The piezoelectric pressure sensor is based on the piezoelectric principle discovered by Jacques and Pierre Curie in 1880. They found that the surfaces of certain crystals – including quartz – become electrically charged when the crystal is subjected to mechanical stress. This charge is exactly proportional to the force acting on the crystal and is measured in picocoulombs (1 pc = 10^-12 coulomb).

As active devices, piezoelectric pressure sensors can only measure quasistatic as opposed to truly static processes. This type of pressure sensor is therefore ideal for dynamic measurement. It is used wherever rapid changes in pressure at temperatures up to 400 °C have to be measured and recorded as accurately as possible. An uncooled pressure sensor generally uses quartz or other crystals developed and grown by Kistler. These PiezoStar^® crystals are particularly sensitive and temperature stable.

The Piezoresistive Pressure Sensor

The piezoresistive pressure sensor builds on the semiconductor effect first described by Charles S. Smith (Piezoresistive Effect in Germanium and Silicon) in 1954. Under mechanical stress, the change in the electrical resistance of semiconductors is up to two orders of magnitude greater than in metals. This type of sensor therefore opened up completely new applications compared with the metal strain gage methods of the time. Since then, other similar techniques have been developed, such as thin film on metal and thick layer on ceramic.

Typical Pressure Sensor Applications:

• Dynamic measurement of cylinder pressure and fuel injection pressure in the development of internal combustion engines
• Continuous monitoring of cylinder pressures in large diesel engines
• Measurement of explosion pressures in the development of air bags
• Control of injection molding cavity pressure
  in order to maximize quality and minimize rejection