Hydroponics:  Irrigated Gravel – How It Works 
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How it Works – 1
How it Works – 2
The controls time the interval between and the length of each pump cycle.
The time is up and the pump starts.
How it Works – 3
How it Works – 4
The pump floods the gravel until it overflows.
The pump turns off and the nutrient solution slowly drains back into the holding tank.