Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. Standard thermocouple refers to a thermocouple whose national standard specifies the relationship between its thermoelectric potential and temperature, the allowable error, and has a unified standard graduation meter. It has a matching display instrument for selection. Non-standardized thermocouples are inferior to standardized thermocouples in the scope of use or order of magnitude, and there is generally no unified indexing table, which is mainly used for measurement in some special occasions. Since January 1, 1988 in my country, thermocouples and thermal resistances have all been produced in accordance with IEC international standards, and seven standardized thermocouples, S, B, E, K, R, J, and T, are designated as my country’s unified design thermocouples.
The indexing numbers of thermocouples are mainly S, R, B, N, K, E, J, T and so on. Among them, S, R, B belong to precious metal thermocouples, and N, K, E, J, T belong to cheap metal thermocouples:
S platinum rhodium 10 pure platinum
R platinum rhodium 13 pure platinum
B platinum rhodium 30 platinum rhodium 6
K nickel chromium nickel silicon
T pure copper copper nickel
J iron copper nickel
N Ni-Cr-Si Ni-Si
E nickel-chromium copper-nickel
1. (S-type thermocouple) platinum rhodium 10-platinum thermocouple
The platinum rhodium 10-platinum thermocouple (S-type thermocouple) is a precious metal thermocouple. The diameter of the couple wire is specified as 0.5mm, and the allowable deviation is -0.015mm. The nominal chemical composition of the positive electrode (SP) is platinum-rhodium alloy, which contains 10% rhodium, 90% platinum, and the negative electrode (SN) is pure platinum. Commonly known as single platinum rhodium thermocouple. The long-term maximum use temperature of this thermocouple is 1300℃, and the short-term maximum use temperature is 1600℃.
2. (R type thermocouple) platinum rhodium 13-platinum thermocouple
The platinum rhodium 13-platinum thermocouple (type R thermocouple) is a precious metal thermocouple. The diameter of the couple wire is specified as 0.5mm, and the allowable deviation is -0.015mm. The nominal chemical composition of the positive electrode (RP) is platinum-rhodium alloy, which contains 13% rhodium, 87% platinum, and the negative electrode (RN) is pure platinum. The maximum operating temperature is 1300°C, and the short-term maximum operating temperature is 1600°C.
3. (B type thermocouple) platinum rhodium 30-platinum rhodium 6 thermocouple
The platinum rhodium 30-platinum rhodium 6 thermocouple (type B thermocouple) is a precious metal thermocouple. The diameter of the couple wire is specified as 0.5mm, and the allowable deviation is -0.015mm. The nominal chemical composition of the positive electrode (BP) is platinum-rhodium alloy, which contains 30% rhodium and 70% platinum, and the negative electrode (BN) is platinum-rhodium alloy. The rhodium content is 6%, so it is commonly called double platinum rhodium thermocouple. The long-term maximum use temperature of this thermocouple is 1600℃, and the short-term maximum use temperature is 1800℃.
S, R, and B thermocouples have the highest accuracy, best stability, wide temperature measurement range, and long service life among the thermocouple series. It has good physical and chemical properties, thermoelectric potential stability and good oxidation resistance at high temperatures, and is suitable for oxidizing and inert atmospheres. The shortcomings of these three types of thermocouples are the thermoelectric potential, which has a low thermoelectric potential rate, low sensitivity reading, reduced mechanical strength at high temperatures, and is very sensitive to pollution, and precious metal materials are expensive, so one-time investment is relatively large.
4. (K-type thermocouple) Ni-Cr-Ni-Si thermocouple
Nickel-chromium-nickel-silicon thermocouple (K-type thermocouple) is currently the largest amount of cheap metal thermocouple, and its amount is the sum of other thermocouples. The nominal chemical composition of the positive electrode (KP) is: Ni:Cr=90:10, the nominal chemical composition of the negative electrode (KN) is: Ni:Si=97:3, and its operating temperature is -200~1300℃.
5. (N-type thermocouple) Ni-Cr-Si-Ni-Si thermocouple
Nickel-chromium-silicon-nickel-silicon thermocouple (N-type thermocouple) is a cheap metal thermocouple. It is a kind of the latest international standardized thermocouple. It was successfully developed by the Australian Ministry of Defense laboratory in the early 1970s. It overcomes the K-type thermocouple. Two important shortcomings of the couple: The thermoelectromotive force of the K-type thermocouple is unstable due to the short-range order of the nickel-chromium alloy between 300 and 500℃; the thermoelectromotive force caused by the preferential oxidation of the nickel-chromium alloy at about 800℃ Unstable. The nominal chemical composition of the positive electrode (NP) is: Ni:Cr:Si=84.4:14.2:1.4, and the nominal chemical composition of the negative electrode (NN) is: Ni:Si:Mg=95.5:4.4:0.1, and its operating temperature is -200 ~1300°C.
The K and N thermocouples have the advantages of good linearity, large thermoelectromotive force, high sensitivity, good stability and uniformity, strong oxidation resistance, low price, and not affected by short-range ordering, and their comprehensive performance is excellent The K-type thermocouple is a kind of thermocouple with a promising future. These two types of thermocouples cannot be directly used in sulfur, reducing or reducing, oxidizing alternate atmospheres and vacuum at high temperatures, nor are they recommended for use in weakly oxidizing atmospheres.
6. (E-type thermocouple) nickel-chromium-copper-nickel thermocouple
Nickel-chromium-copper-nickel thermocouple (type E thermocouple), also known as nickel-chromium-constantan thermocouple, is also a kind of cheap metal thermocouple. The positive electrode (EP) is: nickel-chromium 10 alloy, the chemical composition is the same as that of KP, the negative electrode (EN) is a copper-nickel alloy with nominal chemical composition: 55% copper, 45% nickel and a small amount of manganese, cobalt, iron and other elements. The operating temperature of this thermocouple is -200~900℃.
The E-type thermocouple has the largest thermoelectromotive force and the highest sensitivity among all thermocouples. It is suitable to be made into a thermopile to measure small temperature changes. It is not very sensitive to corrosion in high-humidity atmospheres and is suitable for high-humidity environments. E thermocouple also has the advantages of good stability, better oxidation resistance than copper-constantan and iron-constantan thermocouples, and low price. It can be used in oxidizing and inert atmospheres and is widely used by users. E-type thermocouples cannot be directly used for sulfur at high temperatures, and the uniformity of thermoelectric potential is poor in a reducing atmosphere.
7. (J-type thermocouple) iron-copper-nickel thermocouple
Iron-copper-nickel thermocouple (J-type thermocouple), also known as iron-constantan thermocouple, is also a cheap metal thermocouple. The nominal chemical composition of its positive electrode (JP) is pure iron, and the negative electrode (JN) is a copper-nickel alloy, which is often vaguely called constantan. Its nominal chemical composition is: 55% copper, 45% nickel, and a small amount But very important elements such as manganese, cobalt, iron, etc., although it is called constantan, it is different from the constantan of nickel-chromium-constantan and copper-constantan, so EN and TN cannot be used to replace it. The coverage measurement temperature range of the iron-constantan thermocouple is -200~1200℃, but the temperature range usually used is 0~750℃.
