Why are so many different diaphragm diameters offered?

PIC designs each seal to accommodate the largest diaphragm possible thereby affording maximum sensitivity.

Larger diaphragms are more sensitive resulting in more accurate remote seal systems. Larger diaphragms also have lower spring rates that directly effects the measurement accuracy when the ambient or process temperature changes. Lower spring rates less measurement error. The diaphragm diameter also determines the remote seals displacement capability. If the diaphragm it too small changes in ambient temperature acting on long, large diameter capillary lines can cause the fill fluid to contract which in turn will cause the diaphragm to bottom out.  When the diaphragm bottoms out it no longer has the ability to sense process pressure causing erroneous readings.

PIC offers an alternative to using "GAUGE" seals on transmitter applications and compromising the system's sensitivity and accuracy.

A diaphragm seal is an isolation device designed to separate a pressure instrument from the process media while allowing the instrument to sense the process pressure. The diaphragm seal is attached to the instrument directly (i.e. close-coupled) or remotely (i.e. capillary line).

For smaller flanged or threaded diaphragm seals, the basic construction consists of a flexible membrane sandwiched between an upper housing and a lower housing. The lower housing is designed to adapt to the process connection and contain the process medium as it acts against the diaphragm. For larger flanged process connections and hygenic style seals the diaphragm is welded to the upper housing not requiring the use of a lower housing. In all cases the upper housing is designed to attach to the pressure instrument and be filled solid (i.e. without air space) with a fluid which will act as a hydraulic medium.

The pressure is hydraulically transmitted to the sensing element of the pressure instrument. The sensing element moves as a result of the pressure, thereby accommodating some of the fill volume. The diaphragm is displaced an amount equal to the volume accommodated by the sensing element of the instrument. The higher the pressure, the more fluid accommodated by the instrument, resulting in more displacement by the diaphragm. So it is important that the diaphragm seal be capable of displacing at least as much fluid volume as the instrument requires for normal operation.


Diaphragm seals may be applied to most pressure instruments regardless of the sensor technology employed.

Some common sensor technologies include:

  • bourdon tube
  • bellows
  • piezoelectric
  • capacitance
  • strain gauge
  • force balance mechanism

Typical pressure instruments include:

  • gauges
  • transmitters
  • switches
  • controllers
  • recorders

Many process applications necessitate the use of a diaphragm seal to protect the instrument. These may include:

  • Corrosive and/or hot processes that would damage the instrument.
  • Viscous processes that would clog the instrument and restrict or disable the sensor.
  • Processes that might freeze or solidify in the instrument due to ambient temperature effects.
  • Sanitary processes requiring easily cleaned process contact surfaces.

Pressure and Temperature Limits

PIC offers a standard line of diaphragm seals that may operate at pressures from full vacuum to 10,000 PSI (690 bar) and temperatures between -100 to +570 degrees F (-73 to +299 degrees C). Maximum operating pressures are a matter of design and are dependent upon the temperature of operation. Increasing the temperature will lower the maximum operating pressure. All PIC diaphragm seals have maximum operating pressures base rated at 100 degrees F (38 degrees C). Materials of construction will also affect the working pressure and temperature. The maximum operating temperature is limited by the sealed system component with the lowest maximum temperature. The limiting component may be, but is not limited to, any one of the following: housings, diaphragm, gaskets, O-rings, or fill fluid.

Vacuum applications reduce the maximum application temperature of a fill fluid. Special fill preparation and procedures are required for high vacuum systems.