I'll respectfully offer a dissenting opinion. I often leave powder in my Lee auto-disc measure for days at a time without a problem. In fact I'm sure I have left powder in there for a week or two at times. its not unusual for me to be loading and shooting mostly one caliber at a time...load 150 rounds, shoot 100, etc. Eventually I run out of cases to load and switch to a different caliber. It's got nothing to do with neatness or safety...I still exercise the precautions of having only one powder, one primer type, and one bullet type on the bench at one time, and I've never had any issues with throw weight or caking. I was loading .38 target rounds Saturday and was throwing 3.0 grains of BE and when I go back to the bench tonight or tomorrow it will be throwing 3.0 grains. I keep the lid on the hopper but its not a particularly tight fit.