Common Thermocouple Types and Characteristics

Posted by freeamfva on April 4th, 2023

Common Thermocouple Types and Characteristics

There are several key types of thermocouple wire graduation numbers, such as S, R, B, N, K, E, J, T, etc. Among them, S, R, B belong to precious metal thermocouples, and N, K, E, J, T belong to cheap metal thermocouples.Get more news about K Type PEEK THERMOCOUPLE wire,you can vist our website!

The k-type thermocouple extension wire and cable is a base metal thermocouple with strong oxidation resistance, which can measure the temperature of the medium from 0 to 1300 °C. It is suitable for continuous use in oxidizing and inert gases, with a short-term use temperature of 1200 °C and a long-term use temperature of 1000 °C. 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 alternate redox atmospheres. When the oxygen partial pressure is low, the chromium in the nickel-chromium electrode will be preferentially oxidized, which will greatly change the thermoelectric potential, but the metal gas has little effect on it. Therefore, metal protection tubes are mostly used.
Disadvantages of type K thermocouple:

(1) The high-temperature stability of thermoelectric potential is worse than that of N-type thermocouples wire and precious metal thermocouples, and it is often damaged due to oxidation at higher temperatures (for example, over 1000 °C).

(2) The short-term thermal cycle stability is not good in the range of 250-500 °C, that is, at the same temperature point, in the process of heating and cooling, the thermoelectric potential is different, and the difference can reach 2-3 °C.

(3) The negative pole will undergo a magnetic transformation in the range of 150 to 200 °C, so that the graduation value in the range of room temperature to 230 °C often deviates from the graduation table, especially when used in a magnetic field, there is often time-independent thermoelectric interference.

(4) Under the high-flux medium irradiation environment for a long time, due to the metamorphism of manganese (Mn), cobalt (Co), and other elements in the negative electrode, its stability is not good, resulting in a large change in thermoelectric potential.
3. E-type thermocouple (nickel-chromium-copper-nickel [constantan] thermocouple)
E-type thermocouple compensating cable is a relatively new product, the positive electrode is a nickel-chromium alloy, and the negative electrode is a copper-nickel alloy (constantan). Its biggest feature is that among the commonly used thermocouples, its thermoelectric potential is the largest, that is, the sensitivity is the highest. Although its application range is not as wide as that of K-type couplers, it is often selected under the conditions of high sensitivity, low thermal conductivity, and large resistance that can be tolerated. Restrictions in use are the same as type K but are less sensitive to corrosion in atmospheres containing higher humidity.

4. N-type thermocouple (NiCrSi-NiSi thermocouple)
The main features of the thermocouple are strong anti-oxidation ability in temperature regulation below 1300 °C, good long-term stability and short-term thermal cycle repeatability, good nuclear radiation resistance, and low-temperature resistance. In addition, in the range of 400 ~ 1300 ℃, the linearity of the thermoelectric characteristics of the N-type thermocouple wire is better than that of the K-type thermocouple wire. However, in the low-temperature range (-200 ~ 400 ℃), the nonlinear error is large, and at the same time, the material is hard and difficult to process.

5. J-type thermocouple (iron-constantan thermocouple)
J-type Thermocouple extension and compensating cable: The positive electrode of this duplex thermocouple is pure iron, and the negative electrode is constantan (copper-nickel alloy), which is characterized by its low price. It is suitable for reducing or inert atmosphere for vacuum oxidation, and the temperature range is from -200 to 800 °C. However, the commonly used temperature is only below 500 °C, because the oxidation rate of the iron hot electrode is accelerated after this temperature is exceeded. If the wire with a thick wire diameter is used, it can still be used at high temperatures and has a long life. The thermocouple is resistant to hydrogen (H2) and carbon monoxide (CO) gas corrosion, but cannot be used in an atmosphere containing sulfur (S) at high temperatures (eg 500°C).