Changes in temperature during the test duration are almost unavoidable. The mismatch of thermal expansion coefficients between gage wire, backing, and specimen induces so called apparent strain which contributes errors to the strain measurements.
Although the effect of temperature can be handled by either evaluation as a part of the data or by a dummy gage in the wheatstone bridge circuit, it is more desirable to have a gage which can take care of itself, especially when the temperature gradient and/or variation are large or the bridge circuit is unavailable.
There are two types of self-temperature-compensation gages: the selected-melt gage and the dual-element gage.
The selected-melt gage is based on proper processing of alloys, particularly through cold working, such that the gage wire has a very low thermally induced strain (apparent strain) over a wide range of temperatures. Constantan and Karma alloy are two most common gage wire material which have self-compensation property.
The dual-element gage employs two gird elements which have different thermal expansion properties. The net effect of these two elements almost cancels each other.