The three most common pressure sensor principles described below include metal thin-film and ceramic thick-film pressure sensors, which are the two most common implementations of resistive pressure measurement. The third pressure sensor principle is the piezo-resistive pressure sensor. The significant differences between them result from the different materials used and their functional properties.

1 - Metal thin-film pressure sensor

The main body and the diaphragm of metal thin-film pressure sensors are usually made of stainless steel. The pressure sensor is manufactured using automatic precision lathes and then grinding, polishing and then lapping the diaphragm surface. The pressure sensor is applied to the side of the diaphragm not in contact with the media. Insulating layers, strain gauges, compensating resistors and conducting paths are applied using a combination of chemical (CVD) and physical (PVD) vapor deposition processes, and are photo-lithographically structured using chemical etch procedures. These processes take place in specially designed manufacturing plants under clean room conditions. Some production steps require a vacuum or an inert atmosphere to support the production of high purity atomic structures. The resistors and electrical conducting paths applied to the pressure sensor are significantly smaller than a micrometer and are known as thin-film resistors.

The metal thin-film pressure sensor is extremely stable as a result of the materials and manufacturing process used. It is highly resistant to shock, vibration and dynamic pressure changes. Since pressure sensor materials can withstand high temperatures, the pressure sensor is usually welded to the pressure connection providing a hermetically sealed measurement cell that does not require any additional sealing materials. The physical property of the steel provides a pressure sensor with a relatively low overpressure range and a very high burst pressure.

2 - Ceramic thick-film pressure sensor

The body and diaphragm of ceramic thick-film pressure sensors are made of ceramic.  Aluminum oxide (Al2O3) is widely used due to its high stability and ease of manufacture. The four strain gauges are applied as a thick-film paste in a screen-printing process on the back of the diaphragm. This side does not come in contact with the pressure medium. The strain gauges are burned into the ceramic at high temperatures, and then passivated by applying a protective coating. Manufacturing is usually performed in a clean room because impurities during the screen-printing and the burn-in process will dramatically reduce yield.  Only major manufacturers are able to operate their plants with the proper cleanliness levels in order to avoid any cross-contamination and maintain the required high process stability.

The ceramic used for the pressure sensor is highly corrosion-resistant. However, installation of the pressure sensor into the pressure measuring instrument case requires an additional seal for the pressure connection, which will not be resistant to all possible media.  In addition, the ceramic is brittle and the burst pressure is lower in comparison to a metal thin-film pressure sensor.

3 - Piezo-resistive pressure sensor

A piezo-resistive pressure sensor has a far more complex structure than the metal thin-film pressure sensor or ceramic thick-film pressure sensor. The pressure sensor element is made of a silicon chip which deflects under pressure and consists of a diaphragm, structured with piezo-resistive resistors. The chip has a surface area of only a few square millimeters and is much smaller than the diaphragms of metal thin-film or ceramic thick-film pressure sensors.

The piezo chip is susceptible to environmental influences and in most cases must be hermetically encapsulated.  For this reason, it is installed in a stainless-steel case that is sealed using a thin flat stainless-steel diaphragm. The free volume between the piezo chip and the external diaphragm is filled with synthetic oil.  In an encapsulated piezo-resistive pressure sensor, the pressure medium is only in contact with the external stainless-steel diaphragm, which transmits the pressure through the oil to the internal chip. To reduce the influence of thermal expansion of the fill liquid on the pressure measurement, the pressure sensor design must minimize the volume of liquid used. Special displacement disks and a corrugated diaphragm with matching bed are used for this purpose.

A header is normally used for the mounting and electrical connection of the pressure sensor chip. It has integrated glass-to-metal seals for the electrical connection between the inner and outer chambers and can be hermetically welded to the case. The pressure sensor element, glued to the rear side of the header, is connected to the pins using gold bonding wires and transmits electrical signals from the pressure sensor element to the connected electronics in the external chamber of the pressure sensor. A vent tube, which leads to the rear side of the pressure sensor diaphragm, is located in the center of the header. If the chamber behind the pressure sensor element is evacuated and the ventilation tube is sealed, then it is possible to use a piezo-resistive sensor to measure absolute pressure, since the vacuum of the hollow space serves as an absolute pressure reference.

In pressure sensors designed for gauge pressure measurement, the vent tube remains open and ensures continuous venting to the rear side of the diaphragm, so that the measurement is always performed relative to the local atmospheric pressure. The venting is realized either through the outer case or via a ventilated cable to the outside. This vent tube must be carefully protected against contamination, especially moisture ingress, since the pressure sensor may be damaged.

Pressure Sensor Principles by Comparison

There is no ideal pressure sensor technology for all applications, since each has specific advantages and disadvantages. The pressure sensor design that is most suitable for an application is primarily determined by the demands of the application.  It is not just the pressure sensor technology that is critical for the suitability of the pressure sensor, but above all the practicalities of its implementation in a complete pressure transmitter design.  Depending on the application, the pressure sensor principles described might make implementation either easier or more difficult.

The material in contact with the wetted parts and its suitability for certain media are of fundamental importance. One of the disadvantages of the ceramic thick-film pressure sensor, in comparison with the metal thin-film pressure sensor, is that it requires additional sealing between the non-metallic diaphragm material and the case. This almost always prevents universal applicability due to differences in media compatibility.

The product diversity of pressure sensor manufacturers are usually tailored and optimized to fit different applications.  Only universal instruments allow the users themselves to select the suitable pressure sensor principle. Leading pressure sensor manufacturers offer technical application support for this purpose.

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Excerpted from Electronic Pressure Measurement, Basics, Applications and Instrument Selection; Süddeutscher Verlag onpact GmbH; Munich 2010