NS210 / NS211 Series NCC National Controls Corp Liquid Level Boards
Assuming an empty tank, upon application of voltage to the system, the fill output is turned on and the heater output is off. As the liquid fills the tank, and when it reacdhes the low probe (above heater), the heater output is turned on. The level will rise until it reaches the high probe causing the fill output to turn off. As liquid is removed from the tank, 2 to 4 seconds after it breaks contact with the high probe, the fill output is turned on and stays on until the liquid reaches the high probe. This time delay eliminates fill output chatter if the liquid is splashing.
If the tank is non-conductive, a third (common) probe must be installed at a level where it will alway be submerged.