When the gage installations are more than a few steps away from the instrument, it is usually inconvenient to connect a shunt-calibration resistor directly across the gage as shown previously. For such cases, remote shunt calibration is a common practice. The following schematic diagram illustrates a half-bridge circuit with the calibration resistor positioned at the instrument.

Remote shunt calibration of external half-bridge.

In this example, three extra leadwires and a switch permit connecting the shunt across either arm of the half bridge. Since shunt resistors are characteristically in the thousands of ohms, the resistances of the calibration leadwires, although shown in the figure, can usually be neglected in the strain simulation calculations. Equation ( 514.8 ) is then directly applicable to remote shunt calibration. If the leadwire resistance is large enough so that 100 X / is greater than about 1/10 of the required calibration precision (expressed in percent), Eq. (514.8) can be modified as follows to calculate the correct calibration resistance:

  (514.9)

where represents the resistance of one leadwire between the calibration resistor and gage.

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