Quote Originally Posted by mike_biddell View Post
Refining the optical mouse idea, it is much simpler to use a strip of textured plastic about 1/2 inches wide, 1/10 inch thick and an appropriate length. Attach this to a small polystyrene float. Mount the mouse circuitry in a static position at the top of the tank, so that the float strip moves paste the mouse window (feed the strip thru a slot by the mouse read window), as the depth in the tank increases. This would definitely work and give excellent Z resolution at a very cheap price.
the only downside is you have to have as much clear space above your build reservoir as the actual depth of the reservoir so the gauge can have full travel.