SO, the obvious solution was to put together a system to shut the flow of water off when the bucket is full. That way it will be a simple "set it and forget it" system, and we can keep the bucket on the floor.
This is the circuit we came up with:
(thanks to this bit about latching relay circuits and this bit about how to set up the amplifier/water-controlled-relay part)
It's basically a simple amplifier that trips a latching relay when a very small current passes between the 2 contacts. (and yes, I know I draw and write like a 5th grader)
the solenoid valve we are planning to use is the polycarbonate one at the bottom of this page: mcmaster-carr
I ended up using a switch instead of the NC pushbutton (so the thing is harder to turn on by mistake), and using bc123 and 128 transistors instead of the ones in the drawing, because that's what You Do It had in stock. I built the majority of the circuit the other day and it works, just need to box it up and add the 120v. relay and solenoid valve (we are going to use the 120v solenoid valve instead of the 24v one on the diagram) . And maybe a transformer to power it (it runs off a 9v battery at this point).
One thing I was curious about was if the RODI was going to conduct enough electricity to trip the circuit, but I tested it on a glass of RODI water, and it seemed to work fine. Should have the rest of the system put together maybe by the end of this week...
added some diodes to the relays to protect the transistors. also added a circuit breaker and plugged the whole thing into a GFCI outlet. I ended up using a 120v solenoid, so some extra protection seemed like a good idea. Boxed it all up, added a flashing status led, and so far it works like a charm. Will post a few more pics soon.
UPDATE 2: Pics!
here's the box. the switch is power, the blue button starts the system. The LED flashes while it's running, and the black button is a circuit breaker.
here's the solenoid valve:
and here's the whole setup sitting on a shelf in the basement:
One thing to note is that the control box is zip-tied to a pipe ABOVE all water sources, so hopefully in the event of a drip or leak, no water will enter the box.
still need to make a bracket for the water tube and the shutoff contact wire, but other than that, it's running well!
UPDATE 3: Ok, so this thing has been up and running for close to a year now, and i works great. @ more modifications I did since the last update were one: i added about 3 megohm of resistors to the sensor circuit, to cut the current put through the bucket of water and reduce corrosion on the probes. The other was to use some titanium rod (from www.onlinemetals.com) for probes, again, to limit corrosion, and because it's cooler with titanium bits.
We use it on a day to day basis now, and it just works like a charm, really couldn't be happier. Might add an hour meter some day, to help keep track of the age of the various filter cartridges.