The conductive properties of the liquid complete a circuit between a probe and common when the liquid comes in contact with both. These relays compare the value of the measured resistance between probes with the setpoint of the adjustable potentiometer provided on the product. The output of the relay is used to control pumps, solenoids or valves to lower, raise or maintain the level of the liquid in the tank. Probes are pulsed with a DC voltage to prevent potential electroplating issues. A built-in time delay prevents rapid cycling.