THE WHEATSTONE BRIDGE CIRCUIT.

Everything you always wanted to know about WHEATSTONE BRIDGE but were afraid to ask.

Strain gauges exhibit very small resistance changes. A direct resistance measurement with a multimeter would need to measure 350 ohms (a typical gauge resistance) to 4 decimal places. The Wheatstone bridge converts this very small resistance change into a voltage which can be accurately measured. This is an advantage because it creates a change of voltage from zero rather than a small change of a large value. It’s like being presented with a single needle rather than the entire haystack!

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 is due in large measure to its inherent ability to:

1) detect the small resistance changes produced in the strain gage as 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.

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