Constantan
Encyclopedia
Constantan is a copper
-nickel
alloy
usually consisting of 55% copper and 45% nickel.
Also known as Eureka.
Its main feature is its resistivity
which is constant over a wide range of temperatures. Other alloys with similarly low temperature coefficient
s are known, such as manganin
(Cu86Mn12Ni2).
discovered that metals can have a negative temperature
coefficient of resistance, inventing what he called his "Alloy No. 2."
It was produced in Germany where it was renamed "constantan".
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. Constantan is also used for electrical resistivity heating.
of very large strains, 5% (50 000 microstrain) or above, annealed constantan (P alloy) is the grid material
normally selected. Constantan in this form is very ductile
; and, in gauge lengths of 0.125 in (3 mm) and longer, can be strained to >20%. It should be borne in mind, however, that under high cyclic strains the P alloy will exhibit some permanent resistivity change with each cycle, and cause a corresponding zero
shift in the strain gauge. Because of this characteristic, and the tendency for premature grid failure with repeated straining, P alloy is not ordinarily recommended for cyclic strain applications. P alloy is available with S-T-C numbers of 08 and 40 for use on metal
s and plastic
s, respectively.
s with wires made of iron
, copper, or chromel
.
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
-nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
usually consisting of 55% copper and 45% nickel.
Also known as Eureka.
Its main feature is its resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
which is constant over a wide range of temperatures. Other alloys with similarly low temperature coefficient
Temperature coefficient
The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the...
s are known, such as manganin
Manganin
Manganin is a trademarked name for an alloy of typically 86% copper, 12% manganese, and 2% nickel. It was first developed by Edward Weston in 1892, improving upon his Constantan ....
(Cu86Mn12Ni2).
History
In 1887 Edward WestonEdward Weston (chemist)
Edward Weston was an English-born American chemist noted for his achievements in electroplating and his development of the electrochemical cell, named the Weston cell, for the voltage standard...
discovered that metals can have a negative temperature
coefficient of resistance, inventing what he called his "Alloy No. 2."
It was produced in Germany where it was renamed "constantan".
Constantan alloy
Of all modern strain gaugeStrain gauge
A strain gauge is a device used to measure the strain of an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable...
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
Strain (materials science)
In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
sensitivity, or gauge factor
Gauge factor
Gauge factor or strain factor of a strain gauge is the ratio of relative change in electrical resistance to the mechanical strain ε, which is the relative change in length.In practice, the resistance is also dependent on temperature...
, which is relatively insensitive to strain level and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
. Its resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
is high enough to achieve suitable resistance values in even very small grids, and its temperature coefficient
Temperature coefficient
The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the...
of resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
is not excessive. In addition, constantan is characterized by good fatigue life
Fatigue (material)
'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
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
0 (number)
0 is both a numberand the numerical digit used to represent that number in numerals.It fulfills a central role in mathematics as the additive identity of the integers, real numbers, and many other algebraic structures. As a digit, 0 is used as a placeholder in place value systems...
stability of the strain gauge is critical over a period of hours or days. Constantan is also used for electrical resistivity heating.
A-Alloy
Very importantly, constantan can be processed for self-temperature compensation to match a wide range of test material coefficients of thermal expansion. An alloy is supplied in self-temperature-compensation (S-T-C) numbers 00, 03, 05, 06, 09, 13, 15, 18, 30, 40 and 50, for use on test materials with corresponding thermal expansion coefficients, expressed in parts per million by length (or µm/m) per kelvin or degree Celsius or degree Fahrenheit.P alloy
For the measurementMeasurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...
of very large strains, 5% (50 000 microstrain) or above, annealed constantan (P alloy) is the grid material
Material
Material is anything made of matter, constituted of one or more substances. Wood, cement, hydrogen, air and water are all examples of materials. Sometimes the term "material" is used more narrowly to refer to substances or components with certain physical properties that are used as inputs to...
normally selected. Constantan in this form is very ductile
Ductility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...
; and, in gauge lengths of 0.125 in (3 mm) and longer, can be strained to >20%. It should be borne in mind, however, that under high cyclic strains the P alloy will exhibit some permanent resistivity change with each cycle, and cause a corresponding zero
0 (number)
0 is both a numberand the numerical digit used to represent that number in numerals.It fulfills a central role in mathematics as the additive identity of the integers, real numbers, and many other algebraic structures. As a digit, 0 is used as a placeholder in place value systems...
shift in the strain gauge. Because of this characteristic, and the tendency for premature grid failure with repeated straining, P alloy is not ordinarily recommended for cyclic strain applications. P alloy is available with S-T-C numbers of 08 and 40 for use on metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s and plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
s, respectively.
Physical properties
| Electrical resistivity at room temperature Resistivity Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre... |
500 nΩ·m |
| Temperature coefficient Temperature coefficient The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the... at 20 °C |
8 ppm/K-1 |
| Temperature coefficient -55 to 105 °C | ±40 ppm/K-1 |
| Density Density The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight... |
8.9 × 103 kg/m³ |
| Melting point Melting point The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure... |
1221 to 1300 °C |
| Specific heat capacity | 0.39 J/(g·K) |
| Thermal conductivity Thermal conductivity In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction.... at 23°C |
19.5 W/(m.K) |
| Linear coefficient of thermal expansion at 25-105°C | 14.9 × 10-6 K-1 |
| Tensile strength Tensile strength Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract... |
455-860 MPa |
| Elongation at fracture Fracture A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal... |
<45% |
| Elastic modulus Elastic modulus An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically when a force is applied to it... |
162 GPa |
Temperature Measurement
Constantan is also used to form thermocoupleThermocouple
A thermocouple is a device consisting of two different conductors that produce a voltage proportional to a temperature difference between either end of the pair of conductors. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a...
s with wires made of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, copper, or chromel
Chromel
Chromel is an alloy made of approximately 90 percent nickel and 10 percent chromium that is used to make the positive conductors of ANSI Type E and K thermocouples. It can be used up to 1100 °C in oxidizing atmospheres...
.