Which is more effective: Three-Wire or Two-Wire Quarter-Bridge Installation?

Why Opt for Three?

Unraveling the Electrical Complexity Behind the Decision for Three Lead Wires in Quarter-Bridge Gage Installations

From an electrical standpoint, two 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 two 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 opposition. 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 isn’t 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, there will be a reduction in sensitivity to strain (or gage factor desensitization) caused by the inactive lead wire resistance.  

Of even greater concern is if the temperature of the two lead wires changes, the electrical resistance of the wires will also change, causing a variation in strain reading not related to the actual change in strain of the part being tested. When all three wires in a three-wire system are of equal length, then this places proportionate lead resistance into two adjacent arms of the Wheatstone bridge, keeping the bridge balanced with respect to the lead wires.  

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.