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RTDs Home>Resources>Temperature>Sensors>RTDs

IntroductiontoRTDsResistanceTemperatureDetectors(RTDs)relyonthepredictableandrepeatablephenomenaoftheelectricalresistanceofmetalschangingwithtemperature.Thetemperaturecoefficientforallpuremetalsisofthesameorder0.003to0.007ohms/ohm/C.Themostcommonmetalsusedfortemperaturesensingareplatinum,nickel,copperandmolybdenum.Whiletheresistancetemperaturecharacteristicsofcertainsemiconductorandceramicmaterialsareusedfortemperaturesensing(seesectionsonSemiconductorandThermistor),suchsensorsaregenerallynotclassifiedasRTDs.AprecisionRTDTemperatureCalculatorsupportingfourRTDtypescanprovidea"handson"understandingofRTDcharacteristics.

HowareRTDsconstructed?RTDsaremanufacturedintwoways:usingwireorfilm.WireRTDsareastretchedcoiloffinewireplacedinaceramictubethatsupportsandprotectsthewire.Thewiremaybebondedtotheceramicusingaglaze.Thewiretypesaregenerallythemoreaccurate,duetothetightercontrolovermetalpurityandlessstrainrelatederrors.Theyarealsomoreexpensive.

FilmRTDsconsistofathinmetalfilmthatissilkscreenedorvacuumsplutteredontoaceramicorglassysubstrate.AlasertrimmerthentrimstheRTDtoitscorrectresistancevalue.Filmsensorsarelessaccuratethanwiretypes,buttheyarerelativelyinexpensive,theyareavailableinsmallsizesandtheyaremorerobust.FilmRTDscanalsofunctionasastraingaugesodon'tstrainthem!Thealuminaelementshouldbesupportedbygreaseoralightelastomer,butneverembeddedinepoxyormechanicallyclampedbetweenhardsurfaces.

RTDscannotgenerallybeusedintheirbasicsensingelementform,astheyaretoodelicate.Theyareusuallybuiltintosometypeofassembly,whichwillenablethemtowithstandthevariousenvironmentalconditionstowhichtheywillbeexposedwhenused.Mostcommonlythisisastainlesssteeltubewithaheatconductinggrease(thatalsodampensvibration).Standardtubediametersinclude3,4.5,6,8,10,12and15mmandstandardtubelengthsinclude250,300,500,750and1000mm.

CharacteristicsofRTDsMetalRTDshavearesponsedefinedbyapolynomial:

R(t)=R0(1+a.t+b.t2+c.t3)

WhereR0istheresistanceat0C,"t"inthetemperatureinCelsius,and"a","b"and"c"areconstantsdependentonthecharacteristicsofthemetal.InpracticethisequationisaclosebutnotperfectfitformostRTDs,soslightmodificationsareoftenbemade.Commonly,thetemperaturecharacteristicsofanRTDarespecifiedasasinglenumber(the"alpha"),representingtheaveragetemperaturecoefficientoverthe0to100Ctemperaturerangeascalculatedby:

alpha=(R100R0)/100.R0inohms/ohm/C

Note:RTDscoverasufficienttemperaturerangethattheirresponseneedstobecalibratedintermsofthelatesttemperaturescaleITS90.Forassistancewithsuchcalculations,seetheRTDtemperaturecalculator.Itisalsoofinteresttonotethatthetemperaturecoefficientofanalloyisfrequentlyverydifferentfromthatofthe

constituentmetals.Smalltracesofimpuritiescangreatlychangethetemperaturecoefficients.Sometimestrace"impurities"aredeliberatelyaddedsoastoswamptheeffectsofundesiredimpuritieswhichareuneconomictoremove.Otheralloyscanbetailoredforparticulartemperaturecharacteristics.Forexample,analloyof84%copper,12%Manganeseand4%Nickelhasthepropertyofhavinganalmostzeroresponsetotemperature.Thealloyisusedforthemanufactureofprecisionresistors.

TypesofRTDsWhilealmostanymetalmaybeusedforRTDmanufacture,inpracticethenumberusedislimited.

Metal TemperatureRange Alpha Comments

Copper Pt 200Cto260C 0.00427 Lowcost

Molybdenum Mo 200Cto200C 0.003000.00385Lowercostalternativetoplatinuminthelowertemperatureranges

Nickel Ni 80Cto260C 0.00672 Lowcost,limitedtemperaturerangeNickelIron NiFe 200Cto200C 0.00518 Lowcost

Platinum Pt 240Cto660C0.003850.003920.00377

Goodprecision.Extendtemperaturerangeto1000Cavailable

Othermaterialsareusedforspecialistapplications.Forexample,cryogenictemperaturesensors.

PlatinumRTDsPlatinumisbyfarthemostcommonRTDmaterial,primarilybecauseofitslongtermstabilityinair.TherearetwostandardPlatinumsensortypes,eachwithadifferentdopinglevelof'impurities'.ToalargeextenttherehasbeenaconvergenceinplatinumRTDstandards,withmostnationalstandardsbodiesadoptingtheinternationalIEC7511983,withamendment1in1986andamendment2in1995.TheUSAcontinuestomaintainitsownstandard.AlltheplatinumstandardsuseamodifiedpolynomialknownastheCallendarVanDusenequation:

R(t)=R0(1+a.t+b.t2+c.(t100).t3)

PlatinumRTDsareavailablewithtwotemperaturecoefficientsoralphasthechoiceislargelybasedonthenationalpreferenceinyoucountry,asindicatedinthefollowingtable:

Standard Alphaohms/ohm/CR0

ohmsPolynomialCoefficients Countries

IEC751(Pt100) 0.00385055 100

200C

thermoelectriceffect,vibrationtolerance,leadmarkingandsensormarking.Someofthesearediscussedbelow:ThermoelectricEffect:AplatinumRTDgenerallyemploystwometalstheplatinumsensingelementandcopperleadwires,makingitagoodcandidateforathermocouple.Ifatemperaturegradientisallowstodevelopalongthesensingelement,athermoelectricvoltagewithamagnitudeofabout7V/Cwillbegenerated.Thisisonlylikelytobeaproblemwithveryhighprecisionmeasurementsoperatingatlowexcitationcurrents.WiringConfigurationsandLeadMarking:Therearethreewiringconfigurationsthatcanbeusedformeasuringresistance2,3and4wireconnections.

IEC751requiresthatwiresconnectedtothesameendoftheresistorbethesamecoloureitherredorwhite,andthatthewiresateachendbedifferent.Seethemeasurementsectionforthemeritsofeachofthesewiringschemes.SensorMarking:IEC751stipulatesthatasensorshouldbemarkedwithitsnominalR0value,toleranceclass,thewiringconfigurationandtheallowabletemperaturerange.Anexamplemarkingis:

Pt100/A/3/100/+200

correspondingto100Ohmplatinum,classA,3wireconfigurationandwithatemperaturerangefrom100Cto+200C.MeasurementCurrent:Preferredmeasurementcurrentsarespecifiedas1,2and5mA,although5mAisnotallowedwithclassAsensorsduetopotentialselfheatingerrors.

NickelRTDsNickelsensorsarepreferredincostsensitiveapplicationssuchasairconditioningandconsumergoods.Becausecostisanissue,theyaregenerallymanufacturedinhigherresistancevaluesof1kor2kohmssothatasimpletwowireconnectioncanbeused(ratherthanthe3or4wireconnectionscommonwithplatinumtypes).ThereappearstobenointernationalstandardcoveringthenickelRTD,althoughmostmanufacturersappeartofollowIEC751(whichonlydealswithplatinumdevices)whereappropriate.AresultingproblemisthatthereappearstobenowidelyacceptedcalibrationforthenickelRTD.OnemanufacturerofnickelRTDsrecommendsthefollowingpolynomial:

R(t)=R0(1+a.t+b.t2+d.t4+f.t6)

wherea=5.485x103b=6.650x106d=2.805x1011andf=2.000x1017.Thealphaforthispartis0.00672ohms/ohm/CMorecommonforlowtomediumprecisionmeasurementthesimplificationoftheequationisusedwitha=alpha:

R(t)=R0(1+a.t)

whichiseasilyinvertedfortemperature:

t=(Rt/R01)/a=(Rt/R01)/0.00672

where"a"issubstitutedforthealphavalue.Nickelislesschemicallyinertthatplatinumandsoislessstableathighertemperatures.Glasspassivationcanextendtheusefultemperaturerangeto200C,butthenickelRTDisnormallyusedforsensingintheenvironmentaltemperaturerangeandinclearair.

NickelIronRTDsLowerincostthanthepureNickelRTD,theNickelIronRTDfindsapplicationinHVACandothercostsensitiveapplications.Thealpha=0.00518

CopperRTDsCopperisrarelyusedspecificallyasasensingelement,butisoftenemployedwhenacoppercoilexistsforotherpurposes.Forexampleinavibratingwirestaingaugeacoilisrequiredto"pluck"thewireandsenseitsvibrationfrequency.Thesamecoilcanbeusedtosensethetemperatureofthesensorsothatitsreadingsmaybecompensatedfortemperatureinduceddrifts.Anotherapplicationisinmeasuringthetemperatureofelectricmotorandtransformerwindings.Inthesetypesofapplications,wheretemperaturesensingisasecondaryfunction,careshouldbetakeninwindingthecoilsothatthermalexpansionofthesystemdoesnotinducesignificantstraingaugeeffectsinthecopperwirewhichmayaddtotheuncertaintyofthemeasurement.ThereappearstobenointernationalstandardforcopperRTDs,howeveranalpha=0.00427ohms/ohm/Ciscommonlyused.Whenthetemperaturerangeissmall(say0Cto180C)andtheaccuracyneedsarenotgreat,asimplelinearfunctioncanbeused:

t=(Rt/R01)/0.00427

MolybdenumRTDsMolybdenumhasatemperaturecoefficientofexpansionwhichalmostperfectlymatchesthatofalumina,makingitanidealmaterialforfilmtypeofconstruction.Theusefultemperaturerangeistypically200Cto+200Candthematerial'salpha=0.00300ohms/ohm/C.MolybdenumRTDsarealsoavailablewithanalpha=0.00385ohms/ohm/C(achievedbydopingwithothermetals)whichmakesitcompatiblewiththestandardPt100devicesoverareducedtemperaturerangeandatareducedcost.

ProprietarytypesAnumberofcompaniesmanufactureproprietaryRTDsensortypesanddonotnecessarilypublishdetailsaboutthesensingmaterialused.OnesuchdeviceistheTDSeriesoftemperaturesensorsfromHoneywell.Theseappeartobeconstructedbydepositingathinmetalfilmonasiliconsubstrateandtrimmingbylaser.Thebenefitsarearelativelylowcost,simplicityand0.7Cinterchangeabilityat20C.TheresponseoftheHoneywellpartsissimilartoaplatinumRTD:

R(t)=R0(1+3.84x103.t+4.94x106.t2)

ThefollowingtablerepresentssomeoftheproprietaryRTDsensortypesmanufacturedbyavarietyofcompanies:

Sensor Manuf. Output Tolerance(range) Package Comments

TD4ATD5A Honeywell

8ohms/C(1854ohmsat

0C)2.5C

(40Cto150C)TO92orthreaded

tubeMetalfilmonsiliconsubstrate

KTY81KTY82KTY83KTY84KTY85

Philips1Kor2Kat

25C,+0.8%/CSeebelow

6Cto12C(55Cto+150Csometo300C)

SOD70,SOT23SOD68SOD80

Bulkresistanceofsilicon.Keepexcitationcurrent>0.1mAand