Compensation Methods of Temperature Effect of Lead Wires (3-wire System)
For effective self-temperature-compensation, SELCOM gages adopt the quarter-bridge system. However, if the lead wire cable is the 2-wire system, strain output from the bridge is affected by the temperature effect of the lead wire. To avoid such adverse effect, the 3-wire system is adopted.
If 3 lead wires are connected to a strain gage as shown below, a half lead wire resistance is applied to the adjacent side of a bridge to compensate for the temperature effect of lead wires in bridge output. The temperature effect of the lead wires connected to a measuring instrument outside of the bridge is ignored because the input impedance of the measuring instrument is high.
As a precaution when using the 3-wire system, the 3 lead wires should be the same type, length, and cross-section to equalize temperature effects of each lead wire. If lead wires are directly exposed to sunlight, the coating color should also be the same.