Remedial Measures

By far the best remedy for problems created by a common power-supply leadwire is to avoid using it altogether. However, for those cases in which there is no alternative to the common leadwire, several different techniques can be employed to minimize the difficulties. If, for example, the strain indicator can be balanced at the test temperature, and no further temperature changes occur prior to making strain measurements, there will be, of course, no leadwire temperature errors. Although this leaves only the relatively small errors due to crosstalk between parallel circuits, there may still be a problem if the initial imbalance is beyond the balance range of the instrument in use. The latter problem can be overcome by tying the instrument ends of all dummies together at a common point and inserting a fixed precision resistor in series between points D and C . When all leadwires, including the common leadwire, have the same resistance, a series resistor equal to will nullify the initial imbalance (and reduce crosstalk).

Schematic of parallel Wheatstone bridge circuits with common power supply leadwire.
(Repeated Illustration)

The most effective remedy is one which eliminates the initial imbalance while maintaining leadwire temperature compensation, even under varying test temperatures. This is accomplished by selecting a common power-supply leadwire with a resistance times that of the individual return leadwires. Table 516.1 gives a simple procedure for determining the appropriate wire gauge number to achieve the desired circuit performance. Selection of the common leadwire resistance in this fashion not only solves the initial imbalance and leadwire temperature problems, but also reduces the calibration and crosstalk errors. However, it should be noted that if the resistance of the common leadwire is less than times that of the individual return leadwires, then all of the problems described here will be created in reverse.

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