Page 17 - Instrumentation and Measurement

2.2.4 Piezoelectric Sensor

As shown in figure 2.7, when certain crystals are stretched or compressed, charges appear on their
surfaces. This effect is called piezoelectricity. Examples of such crystals are quartz, tourmaline, and
zirconate-titanate.

FIGURE 2.7 Piezoelectric crystals – crystal structure

A piezoelectric pressure gauge consists essentially of a diaphragm which presses against a
piezoelectric crystal (Figure 2.8). Movement of the diaphragm causes the crystal to be compressed
and so charges produced on its surface. The crystal can be considered to be a capacitor which
becomes charged as a result of the diaphragm movement and so a potential difference appears across
it. The amount of charge produced and hence the potential difference depends on the extent to which
the crystal is compressed and hence is a measure of the displacement of the diaphragm and so the
pressure difference between the two sides of the diaphragm. If the pressure keeps the diaphragm at a
particular displacement, the resulting electrical charge is not maintained but leaks away. Thus, the
sensor is not suitable for static pressure measurements. Typically, such a sensor can be used for
pressures up to about 1000 MPa with a non-linearity error of about ± 1.0% of the full range value.

FIGURE 2.8 Basic form of a piezoelectric sensor.

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