Great idea! But a question: As the level drops, doesn't the outlet valve slowly close which would reduce the flow? To test this, try it again, but this time, use something to hold the outlet float up.mrafal said:One more data point that might be helpful. I set the flow meter to 3, shut off the inlet valve and timed how long it took for the level to drop a half inch. By this crude measure, I get a flow rate of about a 1/3 gallon/minute or 20 gallons an hour.