Hydrometer is a word, just not the one you're looking for... I that I was looking for. I think I might have made the same mistake...

Hydrometer | Define Hydrometer at Dictionary.com
Hygrometer | Define Hygrometer at Dictionary.com

I don't feel the need to consistently monitor the humidity with a hygrometer/hydrometer, but I have one, so I might as well keep in one of my two cages. Why not?

I won't use tape, glad to know.