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Shunt Calibration of Strain Gage Instrumentation

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The procedures described up to this point have referred only to instrument calibration for compressive strains. This seems natural enough, since shunting always produces a decrease in the arm resistance, corresponding to compression. There are occasions, however, when upscale (tension) calibration is more convenient or otherwise preferable. The easiest and most accurate way to accomplish this is still by shunt calibration.

The figure below illustrates the simple Wheatstone bridge circuit again, but with the calibration resistor positioned to shunt the adjacent bridge arm, (usually referred to as the "dummy" in a quarter-bridge circuit).



Upscale (tensile) calibration by shunting adjacent bridge arm.

As demonstrated by Eq. ( 514.1a ), a decrease in the resistance of the adjacent arm will produce a bridge output opposite in sign to that obtained by shunting , causing the instrument to register a tensile strain. Thus, a simulated compressive strain in , generated by shunting that arm, can be interpreted as a simulated tensile strain in . The special subscript notation is temporarily introduced here because the two simulated strains are not exactly equal in magnitude. For calibration at low strain levels, the difference in magnitude between and is small enough that the relationships given in Eqs. ( 514.5 ) through ( 514.7 ) are sufficiently accurate for most practical applications. The error in the simulated tensile strain, in percent, is approximately equal to the gage factor times the strain, in percent.

The foregoing error arises because shunting to produce a simulated compressive strain in that arm, and then interpreting the instrument output as due to a simulated tensile strain in , involves effectively a two-fold simulation which is twice as sensitive to Wheatstone bridge nonlinearity. Accounting for the nonlinearity, as shown in a later section , permits developing a shunt-calibration relationship for precisely simulating tensile strains of any magnitude.



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