Calibration Errors

Shunt calibration of the individual quarter-bridge circuits to adjust the instrument sensitivity would normally be done by shunting the "dummy" in one circuit, under the condition of zero output from the remaining parallel circuits. The use of a common leadwire causes no errors in the actual calibration process itself. However, when subsequent strain measurements are made, the strain-induced resistance changes in the individual gages produce changes in the values of , , and ultimately . Consequently, the changes in will cause the bridge output to vary, even when the resistance of the active gage corresponds to the calibration value. The calibration factor between resistance change and output voltage is then no longer correct for the calibrated circuit and the indicated strains will be in error. Accordingly, the calibration factor is generally correct only for the calibration conditions; namely, when the current through the common leadwire is the same as during calibration. The calibration error produced when the parallel gages are strained does not lend itself to generalization, but is symptomatic of the crosstalk between circuits treated in the following section.

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