What are the types of thermocouples?

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 temperature of the medium from 0 to 1300 ℃. It is suitable for continuous use in oxidizing and inert gases, with a short-term use temperature of 1200 ℃ and a long-term use temperature of 1000 ℃. The relationship between its thermoelectric potential and temperature is approximately linear, and it is currently the most widely used thermocouple. However, it is not suitable for bare wire use in vacuum, sulfur-containing, carbon containing atmospheres, and alternating oxidation-reduction atmospheres; When the oxygen partial pressure is low, chromium in the nickel chromium electrode will preferentially oxidize, causing a significant change in thermoelectric potential. However, the influence of metal gas on it is relatively small, so metal protective tubes are often used.
Disadvantages of K-type thermocouple:
(1) The high-temperature stability of thermoelectric potential is inferior to that of N-type thermocouples and precious metal thermocouples, and it is often damaged due to oxidation at higher temperatures (such as over 1000 ℃);
(2) The short-term thermal cycling stability is not good within the range of 250-500 ℃, that is, at the same temperature point, the thermoelectric potential readings are different during the heating and cooling process, and the difference can reach 2-3 ℃;
(3) The negative electrode undergoes magnetic transformation within the temperature range of 150-200 ℃, resulting in deviations from the calibration table in the temperature range of room temperature to 230 ℃. Especially when used in a magnetic field, time independent thermoelectric interference often occurs;
(4) Long term exposure to high-throughput system irradiation environment can cause significant changes in thermoelectric potential due to the degradation of elements such as manganese (Mn) and cobalt (Co) in the negative electrode, resulting in poor stability.

2. S-type thermocouple (platinum rhodium 10 platinum thermocouple)
The positive electrode of this thermocouple is composed 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 an oxidizing atmosphere, long-term use temperature can reach 1300 ℃. Even in air, pure platinum wire will recrystallize, causing coarse grains and fracture when exceeding 1400 ℃;
(2) High precision, with an accuracy level of * * among all thermocouples, usually used as a standard or for measuring higher temperatures;
(3) Widely applicable, with good uniformity and interchangeability;
(4) The main disadvantages are: the differential thermoelectric potential is small, resulting in low sensitivity; Expensive price, low mechanical strength, not
Suitable for use under reducing atmosphere or conditions with metal vapor.

3. E-type thermocouple (nickel chromium copper nickel [constantan] thermocouple)
E-type thermocouple is a relatively new product, with nickel chromium alloy as the positive electrode and copper nickel alloy (constantan) as the negative electrode. Its * * feature is that in commonly used thermocouples, its thermoelectric potential * *, i.e. sensitivity * *; Although its application range is not as wide as K-type thermocouple, it is often selected under conditions that require high sensitivity, low thermal conductivity, and tolerance for high resistance; The limiting conditions during use are the same as those of K-type, but it is not very sensitive to corrosion in environments with higher humidity.

4. N-type thermocouple (nickel chromium silicon nickel silicon thermocouple)
The main features of this thermocouple are: strong temperature and oxidation resistance below 1300 ℃, good long-term stability and short-term thermal cycle reproducibility, good resistance to nuclear radiation and low temperature performance. In addition, within the range of 400-1300 ℃, the linearity of the thermoelectric characteristics of N-type thermocouples is better than that of K-type thermocouples; However, the nonlinear error is relatively large in the low temperature range (-200~400 ℃), and 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). It is characterized by its low price and is suitable for vacuum oxidation reduction or inert atmosphere. The temperature range is from -200 to 800 ℃, but the commonly used temperature is only below 500 ℃, because beyond this temperature, the oxidation rate of the ferroelectric electrode is accelerated. If thick wire diameter is used, it can still be used at high temperatures and has a longer service life; This thermocouple is capable of withstanding 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 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:
In base metal thermocouples, its accuracy and uniformity of the thermoelectric electrode are good; Its operating temperature is -200~350 ℃. Due to the easy oxidation and detachment of the oxide film of the copper hot electrode, it should generally not exceed 300 ℃ when used in an oxidizing atmosphere. Within the range of -200~300 ℃, they have relatively high sensitivity. Another characteristic of copper constantan thermocouples is that they are inexpensive and are the cheapest among several commonly used standardized products.

7. R-type thermocouple (platinum rhodium 13 platinum thermocouple)
The positive electrode of this thermocouple is made of 13% platinum rhodium alloy, and the negative electrode is pure platinum. Compared with the S-type, its potential rate is about 15% higher, and its other properties are almost the same. This type of thermocouple is widely used in the Japanese industry as a high-temperature thermocouple, while it is less commonly used in China.