Common Thermocouple Types and Characteristics

1. K-type thermocouple nickel-chromium (nickel-silicon (nickel-aluminum) thermocouple)

K-type thermocouple is a base metal thermocouple with strong oxidation resistance. It can measure the medium temperature of 0-1300℃. It is suitable for continuous use in oxidizing and inert gases. The short-term use temperature is 1200℃ and the long-term use temperature is 1000℃. The relationship between its thermoelectric potential and temperature is approximately linear. It is the most widely used thermocouple at present. However, it is not suitable for bare wire use in vacuum, sulfur-containing, carbon-containing atmosphere and redox alternating atmosphere; when the oxygen partial pressure is low, the chromium in the nickel-chromium pole will be preferentially oxidized, causing a large change in the thermoelectric potential, but the metal gas has little effect on it. Therefore, 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. It is often damaged by oxidation at higher temperatures (for example, over 1000°C);

(2) The short-term thermal cycle stability is poor in the range of 250-500°C, 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°C;

(3) The magnetic transformation of its negative electrode occurs in the range of 150-200°C, causing the graduation value in the range of room temperature to 230°C to deviate from the graduation table, especially when used in a magnetic field, thermoelectric potential interference that is not related to time often occurs;

(4) When exposed to high-flux medium-system irradiation for a long time, due to the transformation of elements such as manganese (Mn) and cobalt (Co) in the negative electrode, its stability is poor, resulting in a large change in thermoelectric potential.

2. S-type thermocouple (platinum-rhodium 10-platinum thermocouple)

The positive electrode of this thermocouple is a 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℃, when it exceeds 1400℃, even in air, the pure platinum wire will recrystallize, making the grains coarse and broken;

(2) High precision, the highest accuracy level among all thermocouples, usually used as a standard or to measure higher temperatures;

(3) Wide range of use, good uniformity and interchangeability;

(4) The main disadvantages are: small differential thermoelectric potential, so low sensitivity; expensive price, low mechanical strength, not suitable for use in a reducing atmosphere or under conditions of metal vapor.

3. E-type thermocouple (nickel-chromium-copper-nickel [constantan] thermocouple)

E-type thermocouple is a relatively new product, with the positive electrode being nickel-chromium alloy and the negative electrode being copper-nickel alloy (constantan). Its biggest feature is that among the commonly used thermocouples, its thermoelectric potential is the largest, that is, its sensitivity is the highest; although its application range is not as wide as that of K-type couple, it is often selected under the conditions of high sensitivity, low thermal conductivity, and allowable large resistance; the restrictions in use are the same as those of K-type, but it is not very sensitive to corrosion in atmospheres with high humidity.

4. N-type thermocouple (nickel-chromium-silicon-nickel-silicon thermocouple)

The main features of this thermocouple: strong temperature control and oxidation resistance below 1300℃, good long-term stability and short-term thermal cycle reproducibility, good resistance to nuclear radiation and low temperature. In addition, in the range of 400-1300℃, the linearity of the thermoelectric characteristics of the N-type thermocouple is better than that of the K-type thermocouple; but in the low temperature range (-200-400℃), the nonlinear error is large, 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 cheap and suitable for vacuum oxidation in reducing or inert atmospheres. The temperature range is from -200 to 800℃, but the commonly used temperature is only below 500℃, because after exceeding this temperature, the oxidation rate of the iron thermocouple accelerates. If a thick wire is used, it can still be used at high temperatures and has a longer life; this thermocouple can resist hydrogen (H2) and carbon monoxide (CO) gas corrosion, but cannot be used in high temperature (such as 500℃) sulfur (S) atmosphere.

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 features are:

Among base metal thermocouples, it has the highest accuracy and good uniformity of the thermocouple; its operating temperature is -200~350℃. Because the copper thermocouple is easy to oxidize and the oxide film is easy to fall off, it is generally not allowed to exceed 300℃ when used in an oxidizing atmosphere. In the range of -200~300℃, their sensitivity is relatively high. Another feature of copper-constantan thermocouples is that they are cheap and are the cheapest of several commonly used standard products.

7. R-type thermocouple (platinum-rhodium 13-platinum thermocouple)

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