InterpretingHumidityInstrument
Specifications
Helping you make a better measurement.
WebinarPresenters&HumidityExperts
BruceMcDuffee
MichaelBoetzkes
DavidLove
Agenda&LearningObjectives
• ExaminethemetrologicalmeaningofAccuracy
• SeehowspecificationsusethetermAccuracy
• Lookingbeyond“Accuracy”
• Initialperformance
• LongTermDrift(performanceafteraperiodoftime)
• Measurementsawayfromthecalibrationtemperaturepoints
• Howtocompareinstruments
WhatisAccuracy?
Choosethebestanswer:
A. Theclosenessofameasurementtothetruevalue
B. Closenessofagreementbetweenameasuredquantityvalueandatrue
quantityvalueofameasurand
C. the condition or quality of being true, correct, or exact; freedom from
error or defect; precision or exactness; correctness
D. Anumericalindicationoftheoverallperformanceofaninstrumentduringthe
manufacturer’srecommendedcalibrationinterval
AccurateDefinitionofAccuracy
TheVIM(BIPM- Internationalvocabularyofmetrology–
Basicandgeneralconceptsandassociatedterms)
“closenessofagreementbetweenameasuredquantity
valueandatruequantityvalueofameasurand”
Bureau international des poids et mesures
AccurateDefinitionofAccuracy
“closenessofagreementbetweenameasured
quantityvalueandatruequantityvalueofa
measurand”
• Measurand - quantityintendedtobemeasured
• MeasuredQuantityValue- quantityvaluerepresentingameasurementresult
• TrueQuantityValue- quantityvalueconsistentwiththedefinitionofaquantity
• QuantityValue- numberandreferencetogetherexpressingmagnitudeofa
quantity,forexample5°C,543meters
Bureau international des poids et mesures
WhatdoesNISTsay?
TN1297AppendixD(referencetoVIM)
• “closenessoftheagreementbetweentheresult
ofameasurementandthevalueofthe
measurand”
NISTComments:
• Accuracyisaqualitativeconcept
• Oneshouldnotuseaccuracyquantitatively,that
is,associatenumberswithit;numbersshould
beassociatedwithmeasuresofuncertainty
instead.
CommonMisconceptions
• Accuracyspecificationcoverseverything.
• Theothervaluesonthedatasheetareall
includedintheaccuracyspecification.
• Instrumentsaysaccuracyis+/-1%RHthenIcan
expecttheerrortobelessthan1%RHwhenI
senditinforcalibrationnextyear.
• Theinstrumentcanbeusedatanytemperature
andstillhasanaccuracyof1%RH.
U = k u12 + u 22 + u32 + u 42 + u52........
Whatdoestheaccuracyspecificationmean?
ManufacturinglimitofdifferencebetweentheUnitUnderTestandtheReference
• Typicallyincludes
• Linearity
• Repeatability
• Hysteresis
• Typicallyignores
• Referenceuncertainty
• Longtermdrift
Comments&Questions
Ifwedon’t gettoyourquestion today,we’llrespond viaemail
afterthewebinar.
LookingBeyondAccuracy
• Otherkeyparameters
• Calibrationuncertainty
• Longtermdrift
• Temperaturecompensation
• Oftenneedtolookbeyondthe
webpagespecifications
•
•
•
•
Datasheets
Product/Usermanuals
Askthemanufacturer
Calibrationcertificate
CalibrationUncertainty
• Providescontextto“Accuracy”specification.
• Everymeasurementhasuncertainty(evenNIST)
• Akeypartofthetraceabilityofameasurement
• Itistheindicationofhowtheaccuracyof
measurementshavedegradedthroughthe
chainofcalibrationsgoingbacktoNISTand
essentiallybacktoSI.
Drift
• Allinstrumentswilldriftbut
notallbythesameamount
• Notalwaysonthedatasheet
CommonCausesofDriftinRelativeHumidity
Particulate
Vapor
• Dust - Slowingdownofresponsetime
(increasehysteresis)
• Salts - Createasaltsolutiononsensor
surfaceincreaseslocalhumidity
• Outgassingofproductsinchambersmay
effectdielectricinsensors,orcause
delamination.
• Protection - mechanicalfilters
• Protection - highqualitysensorstested
forthechemical
DriftinRHSensorsCausedbyChemicals
• Chemicalresistanceguides
• Specifyallowable
concentrationsofchemicals
• Foundinusermanuals
TemperatureCompensation
• Specificationsusuallyprovideaccuracyatasingle
temperatureorsmallrange(+/-5°C)
• Errorincreasesoutsideofthisrange
• Examples:
• +0.02%RHperKelvindependentontheprocessand
electronicstemperature(foradeviationof25°C)
• At-20…+40°C±(1.0+0.008xreading)
• At-40…+180°C±(1.5+0.015xreadings)
Comments&Questions
Ifwedon’t gettoyourquestion today,we’llrespond viaemail
afterthewebinar.
SampleSpecificationsfor65%RHat23°C
Specification
InstrumentA
Accuracy
Accuracy
Includes:
- Non-linearity
- Hysteresis
- Repeatability
InstrumentB
AnnualDrift
InstrumentD
±0.8%RH
±1.3%RH+0.003*mv
±1%RH
(± 1.495%RH)
Measurement Uncertainty
Includes:
- Non-linearity
- Hysteresis
- Repeatability
- CalibrationUncertainty
CalibrationUncertainty
InstrumentC
±1%RH+0.007*mv
(± 1.46%RH)
±1%RH
±0.5%RH
<1%RH
NormalizingSpecifications
InstrumentA
InstrumentC
Standard
Uncertainty Comment
Parameter
U
Distribution
Divisor
Accuracy(Linearity,
Repeatability,Hysteresis)
1
Rectangular
1.732
0.577
FromDataSheet
CalibrationUncertainty
1
Normal
2
0.500
FromDataSheet
AnnualDrift
1
FromHistorical
DataSheet
Rectangular
1.732
0.577
CombinedUncertainty
0.957
ExpandedUncertaintyk=2 ±2.0%RH
Parameter
U
Measurement Uncertainty
(Linearity,Repeatability,
Hysteresis,Calibration
Uncertainty)
1.46
AnnualDrift
1
Distribution
Divisor
Standard
Uncertainty Comment
Normal
2
0.730
FromDataSheet
Rectangular
1.732
0.577
BestEstimate
CombinedUncertainty 0.9307
ExpandedUncertaintyk=2 ±1.9%RH
InstrumentD
InstrumentB
Parameter
Accuracy
U
0.8
CalibrationUncertainty
AnnualDrift
0.5
1
Distribution
Rectangular
Divisor
1.732
Standard
Uncertainty Comment
0.462
FromDataSheet
FromCalibration
Certificate
Normal
2
0.250
Rectangular
1.732
0.577
FromDataSheet
CombinedUncertainty
0.780
ExpandedUncertaintyk=2 ±1.6%RH
Parameter
U
Distribution
Divisor
Accuracy(Linearity,
Repeatability,Hysteresis)
1.5
Rectangular
1.732
CalibrationUncertainty
AnnualDrift
0.6
1
Standard
Uncertainty Comment
0.863
Normal
2
0.300
Rectangular
1.732
0.577
CombinedUncertainty
1.081
ExpandedUncertaintyk=2 ±2.2%RH
FromDataSheet
FromAssociated
CalibratorData
Sheet
BestEstimate
Takeaways
1. TheAccuracySpecificationdoesnotprovidethewholepictureof
instrumentperformance,especiallyforlongtermperformance(1
Year).
2. DataSheetsareonlythebeginningtounderstandinginstrument
performance,theydonotalwayscontainalltheinformationrequired
tomakeaninformeddecision.
3. Themostimportantvalueofinstrumentperformanceisthe1year
performancevalue.
Comments&Questions
Ifwedon’t gettoyourquestion today,we’llrespond viaemail
afterthewebinar.
HumidityAcademy
• Resourcesformakingabettermeasurement
•
•
•
•
•
Psychrometric charts
Technicalnotes
Humiditycalculator
Applicationnotes
more
www.rotronic-usa.com/humidity-academy
NextWebinar
Traceability– WhatisitandHowtoAchieveit?
Thursday,January14th 1:00PMEST
Registeratwww.rotronic-usa.com/humidity-webinars
Takeaways
• Howdoestraceabilityaffectthemeasurementsthatwetake?
• HowcanIprovetraceabilityofmymeasurements?
• HowcanIensurethatmysuppliersareactuallyprovidingtraceablecalibration
results?
Helpingyoumakeabetterhumidity
measurement– andmore.
Transmitters,portablemetersand
loggersfor:
•
•
•
•
•
•
•
Humidity
CarbonDioxide
LowDewPoint
WaterActivity
DifferentialPressure
MonitoringsystemsforcGMP
ISO17025calibrations(humidityand
temperature)
Postwebinarsurvey
Thankyou!
email:[email protected]
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