Catching The Drift- What Your RH Measurement System's Specs Aren't Telling You

July 2009

Catching the DriftWhat RH Measurement Spec Sheets

aren’t telling youaren’t telling you...

http://www.veriteq.com/download/whitepaper/catching-the-drift.htm Kevin Bull

Veriteq Instruments © 2009 1

DriftFew Industrial Measurements areFew Industrial Measurements are more prone to drift than Relative HumidityInternal Structure is in Direct Contact with the Environment

“Air Breather”Focus on Capacitive RH Sensor

Most Common Industrial RH Sensor

Initial UncertaintyInitial Measurement UncertaintyInitial Measurement Uncertainty

(As Left) is Important

But…

It’s the As Found that you have toLive With

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RH Measurement is ComplexNMI / Primary Lab Capabilities:

Measurement UncertaintyFrequency 5x10-15

DC Voltage 5x10-10

Mass 5x10-8

L th 1 10 7

Measurement UncertaintyFrequency 5x10-15

DC Voltage 5x10-10

Mass 5x10-8

L th 1 10 7Length 1x10-7

Temperature 6x10-5Length 1x10-7

Temperature 6x10-5

Relative Humidity 0.2%Compared to other Parameters, RH has High measurement uncertainty

RH DefinitionRelative Humidity:Relative Humidity:

Amount of Water Vapor present as Compared to the Maximum Possible.Very Temperature Dependent

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Amount of Water Vapor at Saturation(100% RH)

*

*grams of H20 per Liter of Air

Amount of Water Vapor at Saturation(100% RH)

Previous Graph Zoomed In

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Relative HumidityFrom Graph at 22°C (72°F):From Graph at 22 C (72 F):0.019 g/L: Saturation 100% RH0.0097g/L: 50% of Saturation: 50% RH0.0048 g/L: 25% of Saturation: 25% RH

Sensor Operation

Electrical Capacitor 2 Conductors Separated by an InsulatorAbility to Store Charge Proportional to Dielectric Constant

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Sensor Operation

Designed to allow Water Vapor to Enter Dielectric

Sensor OperationIncreasing Relative Humidityg y

Ambient Water Molecules Enter and Fill the Dielectric (Polymer) -Increasing the Electrical Capacitance

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Sensor OperationDecreasing Relative Humidityg y

Ambient Water Molecules escape from the Dielectric (Polymer) -Decreasing the Electrical Capacitance

What Can Go Wrong?ContaminationContamination

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What Can Go Wrong?Surface ContaminationSurface Contamination

Alters the RH Environment that the Sensor is Exposed to –Improper ReadingSalts are a Common Problem- Impure Water SourceDirectly on Sensor or just in proximity

Sensor ContaminationSingle Point Cal – Looks OKSingle Point Cal Looks OK

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Sensor ContaminationSurface Contamination - RealitySurface Contamination Reality

What Can Go Wrong?Internal ContaminationInternal Contamination

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What Can Go Wrong?Internal ContaminationInternal Contamination

Interferes with Absorption and Diffusion of Water Vapor within SensorShifts RH readingsSometimes it is ReversibleSometimes it is Permanent

HysteresisNot Drift but a Significant SourceNot Drift, but a Significant Source of Error in RH Measurements

Measurement does not return to original value after a large excursionWater Molecules are ‘Trapped’ in DielectricOften it is Reversible – at least partially

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HysteresisR

H

ΔRH

What can You do About Drift?Drift in RH Measurements is InevitableDrift in RH Measurements is Inevitable- Plan for itStart off with better than is required:‘Drift Headroom’ Control what you Can

CalibrationMeasurement System (Electronics)RH Sensor – Select a Good One

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RH CalibrationThe air in the Calibration ChamberThe air in the Calibration Chamber is the ReferenceCompensate for Temperature GradientsAt 25°C and 50% RH:0.1°C Gradient → 0.3% RH Error

At 40°C and 75% RH:0.1°C Gradient → 0.4% RH Error

Chamber Temperature Gradients

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Dew Point is ConstantDew Point is ~constant in a chamber Use Local Temperature and calculate RH

Calibration RangeCalibrate over the Entire OperatingCalibrate over the Entire Operating RangeRH Sensors change with Temperature

A Device Calibrated at 25°C may be Out Of Tolerance if operated at 40°C

Calibration over Limited RH Ranges may not show Sensor Contamination

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RH Sensor Temperature Response

Significant Variations with Temperature

Calibration Coverage

80

90

30

40

50

60

70

80

%RH80-9070-8060-7050-6040-5030-4020 30

9181182

14461710

19742238-25

-105

2035

500

10

20

Raw DataTemperature (°C)

20-3010-200-10

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CalibrationLine Fit:Line Fit:25°C Calibration yields non-ideal fit at RH extremesPoor Performance over TemperatureSurface Fit:Surface Fit:Uses a Multi-Point Calibration to fit the ‘Surface’ (3 RH Points at 25°C and 2 Temperatures at 50% RH)

Measurement SystemRH Sensors are InsensitiveRH Sensors are Insensitive - Accurate and Stable Measurements are Challenging

Must be Repeatable over Time & TemperatureImpervious to Humidity

Long Time Constants will Not be Calibrated Out

Auto-Zero is Desirable

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Sensor ElementNone are PerfectNone are PerfectChoose the Best Available for a Specific ApplicationProven Long Term Drift Record- All Sensors are Not Created EqualResistance to Common ChemicalsLow HysteresisRecovery from Saturation

PreventionKeep Sensor and Probe BodyKeep Sensor and Probe Body away from Salts and other Surface ContaminantsDo not expose to VOCsMinimize exposure to High p gTemperature / High RHDo not Saturate

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Instrument SpecificationsOperating RangeOperating Range

Ensure Calibration is over the Entire Operating RangeTemperature - RH Sensors Respond Differently over TemperatureRelative Humidity RH Sensors areRelative Humidity - RH Sensors are Not Linear (Particularly at Extremes)

Look for ‘Realistic’ Calibration Uncertainties

ConclusionIf you do everything Really WellIf you do everything Really Well you can achieve +/-1% InitiallyLook for a 1 year (or other Cal Interval) SpecificationThat’s what you have to live with...

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Further Resources:Chart Recorder Replacement — ROIChart Recorder Replacement ROI Calculatorveriteq.com/calculator/chart-recorder-replacement-calculator.php

“Switching from Chart Recorders to DataSwitching from Chart Recorders to Data Loggers”veriteq.com/download/dataloggers-vs-chart-recorders.pdf

Further Resources:“Catching the Drift: What theCatching the Drift: What the Specifications of Your Humidity Measurement System Might be Missing”veriteq.com/download/whitepaper/catching-the-drift.pdf

“How Hard Could That Be? Practical Humidity Calibration Experiences”veriteq.com/download/practical-rh-experiences.pdf

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Further Resources:“The Trouble with Humidity: The HiddenThe Trouble with Humidity: The Hidden Challenge of RH Calibration”veriteq.com/download/trouble-with-humidity.pdf

“Methods of Accurately MeasuringMethods of Accurately Measuring Capacitive RH Sensors”veriteq.com/download/methods-of-accurately-measuring-capacitive-rh-sensors.pdf

ThankThank You

Veriteq Instruments

www veriteq comwww.veriteq.com(800) 683-8374