Diaphragm Seals for Pressure Sensors

April 10, 2008
When measuring pressure, it is sometimes necessary to isolate the sensing element from the material being sensed.

For example, sensing mechanisms can be damaged or destroyed by corrosive fluids or debris can clog a sensor port. For these and other situations a diaphragm seal protects the sensing element by creating a barrier between the pressure sensor and the media it’s measuring.

A diaphragm seal is composed of three main parts: the housing, an isolating fill fluid, and the diaphragm itself. The housing contains the process and sensor connections for the diaphragm and holds the fill fluid. The fill fluid is application specific, thus it varies for food and beverage or industrial applications. The diaphragm is a metal plate approximately 3-mm thick that separates the fill fluid from the process material. The pressure transfers from the line or tank through the diaphragm seal to the fill fluid and finally to the sensor’s measuring element. The measuring element is thus completely isolated from the material whose pressure is being measured.

A diaphragm seal is made from materials that are more chemically resistant to the process fluid under pressure. So the actual material will vary with the application. For example, 316 L stainless steel, Hastelloy C276, or Inconel are the materials most commonly used in chemical applications.

A flush-diaphragm seal is used to prevent particles from clogging the small measuring port of a standard sensor. They’re also used with glue, wastewater, resins, paint, and other materials that typically clog standard sensors. And they make a good option in areas where water may freeze and expand in the sensor port.

Finally, diaphragm seals are a good choice in applications where different process connections are necessary or desired. For example, the food and beverage industry demands specific connections such as 3A-rated fittings. The 3A standard was created by the dairy industry as a voluntary benchmark for product performance and sanitary safety. Connector threads on a standard sensor provide a space where bacteria could grow. A diaphragm seal with a triclamp fitting is a 3A-rated sanitary process connection because it has no thread to harbor bacteria.

Turck Inc. (turck.com) supplied information for this column.

Pressure-diaphragm seals isolate pressure sensors from process fluids. A metal diaphragm transfers process pressure to the sensor via an isolating fluid to prevent any process material from reaching and damaging the sensor.

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