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Quarter-Bridge Circuit
The quarter-bridge circuit, with a single active gage, is widely used in experimental stress analysis. When instrument scaling is done by connecting a shunt-calibration resistor directly across the gage, the simulation of compressive strain is exact at all strain levels. This is true because the nonlinearity in shunt calibration is the same as that during strain measurement. For such cases, the proper shunt-calibration resistor to simulate a given strain magnitude can be obtained directly from Table 1 , or calculated from Eq. ( 514.7 ). After instrument scaling, the indicated strain will be correct at the magnitude of the calibration strain, but slightly in error at other strain levels because of the nonlinearity. For most practical applications, the corrected strain at any different strain level can be calculated from:

     (514.18)

where (with signs):

  = corrected strain
  = calibration strain
  = indicated strain
  = gage factor of strain gage

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