What causes Hydrometers to Un-Calibrate?

[shilala]

Originally Posted by RWhisenand

Entropy, it's not what it used to be.

Nerd joke!!!

I should add that I use "my hygrometer says something stupid" as battery changing time. Much like the smoke detector beeping.
Rob explained it perfectly. I can add a little nugget to that...
The Hygroset II's that most of us use have sticky crap on the board that reads the RH. Get a bead stuck on there and it'll read any kind of bizarro stuff it wants to. I regularly look in the grill and see if anything is stuck to the board. Most guys probably won't have that problem, but it happens. To me, it happens a lot.