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Indirect Calibration
The more practical and widely used approach to either instrument verification or scaling is by indirect calibration; that is, by applying a simulated strain gage output to the input terminals of the instrument. It is assumed throughout this publication that the input to the instrument is always through a Wheatstone bridge circuit as a highly sensitive means of detecting the small resistance changes which characterize strain gages. The behavior of a strain gage can then be simulated by increasing or decreasing the resistance of a bridge arm.

As a rule, strain gage simulation by increasing the resistance of a bridge arm is not very practical because of the small resistance changes involved. Accurate calibration would require inserting a small, ultra-precise resistor in series with the gage. Furthermore, the electrical contacts for inserting the resistor can introduce a significant uncertainty in the resistance change. On the other hand, decreasing the resistance of a bridge arm by shunting with a large resistor offers a simple, potentially accurate means for simulating the action of a strain gage. This method, known as shunt calibration , places no particularly severe tolerance requirements on the shunting resistor, and is relatively insensitive to modest variations in contact resistance. It is also more versatile in application and generally simpler to implement.

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