Thermocouples are commonly used temperature measuring components in temperature measuring instruments and are widely used in daily life.
This article is mainly about the introduction of thermocouples, and an in-depth discussion on thermocouple models and temperature measurement ranges.
Thermocouple type classification
Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to the thermocouple whose thermoelectric power and temperature are stipulated in the national standard, the allowable error, and there is a unified standard graduation table. It has a matching display instrument for selection. Non-standardized thermocouples are inferior to standardized thermocouples in terms of use range or order of magnitude, and there is generally no unified indexing table, which is mainly used for measurement in some special occasions. Standardized thermocouples. Since January 1, 1988, all thermocouples and thermal resistances have been produced in accordance with IEC international standards, and seven standardized thermocouples of S, B, E, K, R, J, and T are designated as my country’s unified design. Thermocouple.
The indexing numbers of thermocouples are mainly S, R, B, N, K, E, J, T and so on. Among them, S, R, and B belong to precious metal thermocouples, and N, K, E, J, and T belong to low-cost metal thermocouples.
The following is an explanation of the thermocouple index number
S platinum rhodium 10 pure platinum
R platinum rhodium 13 pure platinum
B platinum rhodium 30 platinum rhodium 6
K NiCr NiSi
T Pure copper Copper nickel
J iron copper nickel
N NiCrSi NiSi
E nickel-chromium copper-nickel
(S-type thermocouple) platinum rhodium 10-platinum thermocouple
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℃.
S-type thermocouple has the highest accuracy, best stability, wide temperature measurement zone, long service life, etc. in 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. Because of the excellent comprehensive performance of the S-type thermocouple, the S-type thermocouple that meets the international temperature scale has long been used as an interpolating instrument for the international temperature standard. Although "ITS-90" stipulates that it will no longer be an internal inspection instrument for the international temperature standard in the future, However, the International Temperature Advisory Committee (CCT) believes that S-type thermocouples can still be used to approximate international temperature standards.
S-type thermocouple has the disadvantage of thermoelectric potential. The thermoelectric potential rate is small, the sensitive reading is low, the mechanical strength decreases at high temperature, it is very sensitive to pollution, and the precious metal materials are expensive, so the one-time investment is relatively large.
(R type thermocouple) platinum rhodium 13-platinum thermocouple
Platinum rhodium 13-platinum thermocouple (R 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 (RP) is platinum-rhodium alloy, which contains 13% rhodium, 87% platinum, and the negative electrode (RN) is pure platinum. The maximum use temperature is 1300℃, and the short-term maximum use temperature is 1600℃.
R-type thermocouple has the highest accuracy, best stability, wide temperature measurement range, and long service life in 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. Because the comprehensive performance of R-type thermocouple is equivalent to that of S-type thermocouple, it has been difficult to promote in our country. Except for temperature measurement on imported equipment, domestic temperature measurement is rarely used. From 1967 to 1971, the three major research institutions of the United Kingdom NPL, the United States NBS and the Canadian NRC conducted a cooperative study. The results showed that the stability and reproducibility of the R-type thermocouple are better than those of the S-type thermocouple. Research in this area has not yet been carried out.
R-type thermocouple has the disadvantage of thermoelectric potential. The thermoelectric potential rate is small, the sensitive reading is low, the mechanical strength decreases at high temperature, it is very sensitive to pollution, and the precious metal materials are expensive, so the one-time investment is relatively large.
(B-type thermocouple) platinum rhodium 30-platinum rhodium 6 thermocouple
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℃.
B type thermocouple has the highest accuracy, best stability, wide temperature measurement range, long service life, high temperature measurement upper limit, etc. in the thermocouple series. It is suitable for oxidizing and inert atmospheres, and also for short-term use in vacuum, but not suitable for reducing atmospheres or atmospheres containing metal or non-metal vapors. An obvious advantage of B-type thermocouple is that it does not need compensation wire for compensation, because the thermoelectric potential is less than 3μV in the range of 0~50℃.
B type thermocouple's shortcomings are the thermoelectric potential, the thermoelectric potential rate is small, the sensitive reading is low, the mechanical strength decreases at high temperature, it is very sensitive to pollution, and the precious metal materials are expensive, so the one-time investment is relatively large.
(K-type thermocouple) Ni-Cr-Ni-Si thermocouple
Ni-Cr-Ni-Si thermocouple (K-type thermocouple) is currently the largest amount of low-cost 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℃.
K-type thermocouple has the advantages of good linearity, large thermoelectromotive force, high sensitivity, good stability and uniformity, strong oxidation resistance, and low price. It can be used in oxidizing inert atmosphere. Widely adopted by users.
K-type 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.
(N-type thermocouple) Ni-Cr-Si-Ni-Si thermocouple
Nickel-chromium-silicon-nickel-silicon thermocouple (N-type thermocouple) is a low-cost 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℃.
The N-type thermocouple has 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. Its comprehensive performance is better than K Type thermocouple is a kind of thermocouple with a promising future.
N-type 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.
(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 low-cost metal thermocouple. The positive electrode (EP) is: nickel-chromium 10 alloy, the chemical composition is the same as KP, the negative electrode (EN) is a copper-nickel alloy with a nominal chemical composition of 55% copper, 45% nickel and a small amount of manganese, cobalt, iron and other elements. The operating temperature of this thermocouple is -200~900℃.
E-type thermocouple has large thermoelectromotive force and high sensitivity, which is the best among all thermocouples. It is suitable to be made into 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 thermocouple cannot be directly used for sulfur at high temperature. In a reducing atmosphere, the uniformity of thermoelectric potential is poor.
(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 instead. The coverage measurement temperature range of iron-constantan thermocouple is -200~1200℃, but the temperature range usually used is 0~750℃
J type thermocouple has the advantages of good linearity, large thermoelectromotive force, high sensitivity, good stability and uniformity, and low price, and it is widely used by users.
J-type thermocouple can be used in vacuum, oxidation, reduction and inert atmospheres, but the positive electrode iron oxidizes faster at high temperatures, so the use temperature is limited, and it cannot be used directly and unprotected in a vulcanizing atmosphere at high temperatures.
(T-type thermocouple) copper-copper-nickel thermocouple
Copper-copper-nickel thermocouple (T-type thermocouple), also known as copper-constantan thermocouple, is also the best thermocouple for measuring low temperature and cheap metals. Its positive electrode (TP) is pure copper, and the negative electrode (TN) is a copper-nickel alloy, which is often constantan. It is commonly used with Ni-Cr-Constantan constantan EN, but not with iron-Constantan constantan JN. Although they are called constantan, the measuring temperature range of the cover of the copper-copper-nickel thermocouple is -200~350℃.
T-type thermocouple has the advantages of good linearity, large thermoelectromotive force, high sensitivity, good stability and uniformity, and low price. Especially used in the temperature range of -200~0℃, it has better stability and annual stability. The performance can be less than ±3μV, and it can be used as a second-class standard for low temperature value transfer after low temperature verification.
T-type thermocouple positive copper has poor oxidation resistance at high temperatures, so the upper limit of the operating temperature is limited.
Thermocouple temperature measurement range
1. The temperature range of platinum rhodium-platinum thermocouple is 630.74~1064.43℃
2, Ni-Cr-Ni-Si thermocouple temperature range 0~900℃
3, tungsten-rhenium thermocouple temperature measurement range 300~2000℃
4, nickel-chromium-copper thermocouple temperature measurement range 0~600℃
5. Copper-Constantan thermocouple temperature measurement range -200~100℃
S-type thermocouple has the highest accuracy, best stability, wide temperature measurement range, and long service life in 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. Because of the excellent comprehensive performance of the S-type thermocouple, the S-type thermocouple, which conforms to the international temperature scale, has been used as an interpolating instrument for the international temperature scale for a long time. Although the rules of "ITS-90" will no longer be used as an internal inspection instrument for the international temperature scale, However, the International Temperature Advisory Committee (CCT) believes that the S-type thermocouple can still be used to approximate the international temperature scale.
S-type thermocouple lacks thermoelectric potential, low thermoelectric potential rate, flexible reading, low mechanical strength under high temperature, very sensitive to pollution, expensive precious metal materials, so one-time investment is relatively large. The information here is provided by Jiangsu Jinhu Automate Automation Instrumentation Factory for free, and I hope it can bring greater help to those in need
The temperature measurement range of various types of thermocouples
1. Type S thermocouple: platinum rhodium 10-platinum thermocouple, temperature range 0～1300℃; advantages: 1. heat resistance, stability, good reproducibility and superior accuracy; 2, oxidation and corrosion resistance Good turbidity; 3, can be used as a standard. Disadvantages: 1. The thermoelectromotive force value is small, and the compensation wire error is large; 2, the price is high; 3, it is more fragile in the regenerating gas environment. (Especially hydrogen, metal vapor)
2. R type thermocouple: platinum rhodium 13-platinum thermocouple, temperature range 0～1300℃; advantages: 1, heat resistance, stability, good reproducibility and superior accuracy; 2, oxidation resistance, corrosion resistance Good turbidity; 3, can be used as a standard. Disadvantages: 1. The thermal electromotive force value is small, and the compensation wire error is large; 2. It is fragile in the reducing gas environment (especially hydrogen and metal vapor); 4. The price is higher than the S scale.
3. Type B thermocouple: platinum rhodium 30-platinum rhodium 6 thermocouple, temperature range 0～1600℃; advantages: 1, oxidation resistance, good corrosion resistance; 2, thermoelectromotive force is very small under normal temperature environment, no need Compensation wire; 3, heat resistance and mechanical strength are better than R type. Disadvantages: 1. The thermoelectromotive force in the medium and low temperature range is extremely small, and the temperature measured below 600℃ is not accurate; 2, the thermoelectromotive force value is small, and the linearity of the thermoelectromotive force is not good; 3, the price is more expensive than S division;
4. K-type thermocouple: nickel-chromium-nickel-silicon thermocouple, temperature range 0～1300℃; advantages: 1. Good linearity of thermoelectromotive force; 2, good oxidation resistance below 1000℃; 3, in metal thermocouple Stability is good. Disadvantages: 1. The thermoelectromotive force changes greatly when compared with precious metal thermocouples; 2, it is not suitable for reducing gas environments; 3, errors will occur due to the influence of short-range sequencing.
V. N-type thermocouple: Ni-Cr-Si-Ni-Si thermocouple, temperature range -270～1300℃; advantages: 1. Good oxidation resistance below 1200℃. 2. The linearity of thermoelectromotive force is good. Disadvantages: 1. Not suitable for reducing gas environment. 2. The thermoelectromotive force changes greatly when compared with the precious metal thermocouple.
6. E-type thermocouple: Ni-Cr-Si-Constantan thermocouple, temperature range -270～1000℃ Advantages: 1. The best sensitivity among thermocouples; 2. Compared with J thermocouple, it has good heat resistance; 3. Suitable for oxidizing gas environment. 4. Low price Disadvantages: not suitable for reducing gas environment
7, J-type thermocouple: iron-constantan thermocouple, temperature range -210～1000℃; advantages: 1. It can be used in a reductive gas environment 2. The thermoelectromotive force is 20% larger than that of K thermocouple. 3. The price is cheaper and it is suitable for medium temperature area. Disadvantages: easy to rust, poor reproducibility.
8. T-type thermocouple: copper-constantan thermocouple, temperature range -270～400℃; advantages: 1. Good linearity of thermoelectromotive force. 2, good properties at low temperature 3, good reproducibility, high precision. Disadvantages: 1. The use temperature limit is low. 2. Large thermal conductivity error.
9. PT100 thermal resistance: platinum resistance, temperature range -200～500℃; The advantages of platinum materials are good chemical stability, high temperature resistance, and easy production of pure platinum. Its disadvantages are: in the reducing medium, especially in the It is easy to be contaminated by the steam reduced from the oxide at high temperature, making the platinum wire brittle and changing the resistance and