetalstothemeasurementgrade,butarechosentohaveamatchingresponseoveramuchreducedtemperaturerange-typically-40°

Cto120°

C.Thereasonforusingextensionwireisreducedcost-theycanbe20%to3