Thermocouple chromel-alumel (type K) consists of two connected to one end of the conductor of different materials is part of a device for measuring temperature. There are different combinations of the conductors, but the most optimal thermoelectric characteristics demonstrates the connection of two Nickel alloys chromel and alumel. The fact that these materials demonstrate the most similar to direct thermoelectric characteristics, which is necessary for high precision measurements.
Chromel (NH9,5) is an alloy of 89-91% Nickel and 8.7-10% chromium. The rest is impurities of materials such as silicon, copper, manganese and cobalt, and the content of the latter ranging from 0.6 to 1.2%.
Alumel (Nmcak2-2-1) consists of 93-96% Nickel and 1.8 to 2.5% aluminum, with high content of manganese (1,8-2,2%) and silicon (0.8 to 1.2%). This alloy is good enough magnetic in contrast to the low chromel.
The high Nickel content leads to the fact that is made of chromel and alumel thermocouple wire has a high resistance to oxidation. And, considering the fact that the thermocouple of these materials have almost linear dependence of thermal electromotive force (thermal EMF) in the range from 0 to 1000°C, thermocouple chromel-alumel is often used in various thermostats.
The material for the electrodes of the thermocouples is thermo electrode wire size range which consists of the following diameters : 0,2; 0,5; 0,7; 1,2; 3,2 mm. At a temperature of 1300 °C wire is used only diameter 1.2 and 3.2 mm.
One of the main advantages thermo electrode wires of chromel and alumel (type K) is their oxidation stability, which allows them to work reliably in an oxidizing environment due to the appearance when heated, a thin protective film which prevents the penetration of oxygen into the metal. But it should be noted that prolonged operation of the thermocouple chromel-alumel in the oxidative environment may lead to some change of thermo-EMF, especially at temperatures of 300-500 and 800-1000°C. It is not recommended to use them in the atmosphere with high sulfur content, which affects both material chromel and alumel. Thermocouple chromel-alumel (type K) popular because of their availability cost and complexity of manufacturing and a wide range of the measured temperature.
At the top are tables graduations (calibration charts) of thermocouple chromel alumel (type K)