I had asked Kevin Bull (Veriteq Instruments) a question on the behavior of the Measurement Uncertainties at low RH Values like those we experience in the Phoenix-Tucson area. His excellent answer is posted in the Discussion Forum.
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Charlie,
You had a question that I was unable to answer with regard to RH measurement uncertainties at lower RH values.
Here is something we put together and I hope is of value:
A ‘typical’ RH calibration system consists of a controlled environment (RH chamber) measured with a Chilled Mirror Hygrometer (CMH).
The uncertainty of a CMH is usually dominated by a temperature measurement - that of the actual mirror. This translates into a relatively constant Dew Point/Frost Point error, but when measuring RH, the equivalent RH error is not constant.
Let’s assume, that we have a Dew/Frost error of 0.1°C (a good CMH). This would contribute the following RH errors:
Temperature (°C ) RH (%) RH Error (%RH)
25.0 10.0 0.1
25.0 50.0 0.3
25.0 90.0 0.5
As we can see, the error is actually greater at higher RH levels.
This does not include the practical difficulties from condensation that arise at the higher RH levels.
Temperature gradients within a chamber are another significant source of error and are approximately the same magnitude as above for every 0.1°C of chamber gradient.
As a chamber is operated further from ambient temperature, the temperature gradients within the chamber almost always become greater.
At 40°C and 80% RH (a common calibration point for the pharmaceutical industry) each 0.1°C of chamber gradient will result in an error of nearly 0.5% RH yet, for comparison, a 0.1°C chamber gradient at 40°C and 10% RH will introduce an error of 0.1% RH.
Another calibration method is the Two Pressure system (Thunder Scientific, for example, makes these). In this case, neither RH nor Dewpoint is directly measured. Instead, temperature and pressure are measured and RH is calculated.
Thunder provides a very detailed uncertainty analysis and states the following uncertainties:
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As can be seen, again, the lowest uncertainties are actually at the low RH values.
I hope this helps to answer your question. Please do not hesitate to contact me if you should have any additional questions.
Sincerely,
Kevin Bull
Veriteq Instruments, Inc.
When Humidity and Temperature are Critical
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