A copper-nickel alloy that contains specific minor amounts of additional elements to achieve precise values for the temperature coefficient of resistivity.
Constantan is a copper-nickel alloy usually consisting of 55% Copper and 45% Nickel. Its main feature is its resistance which is constant over a wide range of temperatures. Other alloys with similarly low temperature coefficients are known, such as Manganin (Cu86Mn12Ni2).
Of all modern strain gauge alloys, Constantan is the oldest, and still the most widely used. This situation reflects the fact that constantan has the best overall combination of properties needed for many strain gauge applications. This alloy has, for example, an adequately high strain sensitivity, or gauge factor, which is relatively insensitive to strain level and temperature. Its resistivity is high enough to achieve suitable resistance values in even very small grids, and its temperature coefficient of resistance is not excessive. In addition, constantan is characterized by good fatigue life and relatively high elongation capability. It must be noted, however, that constantan tends to exhibit a continuous drift at temperatures above 65 °C (150 °F); and this characteristic should be taken into account when zero stability of the strain gauge is critical over a period of hours or days.
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