Nonlinearities in Shunt Calibration

The nonlinearity error described in the preceding sections of this publication must always be kept in mind during the shunt calibration of a static strain indicator or signal conditioner. In conventional practice, the strain gage is momentarily shunted by a large resistance, the magnitude of which is selected to produce a decreased resistance in the bridge arm corresponding to a predetermined compressive strain in the gage (at a specified gage factor). When this is done, the strain indicated by the instrument will be in error by the amount calculated from Eq. ( 507.3 ) (for compression), and should be corrected accordingly. If the Wheatstone bridge has an initial resistive unbalance, correction for the nonlinearity must be made as demonstrated in the preceding second and third examples , in order to properly account for this condition. Micro Measurements Catalog A-110 includes a tabular list of precision shunt-calibration resistors for simulating different compressive strain magnitudes from 100 microstrain to 10 000 microstrain.

In order to calibrate a strain-indicating instrument for tensile strains, the adjacent inactive arm of the bridge can be shunted by resistances having special magnitudes selected for this purpose. In such cases, the instrument nonlinearity error must be corrected for the tension case.

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