Nearly all strain-gage instrumentation and transducer readouts incorporate the Wheatstone bridge circuit. Typically they contain a power supply for providing a voltage to the power corners of the bridge, an amplifier for increasing the output from the bridge to useful levels, and, in many cases, a readout device for indicating the strain or other units of measurement. The resistive components in the arms of the bridge may be either bridge completion resistors, or one or more active strain gages in a transducer or installed on a test part.

Most strain-gage-based transducers and bridge completion modules are designed to present a combination of resistive components to the Wheatstone bridge circuit that will produce zero output when no loads or load-induced strains are present. Few, if any, completely achieve this goal because of tolerances in the resistive values of the components. Some small -- and normally insignificant -- output will usually be present. However, incorrect gage installations or damage to components in the bridge circuit can lead to large imbalances requiring attention before accurate measurements can be made.

The extent of any initial zero imbalance can be readily determined, of course, from the values indicated by strain-gage instrumentation or the transducer readout. For this to be an accurate indication of bridge balance, however, the output of the instrument itself must be zero when the output from the signal corners of the bridge is zero. The star bridge shown here is commonly used for achieving that condition. The resistors between S+ and S- simulate the impedance of the Wheatstone bridge circuit while providing a zero signal to the amplifier input. The pair of resistors between P+ and P- places a test load on the power supply and provides a reference to circuit common for output circuits requiring one.