Strain Gage Connections and Bridge Circuits

Since the resistance change of a strain gage is very small, it requires the use of a Wheatstone bridge circuit. The simplest method is to place a strain gage into one arm of the bridge circuit, while placing fixed resistors into the remaining three arms. This configuration is called a Quarter bridge. Using a 3-wire lead wire to connect the strain gage to the strain instrumentation will cancel any change in resistance in the lead wire due to a change in temperature. This is called a Quarter bridge 3-wire system. Placing strain gages in two of the bridge arms results in a Half bridge. This method has the advantage of either doubling the output voltage, or compensating for thermal outputs of the gages, depending on the strain gage application. In a Full bridge connection, all four arms are replaced by strain gages, yielding re-doubled output and compensation of temperature effects. TML strain meters have functions for arranging the bridge circuits, applying excitation voltage and amplifying and reading the resulting output voltages. Bridge circuits can be completed by connecting the strain gages and setting the instrument switches according to the desired configuration for each strain meter.