Constantan is a Nickel-Copper based alloy wire that has a high resistivity and is mainly used for thermocouples and electrical resistance heating. It has a constant resistivity over aa wide range of temperature.
Primarily, constantan is composed of Nickel and Copper. It contains 60% of nickel and 40% copper.
Constantan has a number of special characteristics due to which it is known as the most versatile alloy that is available. Some of its important characteristics are:
- High specific heat resistance
- Negligible temperature Coefficient
- Easily ductile
- Resistant to atmospheric corrosion
- Can be easily soldered and molded
Constantan Physical Properties
The physical properties of constantan are:
Boiling Point – no data
Melting point – 1225 to 1300 oC
Specific Gravity – 8.9 g/cc
Solubility in Water – Insoluble
Appearance – A silver- white malleable alloy
Odor – Odorless
Electrical resistivity- at room temperature: 52.0 µΩ/cm
At 20°c – 490 µΩ/cm
Density – 8.89 g/cm3
Temperature Coefficient ±40 ppm/K-1
Specific heat capacity 0.39 J/(g·K)
Thermal Conductivity 19.5 W/(m.K)
Elastic Modulus 162 GPa
Elongation at fracture – <45%
Tensile strength – 455 to 860 MPa
Linear Coefficient of thermal Expansion 14.9 × 10-6 K-1
Constantan Wire Resistance
The resistance of constantan wire is 15.94 ohm-cmil/ft. Since the resistance of constantan does not change with temperature, it is used for the purpose of making thermocouples and other such appliances where the resistance should remain constant for a particular range of temperature. It is an electric resistance wire.
Uses of Constantan
Constantan is used for the measurement of temperature. It is used for the formation of thermocouple, along with the wires of other metals such as copper, iron and chromel. It is especially used for resistance purpose since its resistance does not change much with the change in its temperature. It is used for DC current shunts. In the DC current shunts, the precision low value resistance wire is placed in series with the load in a direct current circuit, which is already carrying a high current. It one measures the voltage drop; he can easily measure the current.
Constantan is one of the most widely used alloys. This is owing to the fact that it has the best overall composition of the important properties that is needed for many of the strange gauge applications. Constantan has the highest resistivity among all the other alloys, which makes it insensitive to the level of strain and temperature. Its resistivity is so high that it can also achieve suitable resistance levels in very small grids also. The temperature coefficient of resistance is not at all excessive in case of constantan alloy. Therefore it has very many advantages over the other alloys.
Constantan has a good fatigue life and a very high elongation capacity also. All these additional properties make it a highly useful and good alloy. But this alloy shows drift at the temperature range of 65 °C, therefore this point should be kept n mind when one tests the stability strain if the gauge, since its stability strain is critical at this temperature.
The application of constantan falls in three important categories:
Constantan is used for the formation of thermocouples with the wires of copper, iron and chromel. It is used to form Chrome- constantan thermocouples and good iron constantan thermocouples also.
Heavy Duty Industrial Rheostats
Constantan is an ideal alloy for the electric motor starter resistances and for heavy duty industrial rheostats. Since it has a high specific heat resistance, and a good ductile nature, it satisfies the entire important requirement for this category of specifications. Constantan is transformed into wires of large size or this kind of requirements.
Wire Wound Precision Resistors
Constantan is one of the most widely used alloys in wire wound precision resistors, volume control devices and temperature stable potentiometers. It is a main attraction in this field owing to the fact that it has negligible temperature coefficient and high resistance.
Development of a high thermal E.M.F – This is another important property of constantan, that it develops a high thermal E.M.F against other metals. It is therefore used in thermocouple alloys, with other metals like iron and copper.
Constantan alloys are of two types: P- alloys and A-alloys
If one has to measure large strains, then he can use the Annealed constantan to measure it. The constantan that is available in this form is generally very ductile, and measures about 3mm in length. It can be strained for more than 20%. This p-alloy is available with S-T-C numbers of 08 and 40, so that it can be used in metals and plastics.
Constantan alloy can be processed or the compensation of self temperature, so that it can match a wide range of test material of the coefficients of thermal expansion. This alloy is supplied with the self temperature compensations numbers – (S-T-C) of 00, 03, 05, 06, 13, 30, 40 and 50. These self temperature compensation numbers have their corresponding thermal coefficients which are expressed in parts per million, by length per degree Celsius, Kelvin or degree Fahrenheit.
Effects of Constantan on Health
Constantan has certain negative impacts on the health of human beings as well as animals. If the animals inhale the copper dust from this alloy, it can lead to the hemolysis of the red blood cellsand deposition of hemofuscin in the pancreas and the liver.
To a normal human being, exposure to nickel can present some health hazards to them. Since nickel is a confirmed carcinogen, it may cause cancer in the people. Another common problem is the hypersensitivity to nickel. It may cause allergy and dermatitis, pulmonary asthma, inflammatory reactions and conjunctivitis.
It has other acute effects on the other parts of the body as well. Some of its harmful effects are:
Eyes – dust powder may cause abrasive irritation to the eyes
Skin – No chronic heath effect
Ingestion – Nickel has a low oral toxicity, copper may cause vomiting and nausea.
Inhalation – There are certain cases of asthma that have been reported due to the inhalation of nickel and copper dust. There is a high possibility of respiratory sensitization.
There are certain precautions that need to be taken for the storage of constantan. It should be stored in a cool and dry area, in a tightly sealed container. After handling constantan, hands should be washed thoroughly with soap and water. Use gloves to touch the constantan wires, and should not come in direct contact.