How can I keep a smaller water reservoir's water level at half available when being fed from a larger reservoir? I'm trying to create my own ultrasonic humidifier. I ordered the misting part which works great but it only functions correctly in shallow water. So I'd like to feed from a large water reservoir to a smaller one. My question is how can I fill the smaller reservoir to a desired water level? Will I have to use a closing/opening valve or is there a simpler way? (I was thinking a small balloon hooked up to a pulley that opens and closes a latch much like a toilet but I am trying to avoid complexity.) 

 A: I recently saw an auto pet waterer.   The intent of the device is to keep the same water level as the pet drinks the water.  This is accomplished with basically a bottle of water turned upside down and the top of it submerged under the water level.  If the water level falls below the top of the water bottle, then air bubbles make their way up to the top of the bottle, exchanging air for water and keeping the level the same.
This is a very simple solution, and this type of approach may be appropriate for what you're trying to do, but it differs from the other solutions proposed.  There are some drawbacks:


*

*The pressure of the water reservoir has to adjust to accommodate its level

*The device can only make up for lost water - if you add water the level won't remain the same


If you're building a humidifier I doubt the second point would be a problem.  You're only going to be removing water, right?  The first point may actually be more troublesome.  For one, you refilling it isn't trivial.  You need to actually close up the water reservoir, turn it rightside up again, then fill it.  If you just opened a plug, then it would all fall out and make a big mess.
A: I don't think it's possible without moving parts, but here's a way to improvise a valve that would probably work:

When the water level gets too high, the ping-pong ball floats up and blocks the inlet. The main problem with this approach would probably be getting a good seal between the ping-pong ball and the end of the pipe - you'd probably need some kind of rubber thing on the end of the pipe, but maybe you can find something suitable in a hardware store.
A: The possibilities are endless, a couple more options follow...
If you simply connect both reservoirs with some elastic pipe near their bottoms, the water level will be the same in both. So you will need that either the large reservoir rises as it empties, or the smaller one lowers. The former can easily be achieved hanging the large reservoir from apropriate springs. The latter could be easily made to work by having the whole small reservoir float inside the larger one.
