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Constantan is a copper-nickel alloy containing: copper ≤60%, nickel ≤40%, and manganese ≤2%.
- Melting point + 1260 ° C;
- Ud. Weight 8.9 g / cm 3 ,
- Coefficient of thermal expansion 14 x 10 -6 1 / ° C.
The electrical resistance of the constantan does not depend on temperature. At 20 ° C it is equal to 0.48 Ohmmmm 2 / m. After annealing, the thermal expansion coefficient increases to 2 x 10 -6 1 / K, and the electrical resistivity remains the same. The constantan before annealing has the following characteristics:
- Ud. Electrical resistivity ≤0.52 Ohmmm 2 / m;
- Hardness ≤720 MPa.
At the end of the stabilizing annealing, the constantan becomes soft, and its parameters are:
- Specific electrical resistance ≤0.48 Ohmmm 2 / m;
- Alloy hardness ≤ 500 MPa.
Low thermo-emf value, ≤50 μV / ° C. When the constantan and copper interact, contact currents generate currents that distort the readings. Therefore, the use of the alloy is limited, and manganin is used to increase the accuracy of the measurement.
The minimum diameter of a thermocouple wire can be 0.03 mm. The constantan, in pair with copper, has a high thermo-emf, which is used for measuring temperatures up to 500 ° C. In this case, the constantan acts as a negative electrode, whereas copper, chromel or iron are positive. Constantan serves for making compensating wires, rheostats and heating elements.
Constantan is more in demand in the form of a wire than a tape or a circle. The cost of the alloy depends on the price of nickel and copper, as they serve as the main constituent materials. Constantan is supplied in the form of a ribbon with a thickness of 0.1−5 mm, a width of 3 600 mm; Circle with a diameter of 12−100 mm and wire diameter of 0.1−12 mm.
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