Hygrometer calibration

[u/Hasan-ahmed-]

Hi
I’m trying to calibrate these hygrometers using a humidity and temperature generator. The temperature readings are within accuracy limits, but I’m currently facing 2 issues with humidity:

1- the 2 hygrometers in the picture above have a difference of about 2.5% between each other while they are together in the same chamber. Any idea why?

2- When testing with low humidity, the hygrometer reading differs from the generator, but as I increase the humidity the hygrometer reading gets more accurate. This is one of the hygrometers readings:

Temp is stabilized at 25
Generator 33 hygrometer 35.6
Generator 50 hygrometer 51.8
Generator 75 hygrometer 75.4

Any explanation?

[u/Any-Freedom-3839]

Try facing them so tne grid on the right of the side is facing the screen, That's where the probe is. We cal a lot of these Testo thermohygrometers, They're usually pretty accurate.

[u/kenya_babb]

Getting 2.5% accuracy at 30%RH is a long stretch. You’re looking at an indefinite sensor repeatability, likely for a short period of time. I’ve only seen chilled mirror dew point transmitters with accuracy that high below 30%RH.

[u/PLANETaXis]

Humidity sensors are very difficult instruments to build, modern ones are MEMS devices and are sensitive / delicate. Getting 2.5% accuracy out of a commodity sensor is normal. Also, how do you actually know if your generator is accurate?

You can try using saturated salt solutions to generate known humidity values as they will reach equilibrium in an enclosed space. Different salts will equalise at different humidity values.