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

1. 67%

2. 82%

3.54.2%

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