It is known that a thermocouple 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 (type K). The fact that these materials demonstrate the most similar to direct thermoelectric characteristics, which is necessary for high precision measurements. Before proceeding to the description of the properties of the thermocouples should stop used in these alloys. Chromel 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 comprises 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 made from these alloys 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, it is often used in a variety of temperature controller (see calibration table of the thermocouple).
At short-term measurement of the maximum temperature can reach 1300°C. the material for the electrodes thermocouples (type K) is termoelektrodnye 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 Wire chromel and alumel are isolated from each other and from the case of single-channel ceramic insulators with a ceramic tip on the end that protects from contact with metal case. Thermocouple (type K) of these alloys is characterized by high inertia. With the exception of thermocouple chromel-alumel several types with low inertia. Wire of alumel and chromel has a common resistance to mechanical damage in addition to used in specific thermocouples, which has a high resistance to vibration loads. Mostly thermocouples have a standard head design, but there are options waterproof and sprayproof heads.
One of the main advantages termoelektrodny 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 in oxidizing 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 popular because of their availability cost and complexity of manufacturing and a wide range of the measured temperature-dependent diameter termoelektrodny wire.