Constantan Wire, CuNi40 and CuNi44

Composition

An alloy has got its name from Latin - constants - which means «unchanged», «permanent» because its real resistance virtually does not depend on temperature. Constantan constitutes a silver-yellow alloy that consists of cuprum (58−60%), nickel (38−40%), manganese (1−2%). A melting point is 1260 °C, a linear expansion is 14 • 10^-6 1/°С, a density is 8,9 g/cm³.

Percentage composition of constantan CuNi40

Advantages

It possesses a heat resistance, a plasticity, a constancy of electrical properties. At a temperature of 20 °C, specific electric resistance equals to 0,48 Ohm-mm²/m. After a special process (stabilizing annealing), a coefficient of thermal real resistance rises to 2x10−6 1/К, in an annealed (soft) condition constantan eр = 40 - 50%, sр = 400 - 500 MPa and r = 0,45 - 0,48 Ohm-mm²/m; in hard constantan -eр = 2…5%, sр = 650 - 720 MPa and r = 0,46 - 0,52 Ohm-mm²/m.

Demerits

Such a wire is quite expensive because of nickel presence in its composition. Besides, nickel is prone to damage of a sulfur effect when raising a temperature. The main demerit of alloy is a huge thermoelectromotive force (40−50 mkV/°С) in a combination with cuprum. Thereby, it limits its usage in precise instruments and stable resistors because clue heating of contact elements leads to an appearance of current intensity that impairs the indicated values. In this regard, there can be used manganin if it is necessary to achieve a high accuracy.

Application

Constantan wire is commonly used in a manufacturing of rheostats, compensating lead wires, heating units. The most popular wire is Ø 0,03−5 mm, and a strip of no more than 0,1 mm. A thermocouple made of constantan wire with cuprum differs with significant thermoelectromotive force and is used at a not very high working temperature (up to 500°С). In such thermocouples, cuprum, chromel or ferrum are positive electrodes; negative are copel and alumel (except of constantan). Thermocouple consists of electric conductor fumes, that are soldered in one end and placed into environment oriented for thermometry («hot junction»). Then, exposed ends are going to be placed into a thermostatic regulator (so-called cold junction). An alloying allows withstanding a short-range effect of a high temperature up to 1300 °C while working in aggressive environment. Constantan wire, less often a strip or a rod are utilized for manufacturing heating controllers, compensating lead wires.

An alloy is delivered in the following way:

A rod (Ø 12,0 - 100,0 mm);

A strip (0,1−5,0 mm of thickness; 3−600 mm of wideness).

A wire (Ø 0,10 - 12 mm);

Physical characteristics of constantan CuNi40 at a temperature 20°C