Leadwire Temperature Errors

In a single, isolated, three-wire, quarter-bridge circuit, if the leadwires are of the same resistance and subjected to the same temperature change, there is essentially no false circuit output due to leadwire thermal effects. Since one of the leadwires is in the active arm of the bridge, and the other is in the dummy arm, the effects of the resistance changes cancel. The remaining error, due to a small change in leadwire desensitization of the gage factor, is normally small enough to be ignored. But this situation is entirely different when a common leadwire arrangement is employed.

The single aspect of common leadwire usage with the most potential for producing serious errors is the loss of leadwire temperature compensation. The copper normally used in leadwires has a rather high thermal coefficient of resistance. When the common leadwire and the individual leadwires in each of the parallel circuits are subjected to the same change in temperature, false circuit outputs will usually result. This phenomenon occurs because the common leadwire carries the sum of the currents carried by the individual leadwires. Except for a few unique combinations of leadwire resistances, thermally induced resistance changes in the leadwires will produce a different voltage drop in the common and individual return leadwires, thus unbalancing the bridge. If, for example, the active and dummy gages all have the same initial resistance and the resistances of the common and individual return leadwires are the same, then the voltage drop in the common leadwire will be times as great as in the individual leadwires. Unlike the case of the isolated quarter-bridge circuits, these voltage changes do not cancel one another and, as a result, leadwire temperature compensation is lost.

Output increment due to leadwire resistance change with temperature when the resistance of all dummy and active gages is the same, the common and parallel leadwires are all of the same resistance, and the circuit has been shunt calibrated at 1000 microstrain across the active gages, assuming a gage factor (package data) of 2.000. Note that any thermal output from the strain gages is not included.

As shown in the next section, the magnitude of the false output increases dramatically with both temperature and number of parallel circuits. These outputs are essentially independent of the strain levels to which the gages are subjected. As a result, loss of temperature compensation can lead to very large percentage errors (or, in extreme cases, even the wrong sign!) when measuring typical working strains.

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