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.

1. July 2009
Catching the Drift
What RH Measurement Spec Sheets
aren’t telling you
Kevin Bull

2. Drift
Few Industrial Measurements are
more prone to drift than Relative
Humidity
Internal Structure is in Direct
Contact with the Environment
“Air Breather”
Focus on Capacitive RH Sensor
Most Common Industrial RH Sensor
Initial Uncertainty
Initial Measurement Uncertainty
(As Left) is Important
But…
It’s the As Found that you have to
Live With
Veriteq Instruments © 2009 1

3. RH Measurement is Complex
NMI / Primary Lab Capabilities:
Measurement Uncertainty
Frequency 5x10-15
DC Voltage 5x10-10
Mass 5x10-8
L th
Length 1 10-7
1x10 7
Temperature 6x10-5
Relative Humidity 0.2%
Compared to other Parameters, RH
has High measurement uncertainty
RH Definition
Relative Humidity:
Amount of Water Vapor present as
Compared to the Maximum
Possible.
Very Temperature Dependent
Veriteq Instruments © 2009 2

4. 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
Veriteq Instruments © 2009 3

5. Relative Humidity
22 C (72 F):
From Graph at 22°C (72°F):
0.019 g/L: Saturation 100% RH
0.0097g/L: 50% of Saturation: 50% RH
0.0048 g/L: 25% of Saturation: 25% RH
Sensor Operation
Electrical Capacitor
2 Conductors Separated by an Insulator
Ability to Store Charge
Proportional to Dielectric Constant
Veriteq Instruments © 2009 4

6. Sensor Operation
Designed to allow Water Vapor to
Enter Dielectric
Sensor Operation
Increasing Relative Humidity
g y
Ambient Water Molecules Enter and
Fill the Dielectric (Polymer) -
Increasing the Electrical Capacitance
Veriteq Instruments © 2009 5

7. Sensor Operation
g y
Decreasing Relative Humidity
Ambient Water Molecules escape
from the Dielectric (Polymer) -
Decreasing the Electrical Capacitance
What Can Go Wrong?
Contamination
Veriteq Instruments © 2009 6

8. What Can Go Wrong?
Surface Contamination
Alters the RH Environment that
the Sensor is Exposed to –
Improper Reading
Salts are a Common Problem
- Impure Water Source
Directly on Sensor or just in
proximity
Sensor Contamination
Single Point Cal – Looks OK
Veriteq Instruments © 2009 7

9. Sensor Contamination
Surface Contamination - Reality
What Can Go Wrong?
Internal Contamination
Veriteq Instruments © 2009 8

10. What Can Go Wrong?
Internal Contamination
Interferes with Absorption and
Diffusion of Water Vapor within
Sensor
Shifts RH readings
Sometimes it is Reversible
Sometimes it is Permanent
Hysteresis
Not Drift, but a Significant Source
Drift
of Error in RH Measurements
Measurement does not return to
original value after a large
excursion
Water Molecules are ‘Trapped’ in
Dielectric
Often it is Reversible – at least
partially
Veriteq Instruments © 2009 9

11. RH Hysteresis
ΔRH
What can You do About Drift?
Drift in RH Measurements is Inevitable
- Plan for it
Start off with better than is required:
‘Drift Headroom’
Control what you Can
Calibration
Measurement System (Electronics)
RH Sensor – Select a Good One
Veriteq Instruments © 2009 10

12. RH Calibration
The air in the Calibration Chamber
is the Reference
Compensate for Temperature
Gradients
At 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
Veriteq Instruments © 2009 11

13. Dew Point is Constant
Dew Point is ~constant in a chamber
Use Local Temperature and calculate RH
Calibration Range
Calibrate over the Entire Operating
Range
RH 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
Veriteq Instruments © 2009 12

14. RH Sensor Temperature Response
Significant Variations with Temperature
Calibration Coverage
90
80
70
60
80-90
50 70-80
%RH 60-70
40 50-60
30 40-50
30-40
20 20-30
20 30
10-20
10 0-10
0
918 50
1182 35
1446 20
1710 5
1974 -10
Raw Data 2238
-25
Temperature (°C)
Veriteq Instruments © 2009 13

15. Calibration
Line Fit:
25°C Calibration yields non-ideal
fit at RH extremes
Poor Performance over
Temperature
Surface Fit:
Uses a Multi-Point Calibration to
fit the ‘Surface’
(3 RH Points at 25°C and
2 Temperatures at 50% RH)
Measurement System
RH Sensors are Insensitive
- Accurate and Stable
Measurements are Challenging
Must be Repeatable over Time &
Temperature
Impervious to Humidity
Long Time Constants will Not be
Calibrated Out
Auto-Zero is Desirable
Veriteq Instruments © 2009 14

16. Sensor Element
None are Perfect
Choose the Best Available for a
Specific Application
Proven Long Term Drift Record
- All Sensors are Not Created Equal
Resistance to Common Chemicals
Low Hysteresis
Recovery from Saturation
Prevention
Keep Sensor and Probe Body
away from Salts and other
Surface Contaminants
Do not expose to VOCs
p g
Minimize exposure to High
Temperature / High RH
Do not Saturate
Veriteq Instruments © 2009 15

17. Instrument Specifications
Operating Range
Ensure Calibration is over the Entire
Operating Range
Temperature - RH Sensors Respond
Differently over Temperature
Relative Humidity - RH Sensors are
Not Linear (Particularly at Extremes)
Look for ‘Realistic’ Calibration
Uncertainties
Conclusion
If you do everything Really Well
you can achieve +/-1% Initially
Look for a 1 year (or other Cal
Interval) Specification
That’s what you have to live with...
Veriteq Instruments © 2009 16

18. Further Resources:
Chart Recorder Replacement — ROI
Calculator
veriteq.com/calculator/chart-recorder-
replacement-calculator.php
“Switching from Chart Recorders to Data
Switching
Loggers”
veriteq.com/download/dataloggers-vs-
chart-recorders.pdf
Further Resources:
“Catching the Drift: What the
Catching
Specifications of Your Humidity
Measurement System Might be Missing”
veriteq.com/download/whitepaper/catchi
ng-the-drift.pdf
“How Hard Could That Be? Practical
Humidity Calibration Experiences”
veriteq.com/download/practical-rh-
experiences.pdf
Veriteq Instruments © 2009 17

19. Further Resources:
“The Trouble with Humidity: The Hidden
The
Challenge of RH Calibration”
veriteq.com/download/trouble-with-
humidity.pdf
“Methods of Accurately Measuring
Methods
Capacitive RH Sensors”
veriteq.com/download/methods-of-
accurately-measuring-capacitive-rh-
sensors.pdf
Thank
You
Veriteq Instruments
www.veriteq.com
www veriteq com
(800) 683-8374
Veriteq Instruments © 2009 18