Low/High Supervisory Pressure Switch for Low Differential Dry and Preaction Valves - Nominal System Pressure 15 psi. The Potter PS15 Series Supervisory Pressure Actuated Switches are designed primarily to detect an increase and/or decrease from normal system pressure in automatic fire sprinkler systems. Typical applications are: air/nitrogen supervision in Dry pipe and pre-action systems, pressure tanks, air supplies, and water supplies. The PS15 has two switches.