Measuring pressure at high temperature

01 June 2006

MEMS technology with intelligent packaging makes this instrument work under extremely adverse conditions.

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 300ºC 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.

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.

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 patents.

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.

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. 3; 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.

The resulting sensor package has impressive specifications: it measures pressures from 100 to 1000 bar at operating temperatures up to 350ºC, 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.

Enquiry Code J242

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