Introduction

The usual, and preferred, practice with multiple quarter-bridge strain gage installations used for either static or combined static/dynamic measurements is to employ a separate three-wire circuit for each gage. However, if a number of such gages are connected to a multiple-channel instrument which simultaneously uses the same power supply for several channels, the associated bridge circuits (each of which contains an active and dummy gage) are effectively in parallel. This arrangement, in itself, need not cause any problems, provided the power supply has sufficient capacity to maintain a constant voltage under varying load. If the two individual current-carrying P+ and P- power supply leadwires in each circuit have the same resistance and are subjected to the same temperature, their only contribution to measurement error is the usual desensitization of the gage factor. But this error can readily be eliminated by shunt calibration.

Schematic of parallel Wheatstone bridge circuits with common power supply leadwire.

Unfortunately, the stress analyst may be motivated under certain circumstances to use a current-carrying leadwire that, as shown here, is shared by, or common to, all the active gages. Savings of leadwire can be realized with this arrangement (sometimes called a "chevron") when the runs between equipment and instrument are long. And, savings in man-hours of installation time will be obtained when the number of installations is large. But, considering the potential problems created by the use of a common leadwire, the only valid motivations are those arising from physical and mechanical limitations. These may include the number of slip rings available for measurements on rotating equipment; the number of pass-through conductors possible in a barrier (like the wall of a pressure vessel) between the instrument and gages; and the use of multiple-grid gages with an integral bus or solder tab (common-tab rosettes and certain types of strip gages).

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