How many types of thermocouples are there?

1. Nickel-chromium-silicon-nickel-silicon-magnesium thermocouple (N-type thermocouple) is a low-cost metal thermocouple and a standardized thermocouple:
1. Chemical composition: 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. Its application temperature range is -20~1300℃.
2. Technology: N-type thermocouple has the advantages of larger thermoelectric potential, higher sensitivity, better stability and uniformity, strong antioxidant performance, and is not affected by short-range ordering. Its comprehensive performance is better than that of K-type thermocouple. N-type thermocouple cannot be used directly in sulfur, reductive or reductive, oxidative alternating atmospheres and vacuum at low temperatures, and is not recommended for use in weak oxidizing atmospheres.
Nickel-chromium-silicon-nickel-silicon-magnesium thermocouple (N-type thermocouple) is a low-cost metal thermocouple and a standardized thermocouple. 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. Its application temperature range is -20~1300℃.
3. Features: N-type thermocouple has the advantages of larger thermoelectric potential, higher sensitivity, better stability and uniformity, strong antioxidant performance, and is not affected by short-range ordering. Its comprehensive performance is better than that of K-type thermocouple. N-type thermocouple cannot be used directly in sulfur, reductive or reductive, oxidative alternating atmospheres and vacuum at low temperatures, and is not recommended for use in weak oxidizing atmospheres.
2. Iron-copper-nickel thermocouple (J-type thermocouple) is also called iron-constantan thermocouple:
1. Chemical composition: The nominal chemical identity of its positive electrode (JP) is pure iron, and the negative electrode (JN) is a copper-nickel alloy, which is often ambiguously called constantan. Its nominal chemical identity is 55% copper and 45% nickel, as well as a small amount of very important elements such as cobalt, iron, and manganese. Although it is called constantan, it is different from constantan of nickel-chromium-constantan and copper-constantan, so it cannot be replaced by EN or TN. The iron-constantan thermocouple covers a measurement temperature range of -210~1200℃, but the temperature range of normal use is 0-750℃.
2. Technology: J-type thermocouple has the advantages of good linearity, large thermoelectric potential, high sensitivity, good stability and uniformity.
3. Features: J-type thermocouples can be used in vacuum, oxidation, recovery and inert atmospheres, but the positive iron oxidizes faster at low temperatures, so the application temperature is limited, and it cannot be used directly in a sulfiding atmosphere at a low temperature of 500°C without maintenance.
3. Nickel-chromium-copper-nickel thermocouple (E-type thermocouple), also known as nickel-chromium-constantan thermocouple:
1. Chemical composition: Nickel-chromium-copper-nickel thermocouple (E-type thermocouple), is also a cheap metal thermocouple. Its positive electrode (EP) is nickel-chromium 10 alloy, with the same chemical identity as KP, and its negative electrode (EN) is copper-nickel alloy, with a nominal chemical identity of 55% copper, 45% nickel, and a small amount of cobalt, manganese, iron and other elements. The application temperature of this thermocouple is -200~900°C.
2. Technology: The E-type thermocouple has the largest electromotive force and the highest sensitivity of all thermocouples. It is suitable for making thermopiles to measure small temperature changes. It is not very sensitive to corrosion in high humidity atmosphere and is suitable for use in environments with high humidity.
3. Features: E-type thermocouples also have the advantages of good stability, better antioxidant performance than copper-constantan and iron-constantan thermocouples, and low price. They can be used in oxidizing and inert atmospheres and are widely adopted by users. E-type thermocouples cannot be used directly in sulfur and reductive atmospheres at low temperatures, and their thermoelectric uniformity is poor.
4. Copper-copper-nickel thermocouple (T-type thermocouple), also known as copper-constantan thermocouple, is a perfect low-cost metal thermocouple for measuring high temperatures:
1. Chemical composition: Its positive electrode (TP) is pure copper and its negative electrode (TN) is copper-nickel alloy, often called constantan. It is compatible with constantan EN of nickel-chromium-constantan, but not with constantan JN of iron-constantan, although they are all called constantan. The measurement temperature range of copper-copper-nickel thermocouple is -200~350℃.
2. Technology: T-type thermocouple has the advantages of good linearity, large thermoelectric potential, high sensitivity, good stability and uniformity, low price, etc. It is especially used in the temperature range of -200~0℃, with better stability. The annual stability can be less than ±3μV. After high temperature calibration, it can be used as a secondary standard for high temperature value reporting.
3. Features: The positive copper of T-type thermocouple has poor antioxidant performance at low temperature, so the lower limit of application temperature is limited.
Nickel-chromium-nickel-silicon (K type) thermocouple
·Nickel-chromium-copper-nickel (Constantan) (E type) thermocouple
·Iron-copper-nickel (Constantan) (J type) thermocouple
·Copper-copper-nickel (Constantan) (T type) thermocouple
·Nickel-chromium-silicon-nickel-silicon-magnesium (N type) thermocouple
4. Function:
Cheap metal thermoelectric alloy has the characteristics of high thermoelectric potential, linear relationship between temperature and thermoelectric potential, good long-term stability of thermoelectric potential, etc., which meets the relevant standards of the International Electrotechnical Commission (IEC).
Main uses:
Widely used as temperature measuring elements in defense, scientific research, metallurgy, chemical industry, electric power, instrumentation and other departments.
Thermocouple temperature measurement range and tolerance
Name Model Maximum application temperature range (℃) Permitted deviation (℃)
Temporary Short-term
Grade
Temperature range and tolerance
Nickel-chromium-nickel-silicon GB/T2614
K 1200
1300 I
-40~1100 ±1.5℃ or ±0.4%t
II
-40~1300 ±2.5℃ or ±0.75%t
Nickel-chromium-silicon-nickel-silicon-magnesium GB/T17615
N 1200 1300 I
-40~1100 ±1.5℃ or ±0.4%t
II -40~1300 ±2.5℃ or ±0.75%t
Nickel-chromium-copper-nickel GB/T4993
E 750 900 I -40~800 ±1.5℃ or ±0.4%t
II -40~900 ±2.5℃ or ±0.75%t
Iron-copper-nickel GB/T4994
J 600 750 I 0~750 ±1.5℃ or ±0.4%t
II ±2.5℃ or ±0.75%t
Copper-copper-nickel GB/T2903
T 300 350 I -40~350 ±0.5℃ or ±0.4%t
II ±1℃ or ±0.75%t
III -200~40 ±1℃ or ±1.5%t
The maximum application temperature range refers to the following specifications: K, N, E, T type: 3.2mm, T: 2.0mm.
K-type thermocouple has the following defects:
1. The low temperature stability of thermoelectric potential is worse than that of N-type thermocouple and precious metal thermocouple. At lower temperatures, it is often damaged by oxidation. In an oxidizing atmosphere, a 3.2mm diameter K-type thermocouple exceeds the 0.75 level tolerance after about 650h at 1100℃ and 1200℃; however, under the same conditions, the maximum change in thermoelectric potential of an N-type thermocouple after 1000h is 96.6μV (2.6℃). It is still within tolerance after 1000h at 1250℃.
2. The short-term thermal cycle stability is poor in the range of 250-550℃. Even at the same temperature point, the thermoelectric potential value is different during the temperature rise and fall process, and the difference can reach 2-5℃.
3. The negative pole of the K-type thermocouple will produce magnetic changes in the range of 150-200℃, so that in the range of room temperature to 230℃, the graduation value often deviates from the graduation table, especially when used in a magnetic field, and time-related thermoelectric potential disturbances often occur.
4. When temporarily exposed to high flux neutron flux, the Mn, Co and other elements in the negative electrode undergo changes, making them less stable.