How to reliably measure humidity?

Maybe take a look at the WL1030 from Technoline, recommended according to tests; we have it and are satisfied. According to tests, the inexpensive Hama device is also recommended.

So one might think that the one from TFA is the one that most likely shows the correct data? However, those from the company Thlevel also mostly receive good reviews. And compared to TFA, there is about an 8% difference.

I have now tried the thing with the damp towel with all three. In doing so, 1) and 2) were each at 89% and 3) at 81% after 30 minutes. Now, 2 hours later, all are back to their usual separation. Very strange.

It has to be really soaking wet to reach 100%.

Well, I had a thermo/hygrometer that was analog. You could calibrate it like that and they are also very accurate because they work with human hair.

With yours, I would suspect that 1 and 2 are probably more accurate and 3 is actually an outlier.