catching the drift- what your rh measurement system's specs aren't telling you
DESCRIPTION
Understanding how humidity measurement devices function and how product specifications can be misrepresented can mean the difference between purchasing an expensive system with inherent measurement flaws and getting a system that measures accurately between calibrations.The ability to scrutinize product specs in greater depth will allow you to select a system that fits the measurement needs of your application. This, in turn can help with meeting compliancy and quality assurance requirements.TRANSCRIPT
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
Veriteq Instruments © 2009 2
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