For a long time I've been lusting after a form of water replenishment that doesn't require overflows. Something that would provide not only top-off, but could eliminate large water changes. For instance, if my math is right, if I had something that could automatically drain then add just 1 gallon to my 75g, 7 times a day, I would be turning over water at the same rate as a weekly 50% water change - no more water changes! And auto top-off just happens as a byproduct.
Great idea IMO - but apparently tough to accomplish without some kind of overflow.
When I set up my current tank, I drilled bulkheads in the bottom, and left one unused to help with this problem. I went and bought a couple of these pressure switches
to help. The idea was easy...
Leave a bulkhead open, connected to the pressure switch. The switch would be configured in a normally-open configuration, and would only close when the pressure (water level) was below the desired height (pressure). Then it's just a matter of putting a normally closed solenoid in-line, and on a timer to drain water returning to the tank for a few minutes. When that's done, have another timer power up the pressure switch, which would detect a low pressure condition, and would turn on a pump from a reservoir until the appropriate water level switched it off.
I bought two of the pressure switches so I could put them in series, so that if either of them thought the water was high enough, the new water shuts off. Redundance seemed like a good idea.
I've started testing these switches, and they do a pretty good job. In a test setup, over many trials, their variance in shutting off at a consistent water height is about 1/4". Good enough if for me, if used redundantly.
I'm writing because I've run into a snag though...
The pressure switches detect a "full" condition very well. The problem is state change. Once they detect the "full" condition, they open the circuit, and the circuit remains open until the pressure drops much, much lower. In my case, they won't close again until half the water is out of the tank, and then they faithfully turn on until they are within 1/4" (+/-) of the desired water height.
Bummer. So much for losing a gallon at a time, and letting these things control the top off.
I've come up with a solution, but it's so complicated, I'm not even going to post it. Too prone to failure. Any ideas? I'm stumped.