Constantan is a copper–nickel alloy consisting usually of 55% copper and 45% nickel and specific minor amounts of additional elements to achieve precise (almost constant) values for the temperature coefficient of resistivity. That means, its main feature is the low thermal variation of its resistivity, which is constant over a wide range of temperatures. Other alloys with similarly low temperature coefficients are known, such as manganin.
This alloy has high electrical resistivity (4.9 x 10−7 Ω·m), high enough to achieve suitable resistance values in even very small grids, the lowest temperature coefficient of resistance, and the highest thermal EMF (also known as the Seebeck effect) against platinum of any of the copper-nickel alloys. Because of the first two of these properties, it is used for electrical resistors, and because of the latter property, for thermocouples. Thermocouples are electrical devices consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature.
For example, constantan is the negative element of the type J thermocouple with iron being the positive. The type J thermocouples are used in heat treating applications. Also, Constantan is the negative element of the type T thermocouple with copper the positive. These thermocouples are used at cryogenic temperatures.
Temperature Coefficient of Resistance of Constantan
The temperature coefficient of resistance (TCR), which describes how much its value changes as its temperature changes, of constantan – 45Ni-55Cu is ± 30 ppm/°C. It is usually expressed in ppm/°C (parts per million per degree Centigrade) units.
Thermal Expansion Coefficient of Constantan
Linear coefficient of thermal expansion of constantan at 25 to 105°C is 14.9 x 10-6 K-1.
Thermal expansion is generally the tendency of matter to change its dimensions in response to a change in temperature. It is usually expressed as a fractional change in length or volume per unit temperature change. Thermal expansion is common for solids, liquids and for gases. Unlike gases or liquids, solid materials tend to keep their shape when undergoing thermal expansion. A linear expansion coefficient is usually employed in describing the expansion of a solid, while a volume expansion coefficient is more useful for a liquid or a gas.
The linear thermal expansion coefficient is defined as:
where L is a particular length measurement and dL/dT is the rate of change of that linear dimension per unit change in temperature.
Electrical Resisitivity of Constantan
Electrical resistivity of constantan – 45Ni-55Cu is 4.9 x 10−7 Ω·m, high enough to achieve suitable resistance values in even very small grids.
Electrical resistivity and its converse, electrical conductivity, is a fundamental property of a material that quantifies how strongly it resists or conducts the flow of electric current. A low resistivity indicates a material that readily allows the flow of electric current. The symbol of resistivity is usually the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm-metre (Ω⋅m). Note that, electrical resistivity is not the same as electrical resistance. Electrical resistance is expressed in Ohms. While resistivity is a material property, resistance is the property of an object.
Thermal Conductivity of Constantan – 45Ni-55Cu
The thermal conductivity of constantan – 45Ni-55Cu is 21 W/(m.K).
The heat transfer characteristics of a solid material are measured by a property called the thermal conductivity, k (or λ), measured in W/m.K. It is a measure of a substance’s ability to transfer heat through a material by conduction. Note that Fourier’s law applies for all matter, regardless of its state (solid, liquid, or gas), therefore, it is also defined for liquids and gases.
The thermal conductivity of most liquids and solids varies with temperature. For vapors, it also depends upon pressure. In general:
Most materials are very nearly homogeneous, therefore we can usually write k = k (T). Similar definitions are associated with thermal conductivities in the y- and z-directions (ky, kz), but for an isotropic material the thermal conductivity is independent of the direction of transfer, kx = ky = kz = k.
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