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Measuringtemperature:PlatinumResistancethermometers 24|06|2015
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MEASURINGTEMPERATURE:PLATINUMRESISTANCETHERMOMETERSAlgorithmsTemperatureWrittenbyAdministrator
PlatinumresistancethermometersWithin a limited range several
metals and alloys have an approximately linear resistivity
alteration depending on temperature.Resistance thermometers are
sensors that allow determining temperature by measuring the
electrical resistance of the
sensor,assumingthatitisinthermalequilibriumwiththemediumtomeasure.Thus,respectivesensorscanbeusedwithintheirspecificrangetocalculatetemperaturebypolynomialapproximation.
Platinum has a positive temperature coefficient, meaning that
electrical resistance increases with raising temperature.
PlatinumResistanceThermometers (PRT)areoftenmanufacturedof
coiledplatinumwireor as film thermometerswith a layer of
platinumappliedonnonconductivesubstrates.Theyareusedintherange200Ct850C.Outertheseboundspolynomialapproximationisnotassuredandhighertemperaturesareoftenproblematicforproperuse.Dependingontheapplicationtheplatinumfreelyextendsintothemediumtomeasure,ismountedonnonconductivematerialsorencapsulatedinhousingsforruggeduse.PRTsensorsarecharacterisedbytheirspecificelectricalresistanceR0at0C.Thecalibrationiseasilyaccomplishedbylengthandthicknessoftheusedwireorcoatingareaandthickness.Commontypesare:
Pt100(R0=100)Pt200(R0=200)Pt500(R0=500)Pt1000(R0=1000)Pt3000(R0=3000)Pt6000(R0=6000)Pt9000(R0=9000)
TheadvantageofPRTs is
theirexchangeabilitywithoutnecessitatingrecalibration.Tomeasuretemperaturetheeasymeasurablepropertyoftheelectricalresistanceneedstobegauged.WithinagiventolerancethisresistancecanbeconvertedtotemperaturefortherespectivePRTtype.TolerancesaccordingtoDINIEC751/DINEN60751(1995)are:
ClassA:(0.15+0.002*||)CClassB:(0.30+0.005*||)CClassC:(1.20+0.005*||)CClass1/3:[1/3*(0.3+0.005*||)]CClass1/5:[1/5*(0.3+0.005*||)]CClass1/10:[1/10*(0.3+0.005*||)]C
AselectricalresistanceofPRTscanbecomeverysmall(e.g.18.53Ohmat200CforaPt100sensor)resistanceofthewiresusedtocablethesensormaybesignificant.Anotherpointtopayattentionforisselfheatingofthesensorduetoacontinuousmeasurementcurrent.
EquationAsPRTs are not completely linear polynomial
approximation is required. In the range 200C
TheaboveequationsareimplementedintwofunctionsfortemperaturesonITS90scale.ForIPTS68coefficientsneedtobechangedtotherespectivevaluestabulatedintable(XYZ).Itcanbeeasilyseenthatconversionissenseless,ifR0=0.Thusbothfunctionscheckforthiscase.
Thefirstfunctionistemp_PRTResistanceValue.ItcalculateselectricalresistanceofaPRToftypeR0()attemperatureaTemp(C,ITS90).
IfaTemp is notwithin thedefined rangeof 200C to+850C function
returnsFALSEanda resistancevalueof 0 in
thevariableResistance.Otherwiseelectricalresistancewillbecalculated.DependingonthevalueinaTemprespectiveequation(XYZ)ischosenforaTempvalues0Candequation(XYZ)foraTempvalues=200)And(aTemp=0THenBeginResistance:=R0*((5.775E7*aTemp+3.9083E3)*aTemp+1);endElseBeginResistance:=R0*((((aTemp100)*4.183E12*aTemp5.775E7)*aTemp+3.9083E3)*aTemp+1);end;endElseBegin//otherwisereturnFalseanddefinedResistancetemp_PRTResistanceValue:=False;Resistance:=0;end;end;
Functiontemp_PRTTemperatureValue(aResistance,R0:Double;VarTemperature:Double):Boolean;//Thefunctiontemp_PRTTemperatureValuecalculatesthetemperature//measuredbyaPlatinumResistanceThermometer(PRT)atgiven//electricalresistanceanddefinedresistanceR0at0Cfora//PRTelement.E.g.foraPt1000elementresistanceR0=1000.//FunctionreturnsTRUEifaResistanceiswithintheranges//polynomialapproximationaredefined,therespectivetemperature//forresistanceaResistanceisfoundinTemperature.////Units://aResistanceInputelectricalresistanceOhm//R0TypeofPRT(electricalresistanceat0C)Ohm//TemperatureReturnedITS90TemperatureC////References://PrestonThomas&Quinn1990Techniquesforapproximating//theinternationaltemperaturescaleof1990Bureau//InternationaldesPoidsetMesures,2ndEdition1997,//respectingtheamendmentsofthe2ndEdition////(c)Dr.JanSchulz,18.December2008,www.code10.infoVarMinResistance:Double;MaxResistance:Double;Begin//expectworstcasetemp_PRTTemperatureValue:=False;Temperature:=0;
//findminandmaxresistancevaluesfordefinedrangeIftemp_PRTResistanceValue(200,R0,MinResistance)Andtemp_PRTResistanceValue(+850,R0,MaxResistance)THenBegin//isaResitancewithindefinedrangeofpolynomialapproximationIf(aResistance>=MinResistance)And(aResistance
