Half-Bridge Circuits (2): Shunt Calibration

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All of the foregoing are examples of half-bridge circuits, since one-half of the Wheatstone bridge is external to the instrument. Aside from differences in the quality of the achievable temperature compensation, they differ principally in their degrees of signal increase. The factor of signal augmentation is usually expressed in terms of the "number of active gages",

. When the gage in the adjacent bridge arm senses no applied strain, but serves solely for temperature compensation,

= 1. With two perpendicular gages, aligned along the principal axes in a uniaxial stress field

= 1 +

, where

is the Poisson's ratio of the test material. In the case of the beam, with gages on opposing surfaces,

= 2, since the gages sense equal and opposite strains, and the bridge output is doubled.

When

is greater than unity, it is obviously necessary to adjust the instrument sensitivity by the factor 1/

if the instrument is to directly register the actual surface strain sensed by the primary active gage. Furthermore, if the gage installations are at a distance from the instrument, additional adjustment of the sensitivity (in the opposite direction) is required to compensate for the signal loss due to leadwire resistance. Shunt calibration can correct for both effects simultaneously, and permit adjusting instrument sensitivity to register the correct surface strain at the primary active gage.

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