When we choose a thermocouple, we mainly choose the thermocouple suitable for our use through our own use environment and the temperature range that we need to measure. In terms of specifications, all thermocouples can basically be customized. The following are several very common thermocouples: 1. K-type thermocouple Ni-Cr (Ni-Si (Ni-Al) thermocouple) K-type thermocouple is a base metal thermocouple with strong oxidation resistance, which can measure 0～1300℃ The temperature of the medium is suitable for continuous use in oxidizing and inert gases. The short-term use temperature is 1200°C and the long-term use temperature is 1000°C. The relationship between the thermoelectric potential and the temperature is approximately linear, and it is currently a large thermocouple. However, it is not suitable for bare wire use in a vacuum, sulfur-containing, carbon-containing atmosphere, and an alternating redox atmosphere; when the oxygen partial pressure is low, the chromium in the nickel-chromium electrode will be oxidized preferentially, causing a great change in the thermoelectric potential , But metal gas has little effect on it, so metal protection tubes are mostly used. Disadvantages of K-type thermocouple: (1) The high-temperature stability of thermoelectric potential is worse than that of N-type thermocouple and precious metal thermocouple, and it is often damaged by oxidation at higher temperatures (for example, over 1000°C); (2) In 250～ The short-term thermal cycle stability in the range of 500℃ is not good, that is, at the same temperature point, during the heating and cooling process, its thermoelectric potential shows different values, and the difference can reach 2～3℃; (3) The negative electrode is at 150～ Magnetic transformation will occur in the range of 200°C, so that the graduation value from room temperature to 230°C often deviates from the graduation table, especially when used in a magnetic field, there is often time-independent thermoelectric potential interference; (4) Long-term high-pass Under the irradiation environment of the medium-quantity system, due to the degradation of elements such as manganese (Mn) and cobalt (Co) in the negative electrode, its stability is not good, resulting in a large change in the thermoelectric potential. 2. S-type thermocouple (platinum rhodium 10-platinum thermocouple) The positive electrode component of this thermocouple is 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 the temperature exceeds 1400 ℃, even in the air, pure platinum wire will Recrystallization, making the crystal grains coarse and broken; (2) High precision, high accuracy level in all thermocouples, usually used as a standard or measuring higher temperature; (3) Wide range of use, uniformity and interchangeability Good performance; (4) The main shortcomings are: the differential thermoelectric potential is small, so the sensitivity is low; the price is more expensive, the mechanical strength is low, and it is not suitable for use in a reducing atmosphere or metal vapor. 3. E-type thermocouple (nickel-chromium-copper-nickel [constantan] thermocouple) E-type thermocouple is a relatively new product, the positive electrode is nickel-chromium alloy, and the negative electrode is copper-nickel alloy (constantan). Its great feature is that among the commonly used thermocouples, its thermoelectric potential is very large, that is, the sensitivity is very high; although its application range is not as wide as the K-type couple, it is required for high sensitivity, low thermal conductivity, and large resistance. Under the conditions, it is often selected; the restrictive conditions in use are the same as the K type, but it is not very sensitive to corrosion in an atmosphere containing a higher humidity. 4. N-type thermocouple (Ni-Cr-Si-Ni-Si thermocouple) The main features of this thermocouple: strong anti-oxidation ability under 1300℃, good long-term stability and short-term thermal cycle reproducibility, resistance to nuclear radiation and The low temperature resistance is good. In addition, the linearity of the thermoelectric characteristics of the N-type thermocouple is better than that of the K-type in the range of 400～1300℃; but in the low temperature range (-200～400℃), the nonlinear error is larger, 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). It is characterized by low price and suitable for vacuum oxidation reduction Or in an inert atmosphere, the temperature range is -200～800℃, but the commonly used temperature is only below 500℃, because after this temperature, the oxidation rate of the iron hot electrode will increase. It can be used in high temperature (for example, 500℃) sulfur (S) atmosphere. The thermocouple can withstand hydrogen (H2) and carbon monoxide (CO) gas corrosion. 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 called constantan). Its main feature is: In the case, its accuracy is high and the uniformity of the thermode is good; its use temperature is -200～350℃, because the copper thermode is easy to oxidize, and the oxide film is easy to fall off, so when used in an oxidizing atmosphere, Generally, it cannot exceed 300°C. In the range of -200 to 300°C, their sensitivity is relatively high. Another feature of copper-constantan thermocouples is that they are cheap. They are a very cheap one of several commonly used stereotypes. 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 electric potential rate is about 15% higher. Others The performance is almost the same. This type of thermocouple is used a lot in the Japanese industry as a high-temperature thermocouple, but it is used less in China.
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