These have to be the least accurate things I have ever seen.
The rectangular one is accurate or accurate enough and has been what I used but I noticed files all had cutouts for these round hygrometers…
Well from my 6 pack 1 is within a margin of error to even be useful.
I get they aren’t expensive but seems like a waste of money for this bad.
The old horsehair types are pretty decent though not precision meters. The numbers will be completely off unless calibrated, but you can make multiple meters agree. Either way, you can see which days they take a massive dive.
Much the same as those bimetallic thermometers.