I have a number of non-mechanical humidistats that I have collected over the years. They are basically the same coin sized disks that are installed in violin cases. Some of them stand alone on feet and others are designed to stick on a surface with adhesive. One of my favorites is actually from the days when my kids kept lizards. I particularly enjoy checking that one since it has an image of a gecko on the face and reminds me of the days when my kids were small.

In an effort to modernize I picked up a cheap digital ThermoPro on Amazon. I stuck it in my flute cupboard and was surprised that the humidity had risen to 65%, So I took out my low tech water glass with wet sponge arrangement and cracked open the door. A day later I was aghast it was reading 27%.

I took one of my trusty disks from another cupboard and set it in there. After a few hours I went back to check and it read my more typical 50%.

Whew!

And beware dying ones. Sometimes old measurement equipment just suddenly ups and dies. That's probably the best thing it can do. But sometimes it just wanders off, all by itself, showing increasingly inaccurate measurements until suddenly you realise "that can't possibly be right"!

So wise to check all measurement devices on receipt and from time-to-time. If you're not relying on extreme precision, then probably enough to put it outside (but in the shade) and compare its reading to your local weather station's. Or compare it to known good one (but again be aware that that one might have degraded since calibration).

But if you need more precision, check out the web for articles on how to calibrate a hygrometer.