What good is a 75% humidity pack for?

Originally Posted by Cigarbud

I would have to agree and say that a good use for it would be calibrating a new hygrometer. Open it, put it in a ziplock bag with the new hygrometer and let it sit for a half day sealed. Calibrate it off of that. Works a lot better than the old salt test.

Keep it half-open? Why not completely sealed?