Introduction

Since the invention of the electrical resistance strain gage more than a half century ago, the Wheatstone bridge has become the sensing circuit of choice in most commercially available strain gage instrumentation. This popularity is due in large measure to its inherent ability to 1) detect the small resistance changes produced in the strain gage when it follows even minute dimensional changes on the surface of a test part under load, 2) produce a zero output voltage when the test part is at rest, and 3) provide for compensation of temperature-induced resistance changes in the strain gage circuit. To varying degrees, each of these factors is essential for accurate strain gage measurements.

In the majority of strain gage applications for the determination of the state of stress on a test-part surface, individual strain gage elements, whether from uniaxial or rosette strain gage configurations, are connected independently to the Wheatstone bridge in a quarter-bridge arrangement. As discussed in the following sections, the wiring scheme chosen to connect the strain gage to the bridge circuit has a significant effect on the accuracy of measured strain data.

In particular, use of a two-wire connection is generally not recommended because it may introduce a significant resistance offset in the strain gage circuit; temperature changes in the leadwire system will introduce errors into measured strain data; and the leadwire system will reduce the sensitivity of the strain gage circuit. Configuring the strain gage input as a three-wire circuit provides for intrinsic "bridge balance" and automatic compensation for the effects of leadwire temperature changes on measured strain data, and reduces the loss in sensitivity present in the two-wire configuration. Consequently, the three-wire connection is the recommended hookup for quarter-bridge strain gage circuits for static strain measurement.

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