I see a lot of people use the inexpensive reptile aquarium hygrometers, like these:
And I have personally used these Chinese ones that are probably most often used in restaurant food storage, but could also be used for reptiles or our purposes:
These might need to be “calibrated” or adjusted by comparing them to a trusted digital hygrometer.
I had to calibrate it for the Chinese cheapoes and I wouldn’t be surprised if the inexpensive reptile one, that are probably made in China also, might also need a bit of an adjustment.
They are easily “calibrated” by carefully accessing the back. There is usually a hole in the middle that gives you access to to a metal frame that has a pair of holes in it and this would normally be for use with the appropriate tool. And if you have one, by all means use it. And if not, I’m sure you can find a pick or something appropriate that can then turn the “adjustment”. This basically turns the box or frame – that the hygrometer coil is housed in – be careful not to touch or damage the coil spring the “dial indicator needle” is attached to – when turning this “box” to adjust the needle to then be pointing at the correct reading on the dial.
You can even get a combo that reads temperature as well and these might be better as far as coming properly calibrated: