Instrument Scaling for Small Strains: Shunt Calibration

Instrument Scaling for Small Strains

Very commonly, when making practical strain measurements under typical test conditions, at least one active bridge arm is sufficiently remote from the instrument that the leadwire resistance is no longer negligible. Under these circumstances, the strain gage instrument is "desensitized"; and the registered strain will be lower than the gage strain to an extent depending on the amount of leadwire resistance. In a three-wire quarter-bridge circuit, for instance, the signal will be attenuated by the factor

, where

is the resistance of one leadwire in series with the gage. The usual way of correcting for leadwire desensitization is by shunt calibration -- that is, by simulating a predetermined strain in the gage, and then adjusting the gage factor or gain of the instrument until it registers the same strain.

This section includes a variety of application examples involving quarter-, half-, and full-bridge strain gage circuits. In all cases treated here, it is assumed that strain levels are small enough relative to the user's permissible error limits that Wheatstone bridge nonlinearity can be neglected. Generalized relationships incorporating nonlinearity effects are given in subsequent sections.

( continued... )

Page 12 of 50