Abstract: A pump operable in response to the level of the liquid to be pumped comprising an impeller, a motor for rotating the impeller, a discharge passage for receiving the liquid pumped by the impeller, and a sensing chamber communicating with the discharge passage between the impeller and the pump outlet so that liquid in the discharge passage can trap gas in the sensing chamber. A pump controller is responsive to the pressure of the trapped gas in the sensing chamber for energizing and di-energizing the motor. Energizing of the motor to cause the impeller to pump liquid into the discharge passage substantially increases the pressure of the trapped gas to thereby assure that the pump remains on.