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K type thermocouple is oxidation resistance strong base metal thermocouple, not appropriate in a vacuum, sulfur, carbon atmosphere and REDOX alternately under the atmosphere of bare wire; When oxygen tension is low, nickel and chromium in the chrome will preferential oxidation, has brought big changes in the thermoelectric potential, but the smaller the influence of metal gas, therefore, using metal protection tube. Type K thermocouple thermoelectric potential shortcomings: (1) the high temperature stability of the N type thermocouple, thermocouple and precious metal at high temperature (over 1000 ℃, for example) is often damaged by oxidation; (2) within the range of 250 ~ 500 ℃ short term thermal cycling stability is bad, that is, at the same temperature in the process of heating cooling, the thermoelectric potential value is not the same, the difference can reach 2 ~ 3 ℃; (3) the negative in 150 ~ 200 ℃ range magnetic transition to occur, causing at room temperature to 230 ℃ range piding value often deviates from the indexing table, especially when used in the magnetic field tend to appear has nothing to do with the time of the thermoelectric potential interference; (4) irradiation in high flux system environment for a long time, due to the negative of the manganese (Mn), cobalt (Co) elements such as disintegration occurs, make its stability is poor, the thermoelectric potential. Great changes have taken place 5, nickel chrome - nickel and silicon thermocouple (index number N) as the main characteristics of the thermocouple is: under 1300 ℃ temperature oxidation resistance is strong, short and long term stability thermal cycling reproducibility is good, resistance to radiation and low temperature performance is good, in addition, within the scope of the 400 ~ 1300 ℃, N type thermocouple thermoelectric properties of linear better than K type I; But in low temperature range (200 ~ 200 ℃), the nonlinear error is larger, at the same time, the material is hard to processing. 6, copper, copper nickel thermocouple (index number for T) T thermoelectric couple, the thermocouple is extremely pure copper, negative extremely copper nickel alloy (also called a constantan), its main features are: in the base metal thermocouple, it has the highest accuracy, good uniformity of thermal electrode; It is the use of temperature - 200 ~ 200 ℃, due to the copper thermoelectric extremely easy oxidation, and the oxide film is easy to fall off, so when used in the oxidizing atmosphere, generally should not exceed 300 ℃, within the scope of the - 200 ~ 300 ℃, they are sensitive degree is higher, copper-constantan thermocouple and a characteristic is cheap, is a common several kinds of finalize the design product in one of the cheapest. 7, iron - constantan thermocouple (index number for J) J type thermocouple, the thermocouple is extremely pure iron, negative extremely constantan (copper nickel alloy), the characteristic is cheap, suitable for vacuum oxidation reduction or inert atmosphere, the temperature range from 200 ~ 200 ℃, but commonly used temperature is below 500 ℃, because when above this temperature, the oxidation rate to speed up the iron is hot electrode, such as use coarse wire diameter wire, can be used in high temperature and long service life; The thermocouple capable of hydrogen (H2) and carbon monoxide (CO) gas corrosion, but not in high temperature (500 ℃), such as sulfur (S) used in the atmosphere. 8, nickel chromium nickel - copper (constantan thermocouple (index number is E) E type thermocouple is a relatively new product, it is nickel chromium alloy anode, the cathode is copper and nickel alloy (constantan), its biggest characteristic is in commonly used thermocouple, the thermoelectric potential is the largest, the highest sensitivity; Its application range is less than K type widely, but the required high sensitivity, low thermal conductivity, big permissible resistance under the condition that is often chosen; Of conditions in use with K type are the same, but for containing high humidity atmosphere corrosion is not very sensitive. In addition to the above eight kinds of commonly used thermocouple, as a standard thermocouple and tungsten-rhenium thermocouples, platinum, rhodium thermocouple, iridium germanium series thermocouple, platinum, molybdenum thermocouple and nonmetallic material thermocouples, etc. Second, insulating tube working side of the thermocouple is firmly welded together, need to be protected with insulating tube heat between the electrodes. Many thermocouple insulation material, generally can be pided into two categories, organic and inorganic insulation, high temperature insulation must adopt inorganic substances, generally choose clay pipe insulation under 1000, under 1300 selects high aluminum tube, choose corundum tube under 1600. Third, protect the pipe protection is to use the thermocouple electrode is not direct contact with the medium under test, it not only can prolong the life of thermocouple, heat may also play a supporting and fixed electrode to increase its strength; Thermocouple protection tubes, and insulation, therefore, to choose whether or not appropriate, will directly affect the service life of the thermocouple and the measurement accuracy, used for protection tubes are two main types of metal and nonmetal materials.