The P74JA Series differential pressure controller is commonly used on water systems to control a pressure differential between supply and return lines. The controller’s SPDT floating action contacts position a modulating valve operated by a motor-driven actuator. In a typical application the valve is located in a bypass line around the system pump. When the system pressure differential increases above the controller’s set point, the valve opens further. More water is diverted through the bypass valve, resulting in a reduction in system pressure differential. A reduction in the pressure differential causes the valve to begin closing and results in an increase in system pressure differential.