A diaphragm seal is a flexible membrane that seals and isolates an enclosure. The flexible nature of this seal allows pressure effects to cross the barrier but not the material being contained. Common uses for diaphragm seals are to protect pressure sensors from the fluid whose pressure is being measured.

Since diaphragm seals need to be highly flexible, elastomers are commonly used, and include a wide variety of both general purpose and speciality rubbers. Elastomers are limited to low pressure applications and those that are chemically compatible with the various plastics and rubbers used.

Depending on the PSI levels of the sealing application, the diaphragm may require fabric reinforcement. Typically, PSI under 5 does not require fabric reinforcement. A PSI between 5 and 10 is application dependent. Anything above 10 almost always needs fabric reinforcement.