I use the Humistat #3 (
https://www.humistat.com/product/hum...model-3-black/ ) to humidify and/or maintain humidity. I have never been in a situation where I needed to de-humidify. Seems the cap on the fill area keeps the water from just flowing through. Sort of like power draw on electrical items - if the device is consuming power, than it's flowing through; if it's off (no need), no power use occurs.
The Humistat is pretty popular with violin users. I also use the small, cylindrical Humistat #1 for flutes. If I have a number of flutes in the same (stationary) box, I have used the beads w/a piece of sponge in a small plastic case (similar to the soap dish type) with small holes in the top.