An alternative approach, which eliminates the effect of leadwire resistance, is to shunt one arm of the internal half-bridge commonly found in conventional strain gage instruments. This procedure requires, of course, that the resistances of the internal bridge arms be known. In addition, it requires that the internal half-bridge be isolated from any balance circuitry which may be present, or that the effects of such circuitry be incorporated in the shunt-calibration calculations. In any case, the instruction manual and circuit diagram for the instrument should be consulted before attempting to calibrate by shunting the internal half-bridge.

The calibration relationship for instrument verification is based on different reasoning than it is for instrument scaling. In scaling applications, the calibration resistor is calculated to develop the same bridge output voltage that would occur when a strain gage of specified gage factor is subjected to a given strain. The instrument gage factor or gain control is then adjusted to register the simulated strain. The effects of signal loss due to leadwire resistance, or signal increase from multiple active gages, are thus compensated for. With this technique, the final setting of the gage-factor or gain control is determined only by the external circuit parameters; and, in the case of a strain indicator, for example, the resulting gage factor setting of the instrument would normally be quite different from that of the strain gage.

In contrast, when calibrating for instrument verification purposes, the instrument gage factor or gain is ordinarily preset to some convenient value. The verification relationship is then written to express the registered strain (in a perfectly accurate instrument) as a function of the shunt resistor used to synthesize the strain signal. It will be seen that the gage factor of the strain gage itself is not involved in this process. Nor are other external circuit parameters, except the initial resistance of the shunted bridge arm, which is usually the nominal resistance of a strain gage.

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