The PS4 is used to sense the static pressure in the supply air duct and maintain the static pressure set point limit that the system will operate at. The PS4 will control an ND or URD damper, to allow an adequate amount of pressure to by-pass from the supply plenum to the return duct or dump zone, and maintain design static pressure and velocity. The PS4 has a range of 0.1" to 4.0" W.C. The PS4 can be used with motor actuators to create model EBD bypass dampers http://www.ewccontrols.com/bypass.htm - 12.3kb