Common Thermocouple Types and Characteristics

1. K-Type Thermocouple (NiCr, NiSi, or NiAl)

The K-type thermocouple is a base metal thermocouple with strong oxidation resistance. It can measure media temperatures from 0 to 1300°C and is suitable for continuous use in oxidizing and inert gases, with a short-term operating temperature of 1200°C and a long-term operating temperature of 1000°C. Its thermoelectric potential (TEV) has a nearly linear relationship with temperature, making it the most widely used thermocouple. However, it is not suitable for bare wire use in vacuum, sulfur-containing or carbon-containing atmospheres, or atmospheres with alternating redox cycles. When the oxygen partial pressure is low, the chromium in the NiCr electrode will preferentially oxidize, causing a significant change in the thermoelectric potential. However, this is less affected by metallic gases, so a metal protective tube is 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℃);

(2) The short-term thermal cycle stability is poor in the range of 250-500℃, that is, at the same temperature point, its thermoelectric potential reading is different during the heating and cooling process, and the difference can reach 2-3℃;

(3) Its negative electrode undergoes magnetic transformation in the range of 150-200℃, resulting in the scale value in the range of room temperature to 230℃ often deviating from the scale, 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 decay 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 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, thus low sensitivity; high price, low mechanical strength, not suitable for use in reducing atmosphere or under conditions with metal vapor.

3. Type E Thermocouple (Nickel-Chromium-Copper-Nickel [Constantan] Thermocouple)

The Type E thermocouple is a relatively new product, with a positive electrode made of a nickel-chromium alloy and a negative electrode made of a copper-nickel alloy (Constantan). Its greatest characteristic is that it has the highest thermoelectric potential (TEV) and sensitivity among commonly used thermocouples. While not as widely used as the Type K couple, it is often chosen when high sensitivity, low thermal conductivity, and a high resistance tolerance are required. Its limitations are the same as those for the Type K couple, but it is less sensitive to corrosion in atmospheres containing high humidity.

4. N-type Thermocouple (NiCrSi-NiSi Thermocouple)

This thermocouple's main features include strong temperature control and oxidation resistance below 1300°C, excellent long-term stability and short-term thermal cycling reproducibility, and excellent resistance to nuclear radiation and low temperatures. Furthermore, within the 400-1300°C range, the linearity of the thermoelectric characteristics of the N-type thermocouple is better than that of the K-type thermocouple. However, in the low-temperature range (-200-400°C), the nonlinear error is larger. Furthermore, the material is relatively hard and difficult to machine.

5. J-Type Thermocouple (Iron-Constantan Thermocouple)

The J-Type thermocouple has a positive electrode made of pure iron and a negative electrode made of constantan (a copper-nickel alloy). It is economical and suitable for vacuum oxidation in reducing or inert atmospheres. Its temperature range is -200°C to 800°C, but it is commonly used below 500°C. Above this temperature, the oxidation rate of the iron thermocouple accelerates. Using thicker wire allows for high-temperature operation and a longer lifespan. This thermocouple is resistant to corrosion from hydrogen (H2) and carbon monoxide (CO) gases, but cannot be used in high-temperature (e.g., 500°C) atmospheres containing sulfur (S).

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 a copper-nickel alloy (also known as Constantan). Its main features are:

Among base metal thermocouples, it has the highest accuracy and best thermocouple uniformity. Its operating temperature is -200 to 350°C. Because copper thermocouples are easily oxidized and the oxide film easily falls off, they are generally not used above 300°C in oxidizing atmospheres. Within the -200 to 300°C range, their sensitivity is relatively high. Copper-Constantan thermocouples are also inexpensive, making them 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 13% platinum-rhodium alloy, and the negative electrode is pure platinum. Compared to the S-type, its electrochemical specific resistance is approximately 15% higher, while other performance is nearly identical. This type of thermocouple is most commonly used in Japanese industry as a high-temperature thermocouple, but is less commonly used in China.