Page 150 - Instrumentation Reference Book 3E

134 Measurement of pressure

Crystal structure (simplified) Crystal structure (simplified)
Unloaded Loaded Unloaded Loaded
'4
r y -------_-_

Figure 9.22 Principle of transverse piezoelectric effect. ++-++Ft ++++
Courtesy, Kistler Instruments Ltd.

Figure 9.24 Charge amplifier associated with
piezoelectric effect sensor.

jected to mechanical stress. Strain gauges, as
described in Chapter 4, involve this phenomenon,
but the particular characteristics of silicon allow
construction of a thin diaphragm that can be
deflected by an applied pressure and can have
resistors diffused into it to provide a means for
Figure 9.23 Principle of longitudinal piezoelectric effect. sensing the deflection. An example of this is the
Courtesy, Kistler Instruments Ltd. Kistler 4000 series as shown in Figure 9.25 for
which the pressure-sensing element is shown in
quartz transducer is virtually completely compen- Figure 9.26.
sated. It can be shown that the output voltage Because the stress varies across the diaphragm,
from the amplifier is -Q/C, where Q is the charge four pairs of resistors are diffused into a wafer
generated by the quartz sensor and C, is the feed-
back capacitance. Thus the system is essentially
insensitive to the influence of the input cable
impedance.
Sensors such as these are characterized by
their high stability, wide dynamic range, good
temperature stability, good linearity, and low
hysteresis. They are available in a very wide
variety of configurations for dynamic pressure
ranges from 200 kPa to 100 MPa.

9.2.4 Pressure measurement by allowing the
unknown pressure to act on an elastic member and
measuring the resultant stress or strain
9.2.4. I Piezo-resistive pressure sensors
b
For many metals and some other solid materials, *- Figure 9.25 Piezo-resistive
pressure transducer. Courtesy,
the resistivity changes substantially when sub- Kistler Instruments Ltd.