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.