Specifying Pressure Ports on the P55 Transducer

The pressure ports are the piping connections on the transducer body that are used to connect the transducer to a pressure source. The standard connection for the P55D is 1/8” female NPT pipe threads. These are standard plumbing connections and adapters may be purchased from a variety of sources to connect to plastic or steel tubing. Swagelok adapter fittings are often used with the P55.

There are two 1/8” NPT ports for the standard P55 – one port for each side of the differential pressure transducer. These are marked + and -. The + port is normally connected to the higher pressure and the – port to the lower pressure in the system to be measured. The transducer so configured will measure the difference between the two pressures. When the – port of a P55D is left open to atmosphere, the + port will measure gage pressure.

The standard P55D also has bleed ports opposite the 1/8” NPT pressure ports. The bleed ports are 8-32 set screws with a small gasket that is used to allow the air inside the sensor cavity to be displaced with liquid. This is useful for making fast dynamic measurements in fluid-filled systems.

An alternate pressure port configuration provides for two 1/8” NPT female threaded ports on
each half of the sensor. In this configuration there is no bleed port but rather a second 1/8”
NPT port. This can be used to connect additional plumbing to the transducer or to flush the  sensor cavity, etc. A pipe plug can also be used to plug off one of the open NPT ports. P55A absolute sensors have a single 5/16” female straight thread port. An adapter is supplied so that a 1/8” male NPT pipe thread interface is provided.

Both the differential and absolute versions can be ordered with a 1” long 1/4” OD tube stub
that can be used to connect directly to a Swagelok fitting. This is useful if threaded connections are not appropriate.