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Shunt Calibration of Strain Gage Instrumentation

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Because of its numerous advantages, shunt calibration is the normal procedure for verifying or setting the output of a strain gage instrument relative to a predetermined mechanical input at the sensor. The subject matter of this publication encompasses a variety of commonly occurring bridge circuit arrangements and shunt-calibration procedures. In all cases, it should be noted, the assumptions are made that the excitation for the bridge circuit is provided by a constant-voltage power supply 1 , and that the input impedance of any instrument applied across the output terminals of the bridge circuit is effectively infinite. The latter condition is approximately representative of most modern strain-measurement instruments in which the bridge output is "balanced" by injecting an equal and opposite voltage developed in a separate network. It is also assumed that there are no auxiliary resistors (such as those commonly used in transducers for temperature compensation, span adjustment, etc.) in either the bridge circuit proper or in the circuitry supplying bridge power.

1 In general, the principles employed here are equally applicable to constant-current systems, but the shunt-calibration relationships will differ where nonlinearity considerations are involved.

Although simple in concept, shunt calibration is actually much more complex than is generally appreciated. The full potential of this technique for accurate instrument calibration can be realized only by careful consideration of the errors which can occur when the method is misused. Of primary concern are:

  1. The choice of the bridge arm to be shunted, along with the placement of the shunt connections in the bridge circuit.
  2. Calculation of the proper shunt resistance to simulate a prescribed strain level or to produce a prescribed instrument output.
  3. Wheatstone bridge nonlinearity (when calibrating at high strain levels).

Because of the foregoing, different shunt-calibration relationships are sometimes required for different sets of circumstances. It is particularly important to distinguish between two modes of shunt calibration which are referred to in this publication, somewhat arbitrarily, as instrument scaling and instrument verification .

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