strain gauge element is also added. Thus, when a bridge circuit is used to measure
the change in the resistance of the strain gauge, the dummy gauge can be placed in
the adjacent arm of the bridge to compensate for the temperature changes. This sec-
ond strain gauge is positioned adjacent to the first, so that it is at the same tempera-
ture. It is rotated 90°, so that it is at right angles to the pressure-sensing strain gauge
element, and therefore will not sense the deformation as seen by the pressure-sensing
element. This structure is also used in load cells [8].
Figure 11.12 shows an alternative use of a strain gauge for measuring the force
applied to a cantilever beam. The force on the beam causes the beam to bend, pro-
ducing a sheer stress. This would be the type of strain encountered in a diaphragm
pressure sensor.
The resistance change in strain gauges is small and requires the use of a bridge
circuit for measurement, as shown in Figure 11.13. The strain gauge elements are
mounted in two arms of the bridge, and two resistors, R1 and R2, form the other two
arms. The output signal from the bridge is amplified and impedance matched. The
strain gauge elements are in opposing arms of the bridge, so that any change in the
resistance of the elements due to temperature changes will not affect the balance of
the bridge, giving temperature compensation. More gain and impedance matching
Figure 11.11 Strain gauge: (a) as a serpentine structure, and (b) as a load sensor.