Measuring pressure at high temperatures

Pressure measurement is a well understood technology in the
manufacturing industries. It is routinely performed by a variety
of robust, reliable instruments. However, there's one application
area where nearly all pressure sensors fail: in high temperature
zones, at 300°C for example. This requires special treatment.

Such high temperatures are encountered in the plastics industry,
for example, where it is necessary to measure the melt pressure
on extruder machines. However, with the increasing sophistication
of batch manufacturing in the food and pharmaceutical industries,
the need for this special type of pressure instrument is broadening
into other markets.

Designed with silicon oxide to withstand
high temperature, Gefran's MEMS sensor
uses piezoresistors connected to each
other in a Wheatstone bridge.

Until now, the best way to measure pressure at high temperature
is to transmit the pressure from the fluid being measured to the
sensor, located some distance away from the heat, by means of
a fluid transmission line. One end of the transmission line is covered
by a thin membrane and inserted into the fluid where the pressure
is to be measured. At the other end, at a comfortable distance from
the heat, is a standard pressure sensor. The transmission line is filled
with a medium that is as inelastic and temperature independent as
possible; this is usually mercury or special types of oil.

These 'melt pressure' sensors have been used for many years. Usually
regarded as commodity items, they are manufactured by a number
of different companies, and they do their work very well. But, they
have two major drawbacks: first, using mercury as a transmission
fluid is considered environmentally unsound and governmental agencies
have demanded that the practice be discontinued. Secondly, the thin
membrane (which is only about 0.1 mm thick) separating the transmission
fluid from the process fluid is prone to rupture. This is caused by the
abrasion due to charged polymers on the membrand. Newer coatings
have made it less vulnerable to failure, and this has improved its
performance. Still, 90% of melt pressure sensor failures are due to
the collapse of the membrane.

Gefran's 'Impact' melt pressure sensor is accurate even at 350 Deg.C.

After years of manufacturing melt pressure sensors, Gefran engineers
thought they could greatly improve the design, and have spent several
years creating a new instrument called 'Impact.'

Impact is radically different from the fluid transmission type of sensors.
The new design requires, in the manufacturing process, extensive use
of lasers and special alloys and the coupling of different materials like
steel and ceramics. In creating the new design, Gefran generated four

A major design commitment was a new and highly sensitive monolithic
piezoresistive sensor, made with MEMS technology. The square silicon
chip contains both the membrane and sensitive element. It is shown
in Fig. 1 and mounted in its carrier on the front of the cylinder in Fig. 2.
The new sensor is so sensitive its maximum deflection is on the order
of one ten-thousandth of a millimetre.



Fig.1: The chip mounted on its carrier

They also designed a much thicker membrane (1.5 mm) to come into
contact with the process fluid, but instead of transmitting the pressure
value by a liquid such as oil or mercury, a solid 'push rod' was designed
to do the job. The membrane and push rod are indicated in red in Fig. 2;
the sensor is mounted just behind the pushrod and connected to it with
a special connector so that the two may be separated during the
installation phases on the machine.

Fig.2: The process contact membrane and push rod are shown in red.

The resulting sensor package has impressive specifications: it measures
pressures from 100 to 1000 bar at operating temperatures up to 350°C,
with a degree of accuracy of 0.25% full scale.

The greater thickness of the membrane-it is 10 to 15 times thicker than
membranes on previous instruments-is the key to the long life of Impact.
There are no longer any concerns about the wear and tear on the
membrane due to charged polymers. It was such an improvement, Gefran
engineers decided they no longer needed to use special coatings.

We acknowledge Gefran for this article...


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