Abstract: The present invention is directed to apparatus for sensing high and low liquid level for the purpose of controlling a circuit. As liquid rises, gas within a first communicating line to a differential pressure switch is relieved through a check valve to atmosphere. When the liquid rises sufficiently, the pressure within a second communicating line causes closure of the switch and the circuit. As the level of liquid recedes, the check valve closes and a vacuum develops in the first connecting line so that the switch and circuit remain closed until the liquid level is below the entry to the first communicating line. At that time, the liquid within the communicating line exits and the pressures are equalized to the switch which then opens thereby opening the circuit.

Abstract: An hydraulic switch (10) for buoyingly controlling a secondary electrical switch (74) is disclosed. A float (68) is retained to move between a first position spaced apart from a vent opening (64) and a second position in sealing contact with vent opening (64). The electrical switch (74) changes state from an open condition to a closed condition as the float (68) approaches its second position.

Abstract: An hydraulic switch (10) and method for controlling the starting and stopping of a pump is disclosed. Switch (10) has an aperture (34) in the base (32) of a reservoir (28). Liquid enters aperture (34) thereby raising float (38) which operates switching mechanism (26) to start pump motor (20). As liquid is pumped passed impeller (22) along flow path A, some of the liquid is forced into conduit (36) leading to reservoir (28). The liquid flowing through conduit (36) keeps reservoir (28) filled. Consequently, float (38) remains in an elevated position keeping switch mechanism (26) on. Float (38) does not descend until liquid ceases to be pumped and stops flowing through conduit (36). Then, as liquid drains from reservoir (28) through aperture (34), float (38) drops thereby causing switching mechanism (26) to switch motor (20) off.