Temperature vs. Humidity Mapping: Two Critical Differences

Woman in a laboratory
Paul Daniel, Vaisala
Senior GxP Regulatory Compliance Expert
Published:
Industrial Manufacturing and Processes
Life Science

During one of our webinars on Environmental Mapping Qualification, we received a question from a participant on the difference between mapping temperature only, and temperature with RH. Senior Regulatory expert Paul Daniel answers below:

Dear Paul,
I recently attended your webinar on Mapping Protocols and I wonder – can I use the same methodology you discuss for temperature and humidity mapping studies as with temperature-only mapping studies? Thanks!
J

 

Dear J,

Short answer: Absolutely, the same methodology would be used for humidity mapping. Almost everything would be the same when considering the protocol document, and the attachments you would use for mapping humidity. Long answer: there are two critical differences in humidity mapping; those are sensor calibration and sensor placement.

Calibration: It is rare that a pre-study or post-study calibration can be done for a humidity sensor unless you have a well-equipped calibration lab. In these cases, there would be a single check before the study to verify that calibration certificates were available for your humidity sensors, and that each sensor was within its calibration interval at the time of use.

Sensor placement: Generally, the easiest practice is to use the same number of humidity and temperature sensors. This is especially easy if you are using dual sensors like our DL2000 logger. However sometimes it can make sense to utilize fewer humidity sensors. If you do use fewer humidity sensors, it is important that you understand humidity well enough to explain to an auditor what the expected humidity values would be (based on the temperature) in the spots that had only temperature sensors.

Our Humidity Calculator is a tool that can help with that. The free Vaisala Humidity Calculator allows you to calculate several humidity parameters from one known value. Make unit conversions on the fly, and see the effects of changing ambient conditions, like temperature and pressure. No more installing software for offline use - just bookmark the calculator page or add it to the home screen on your mobile device.
To learn more about spot checking temperature and humidity, here's a short video explaining the use of our HM70 handheld temperature and humidity meter for spot checking.

Paul Daniel, Vaisala

Senior GxP Regulatory Compliance Expert

Paul Daniel has worked in the GMP-regulated industries for over 25 years helping manufacturers apply good manufacturing practices in a wide range of qualification projects.  His specialties include mapping, monitoring, and computerized systems.  At Vaisala, Paul oversees and guides the validation program for the Vaisala viewLinc environmental monitoring system.  He serves as a customer advocate to ensure the viewLinc environmental monitoring system matches the demanding requirements of life science and regulated applications.  Paul is a graduate of University of California, Berkeley, with a bachelor’s degree in biology.

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Comment

Denis

Feb 24, 2021
Hi Daniel,
Thanks for this note
You write that you can calculate humidity values based on the indications of several sensors.
To do this, we need to take the vapor pressure obtained from several sensors as a constant value for all points in the warehouse. How true is this?

Paul Daniel

Feb 25, 2021
Hi Denis -

To be clear, I wrote that the easiest practice is to use dual sensors that measure both temperature and humidity. This avoids any issues of explaining the measurement to auditors.

If you do choose to use fewer humidity sensors, the goal is to extrapolate the known humidity values from these known locations to those locations that only had temperature sensors. You are correct in that we need to find a temperature independent measure of humidity to use in our calculations. If you know the temperature and relative humidity, you can calculate any number of ways of measuring humidity, including vapor pressure, as you mentioned. I use absolute humidity, but that is just personal preference as I think the units of “grams per cubic meter” gets the idea across to auditors very clearly.

Water vapor is a gas, and we generally assume that because gasses diffuse almost immediately, so the absolute humidity should be constant in any given space, and it shouldn’t matter which sensors we use as reference for this. But better to do a thorough comparison, and include several points to get an average absolute humidity value. This will also show any auditors that your assumption is true that absolute humidity is constant in your space. The last step, is of course, to calculate the Relative Humidity values at the locations that were not monitored with a hygrometer.

Good luck, and let us know how it works out.

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