Initial Imbalance

Because the common leadwire does not affect the voltage across the internal half bridge, is not applied to the 1/ 2 term in Eq. ( 516.3 ). This gives rise to the problem of an initial imbalance in every circuit, even when the active and dummy gages are of the same resistance. To illustrate the magnitude of the initial imbalance, consider the case of parallel circuits in which all active and dummy gages are of the same resistance, ; and all leadwires, including the common leadwire, have the same resistance, . If the instrument gage factor control is set at 2.000, the initial imbalance, in microstrain units, is:

    (516.4)

Equation (516.4) is plotted below for various combinations of the parameters and . As demonstrated by the figure, the imbalance can easily exceed the balance range of commercial strain indicators and signal-conditioning amplifiers.

Initial imbalance produced by the use of a common leadwire when all active and dummy gages are of the same resistance, common and individual leadwires are the same resistance, and the instrument gage factor setting is 2.000

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