Probably the cheapest and most commonly used Hygrometer, not very accurate but better than nothing I suppose
These type of Hygrometers can be found almost anywhere but if you want my advice don't bother.
I use a model made by Testo the 608-H1,I took it out of the hatcher just to take the picture.
All three were placed together and you can see the difference in the reading
imagine what would happen to the eggs if I used the 2nd one and reduced it to 50%, very dry eggs and probably very few hatching