Diaphragm Seals for Pressure Sensors
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.