pneumatic pressure switch testing and repair
pneumatic pressure switch is a three-way block and bleed valve that operates by pressure acting on a piston opposing and adjustable spring force. It functions as either a normally closed or normally open valve depending on the two ports through which the instrument pressure is supplied. The output port, marked "OUT," remains the same for both modes of operation.
When the instrument supply pressure is connected to the "HI IN" port, the switch operates as a normally open valve and is called a high switch. If sense pressure acting on the piston is at normal levels, it is insufficient to overcome the spring force. The middle O-ring on the spool is between the "HI IN" and "OUT" ports, preventing the instrument pressure from passing to the "HI IN" port but allowing it to flow from the "HI IN" port to the "OUT" port. When the pressure acting on the piston becomes large enough to overcome the spring force, the spool shifts and the center O-ring moves between the "HI IN" port and the "OUT" port. Instrument pressure at the "OUT" port and downstream "BLEEDS" to atmosphere through the "LO IN" port while supply pressure at the "HI IN" port is "BLOCKED" from entering the "OUT" port.
To change the sense pressure value when the switch is used as either a high or low, alter the force of the spring, the size of the sense piston diameter, or both. Changes to spring force can be made by turning the spring cap to increase or decrease spring compression, while piston diameter can be changed by adding or removing o-rings or changing the piston.
