Hate to burst anyone's perfect hygrometer bubble... but I've heard that the hygrometers that are commercially available aren't really that accurate anyway.... kind of like how one thermometer varies from another... I would quit worrying so much about what a number says and start looking at your animals or your eggs themselves, look at the enclosures and the egg boxes... take note of condensation.

That being said I use more than one thermometer, even more than two... and compare them to each other and guess what the actual temperature is. It's really likely that a ton of people that are hatching eggs out there at a couple degrees higher or lower than they think just because of their thermometer...