maybe it's too simple but why don't you put your secondary tank on the table behind it. Just connect the bottom of the sec tank to the top of the primary. By gravity the first one will always be full. Their wouldn't be much energy transfer since the water would go up. The only thing you would have to check is make sure that water won't go back to the bong since the sec tank would be higher than the bong itself.
Also, is your water electricaly conductive? (if it's pure water, it might not be enough conductive for what i think)
if so, you could simply put 4 fire in the gage to see the level, 2 at the bottom and 2 at the top. Each wire would be set appart by around 1/2 inch. On the first one, you put a reverse relay (when current is flowing throught the 2 wire, the relay doesn't let current pass and when the current doesn't, the relay will let current pass so you wire youre pump on it.) then on the 2 wire on the top, you use another reverse relay, when current flows, it stops the pump.
I tried to make an image of what i wanted to show
To explain a little, the circuit is set so the relay on top is always closed (ie current flows) and opens when current flow via the 2 little wire on the top of the level metter.
The bottom relay is always open until current doesn't flow via the 2 little wire.
so the end result. relay 1 (the top one) will always let current flow until the level as reached the 2 top wire. Relay 2 will never let current flow until it goes lower then the 2 bottom wire.
Need for info... ask questions and i will answer it
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