Diaphragm seals (also called chemical seals, instrument isolators, and gauge protectors) are isolation devices used to separate pressure instruments from the process media, while allowing the instruments to sense the process pressure. A diaphragm, or bladder, together with a fill fluid, transmits pressure from the process medium to the pressure element assembly of the instrument. Diaphragm seals may be attached directly to the instrument, or remotely, using a capillary line. These devices are intended to keep the process medium out of the pressure element assembly and to prevent damage from corrosion or clogging.  Diaphragm seals are also used to maintain the sanitary requirements of the process medium and to reduce the process temperature to which the pressure element is exposed. Diaphragm seals function using a flexible diaphragm that is attached between the upper and lower housing of the seal. The upper housing is filled with a fluid and attached to the pressure instrument. The lower housing is in contact with the process medium and has the connection to the process system. The process media pressure is transmitted via the diaphragm to the fill fluid in the upper housing and subsequently to the pressure instrument.

There are three common diaphragm seal designs, although custom and specialized varieties are also available.

Welded diaphragm seals are welded (or bonded) directly to the upper housing, permitting removal of the upper housing without a loss of fill fluid.

Clamped diaphragm seals are clamped between the upper and lower housing.  This style of diaphragm may be removed and replaced easily, as it is not welded into place.

Capsule diaphragm seals are internally sealed devices with the fill fluid within the capsule in the upper housing. The main advantage of this type of seal is that the upper housing can be removed without loss of the fill fluid, because the fill fluid is contained within the seal.

When specifying diaphragm seals, there are a number of important points to consider.  These include the type of process fluid and how it will react with the seal materials, the vapor pressure point of the fill, the flexibility of the diaphragm, and that the fill fluid is chemically compatible with the process fluid.  Fill fluids must be chemically compatible with the process fluid to avoid potentially hazardous conditions in the remote possibility of a diaphragm rupture.

Vacuum, compound and low range pressure devices require a highly transmissive, flexible diaphragm to give accurate results.  An important consideration is the vapor pressure point of the diaphragm seals’ fill for a vacuum application. If the combination of pressure and temperature reach the vapor pressure point accuracy will suffer.