Chemical Engineering April 2019 - 36

Cover Story
Endress+Hauser
small changes in pressure. As a result, these
diaphragms can easily be damaged or even
ruptured due to overpressure. In addition,
overpressure or repeated cycling of process
pressure can cause the diaphragm to deform,
resulting in a shift in the measurement.
It's important to remember that this can happen
even at relatively low overpressures due
to repeated cycling of pressure, even within
what may be considered acceptable limits.
FIGURE 4. This remote seal
protects the fill fluid from high
temperatures
FIGURE 5. This ceramic sensor
can withstand three times
the overpressure of an oilfilled
sensor
without the possibility of hysteresis errors (the
difference in output at any measurement point
when approaching the point first with increasing
and then with decreasing pressure) or a
significant risk of failure.
To understand what steps can be taken
by manufacturers, it's important to consider
the two most common types of pressure instrument
sensors on the market: strain-gage
sensor typically used for differential pressure
measurement and ceramic sensors typically
used for static pressure measurement.
Strain-gage sensors come in a variety of
types such as piezoresistive, resonant frequency
or capacitance styles. Regardless of
the sensor type, there are two things common
to strain-gage instruments: metal process
diaphragms and fill fluid.
Process diaphragms prevent direct contact
of process fluid with the sensor. This
can be accomplished both with either internal
process diaphragms or remote seal diaphragms
(Figure 3), each of which can also
protect the entire instrument from elevated
temperatures or aggressive chemicals.
These measuring diaphragms can be
made from a variety of metals, but all are extremely
thin to allow for greater sensitivity to
Endress+Hauser
Expanding fill fluids
Hydraulic fill fluids are non-compressible
and are used to transfer the pressure from
the process diaphragm to the instrument's
sensor. There are a wide variety of fill fluids
that can be used for different applications.
The most common types are silicone oils,
which come in grades to handle different
process temperatures.
In addition, there are inert oils for applications
where silicone is not acceptable, and
food-grade fluids for use in food and beverage
or pharmaceutical applications. All fill
fluids can expand during upset conditions,
such as elevated process or ambient temperatures
or pressures.
This expansion can cause significant overpressures
on both the process diaphragm
and the sensor, which can result in measurement
shifts or irreparable damage to the diaphragm
or sensor.
There are several ways for manufacturers
to design oil-filled pressure instruments to
better withstand overpressure events. These
include taking steps to minimize the amount
of fill fluid in the instrument, and then designing
mechanical systems to absorb the force
of the overpressure and keep it from reaching
the instrument's sensor.
As previously mentioned, fill fluid expands
and contracts according to the process and
ambient temperatures of the application. The
best way, therefore, to minimize the expansion
effects of the fill fluid is to use as little fill
fluid as possible. This requires careful design
of the instrument to minimize the volume of
fill fluid while still having enough fill fluid to
maintain the sensitivity of the measurement.
One way this can be accomplished is by
minimizing the space between the process
diaphragm and the sensor body. Many manufacturers
machine the sensor body to match
the pattern pressed into the diaphragm.
Some even go the added step of hydroforming
the process diaphragm against the machined
sensor body to ensure an exact match
between the diaphragm and the sensor body,
resulting in the smallest possible volume of fill
36
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Chemical Engineering April 2019

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