Three-Wire vs Two-Wire Quarter-Bridge Installations
Why are 3 lead wires used for a quarter-bridge gage installation? Why not use just 2 wires?
Electrically speaking, 2 wires from a quarter -bridge gage installation connected to a Wheatstone bridge based instrument will properly complete the circuit and provide a measurement from the strain sensor. However, the electrical resistance of the 2 lead wires is in series with the strain gage sensor resistance and will be presented to the Wheatstone bridge as part of the bridge-arm resistance. This will result in a large bridge imbalance, which must be compensated using the bridge balance adjustment on the instrument. Also, the lead wire resistance is not active in the strain measurement. Since strain sensitivity for the gage in the bridge circuit is relative to the starting resistance of the installed gage, then there will be a reduction in the gage sensitivity to strain (called gage factor desensitization) caused by the inactive lead wire resistance. Of even greater concern is that when the temperature of the 2 lead wires changes, then the electrical resistance of the wires will change, causing an apparent change in strain reading that is not related to the actual change in strain of the part being tested. When all 3 wires in a 3 wire system are of equal length, then this setup places equal lead resistance into two adjacent arms of the Wheatstone bridge, keeping the bridge balanced with respect to the lead wires. Of even greater importance, if the lead wires remain at the same temperature, even if the temperature changes during the test, then the temperature-induced resistance change of the wires will be cancelled in the bridge and, thus, will not result in an apparent change in strain reading. If the lead wire resistance is high (very long length or small gauge wire), then it is advised to perform a shunt calibration on the circuit to reduce gage factor desensitization effects. Tech Tip TT-612, “The Three-Wire Quarter-Bridge Circuit,” provides additional information on three- vs. two-wire quarter-bridge installations. Application Note VMM-16 discusses the benefits of proper shunt calibration.