Originally Posted by Brutus2600

With the cup of water in there you'll see a RH spike fairly quick, but if you removed that cup it would drop down again. Leaving it in there for a week or two allows the humidor to absorb that water and once the cup is removed it should maintain a consistent RH.

Also, if you're curious if your hygrometer is still any good or how accurate it is, just do a salt test with it