You will calibrate the strain gauges to develop a relationship between the output voltage of the bridge measuring change in resistance, and the strain in the beam.
(Recall from the Bridges section, that since the typical change in resistance of a strain gauge is extremely small (~0.0002%), a bridge circuit is generally used to measure resistance changes of strain gages.)
Resistance strain gauges need to be calibrated in situ, attached to the device whose strain is going to be measured, since moving the strain gauge deforms it and alters its calibration.
One easy way to calibrate strain gauges is to simulate a known strain using a shunt resistor. In practice the resistance strain gauge changes its resistance when it is deformed. Rather than actually deforming the strain gaguge, an additaional resistance is placed in parallel with one strain gauge in the bridges. The change in resistance is indicated by the bridge can then be equated to the equivalent strain of the shunt resistor.
Recall that for resistors in parallel, the equivalent resistance is given by:
(1)where:
In our case the equivalent resistance is given by:
(2)where:
It can be shown that the equivalent strain is given by:
(3)
