The thermocouple with the highest performance and accuracy among common thermocouples

The commonly used thermocouples include the following:
1. K-type thermocouple nickel chromium (nickel silicon (nickel aluminum) thermocouple)
K-type thermocouple is a base metal thermocouple with strong oxidation resistance, which can measure the medium temperature from 0 to 1300 ℃ and is suitable for use in oxygen
Continuous use in chemical and inert gases, with a short-term usage temperature of 1200 ℃ and a long-term usage temperature of 1000 ℃. The relationship between its thermoelectric potential and temperature is approximately linear, making it the most widely used thermocouple currently. However, it is not suitable for bare wire use in vacuum, sulfur-containing, carbon containing atmospheres, and alternating redox atmospheres; When the oxygen partial pressure is low, the chromium in the nickel chromium electrode will preferentially oxidize, causing a significant change in thermoelectric potential. However, the influence of metal gases on it is relatively small, so metal protective tubes are often used.
Disadvantages of K-type thermocouples:
(1) The high-temperature stability of thermoelectric potential is worse than that of N-type thermocouples and precious metal thermocouples, and at higher temperatures (such as super
Over 1000 ℃) is often damaged due to oxidation;
(2) Short term thermal cycling stability is poor within the range of 250-500 ℃, that is, at the same temperature point, during the heating and cooling process
Among them, the thermoelectric potential readings are different, with a difference of up to 2-3 ℃;
(3) The negative electrode undergoes magnetic transformation within the range of 150-200 ℃, resulting in a division value between room temperature and 230 ℃
Often deviating from the graduation table, especially when used in a magnetic field, there is often time independent thermoelectric interference;
(4) Long term exposure to high-throughput medium system irradiation environment, due to the presence of elements such as manganese (Mn) and cobalt (Co) in the negative electrode
Metamorphosis occurs, resulting in poor stability and significant changes in thermoelectric potential.
2. S-type thermocouple (platinum rhodium 10 platinum thermocouple)
The positive electrode of the thermocouple is made of platinum rhodium alloy containing 10% rhodium, and the negative electrode is pure platinum.
Its characteristics are:
(1) Stable thermoelectric performance, strong oxidation resistance, suitable for continuous use in oxidizing atmospheres, and can reach temperatures for long-term use
At 1300 ℃ and above 1400 ℃, even in the air, pure platinum wire will recrystallize, causing coarse and fractured grains;
(2) High precision, with the highest accuracy level among all thermocouples, commonly used as a standard or for measuring higher temperatures;
(3) Wide range of use, good uniformity and interchangeability;
(4) The main drawbacks are: the differential thermoelectric potential is small, resulting in lower sensitivity; The price is relatively expensive, the mechanical strength is low, and it is not
Suitable for use in reducing atmospheres or under conditions with metal vapor.

3. E-type thermocouple (nickel chromium copper nickel [constantan] thermocouple)
The E-type thermocouple is a relatively new product, with a positive electrode made of nickel chromium alloy and a negative electrode made of copper nickel alloy (constantan). Its biggest feature
Among commonly used thermocouples, the thermoelectric potential is the highest, indicating the highest sensitivity; Although its application range is not as wide as that of K-type couples, it is often chosen under conditions that require high sensitivity, low thermal conductivity, and the ability to tolerate large resistances; The restrictions during use are the same as those for K-type, but they are not very sensitive to corrosion in environments with high humidity.
4. N-type thermocouple (nickel chromium silicon nickel silicon thermocouple)
The main characteristics of this thermocouple are strong temperature regulation and oxidation resistance below 1300 ℃, long-term stability, and short-term thermal cycling
Good performance, nuclear radiation resistance, and low temperature resistance. In addition, within the range of 400-1300 ℃, the linear thermoelectric characteristics of N-type thermocouples are better than those of K-type thermocouples; But in the low temperature range (-200~400 ℃), there is a large nonlinear error, and at the same time, the material is hard and difficult to process.
5. J-type thermocouple (iron constantan thermocouple)
J-type thermocouple: The positive electrode of this thermocouple is pure iron, and the negative electrode is constantan (copper nickel alloy), which is characterized by its low price and suitability
Used in reducing or inert atmospheres for vacuum oxidation, with a temperature range of -200 to 800 ℃, but commonly used temperatures are only below 500 ℃ because beyond this temperature, the oxidation rate of the ferroelectric electrode accelerates. If thick wire diameter is used, it can still be used at high temperatures and has a longer lifespan; This thermocouple is resistant to corrosion from hydrogen (H2) and carbon monoxide (CO) gases, but cannot be used in high temperature (such as 500 ℃) sulfur-containing (S) atmospheres.
6. T-type thermocouple (copper nickel thermocouple)
T-type thermocouple: The positive electrode of this thermocouple is pure copper, and the negative electrode is copper nickel alloy (also known as constantan). Its main characteristics are:
Among base metal thermocouples, it has the highest accuracy and good uniformity of thermoelectric electrodes; Its operating temperature is -200~350 ℃. Due to the easy oxidation of copper hot electrodes and the easy detachment of the oxide film, it is generally not allowed to exceed 300 ℃ when used in an oxidizing atmosphere. Within the range of -200~300 ℃, their sensitivity is relatively high. Another characteristic of copper constantan thermocouples is their low price, which is the cheapest among several commonly used standardized products.
7. R-type thermocouple (platinum rhodium 13-platinum thermocouple)
The positive electrode of the thermocouple is a 13% platinum rhodium alloy, and the negative electrode is pure platinum. Compared with the S-type, its potential rate is 15% higher
Left and right, other performance is almost the same. This type of thermocouple is most commonly used as a high-temperature thermocouple in the Japanese industry, while it is less commonly used in China.