I have four presses. Only one of them has ever seen powder stay in the powder measure past midnight the day I poured it in. It's a MEC 600-Jr that I keep "set up" for my boys and I to load 7/8-oz 20-ga loads using Unique.

It's the only powder I have ever used for 20-ga and that's the way it will stay so "no worry about what is in it". There's a hood over the powder bottle to prevent light and also make any temperature changes very minimal.

Everything else gets emptied when I am done for the day.