Technical standards for thermocouples (detailed overview of various types of thermocouples)

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Technical standards for thermocouples (Type S thermocouples): Platinirhodium 10-Platinum thermocouples. The Platinirhodium 10-Platinum thermocouple (Type S thermocouple) is a noble metal thermocouple. The diameter of the wire is specified at 0.5 mm, with an allowable deviation of -0.015 mm. The nominal chemical composition of its positive electrode (SP) is a platinum-rhodium alloy containing 10% rhodium and 90% platinum, while the negative electrode (SN) is pure platinum; hence it is commonly referred to as a single-platinum-rhodium thermocouple. The maximum continuous operating temperature for this thermocouple is 1300°C, while the maximum short-term operating temperature is 1600°C. S-type thermocouples boast the highest accuracy, best stability, a wide temperature measurement range, and a long service life among all thermocouple types. It has good physical and chemical properties, excellent thermoelectric potential stability, and good oxidation resistance at high temperatures; it is suitable for use in both oxidative and inert atmospheres. Due to their excellent overall performance, S-type thermocouples that meet the requirements of international temperature scales have long been used as interpolation instruments for such scales. Although ITS-90 stipulates that they should no longer be used as such instruments, the International Temperature Advisory Committee (CCT) believes that S-type thermocouples can still be utilized to approximate the international temperature scales. The disadvantages of S-type thermocouples include a low thermoelectric potential, low sensitivity, reduced mechanical strength at high temperatures, high sensitivity to contamination, and expensive precious metal materials, resulting in high initial investment costs. (R-type thermocouple) Platinum-rhodium 13-platinum thermocouple. The platinum-rhodium 13-platinum thermocouple (R-type thermocouple) is a noble metal thermocouple. The diameter of the filaments is specified at 0.5 mm, with an allowable deviation of -0.015 mm. The nominal chemical composition of its positive electrode (RP) is a platinum-rhodium alloy containing 13% rhodium and 87% platinum, while the negative electrode (RN) is pure platinum. The maximum operating temperature over the long term is 1300°C, and the maximum operating temperature in the short term is 1600°C. R-type thermocouples boast the highest accuracy, best stability, a wide temperature measurement range, and a long service life among all thermocouple types. It has good physical and chemical properties, excellent thermoelectric potential stability, and good oxidation resistance at high temperatures; it is suitable for use in both oxidative and inert atmospheres. Due to the comparable overall performance of R-type thermocouples to that of S-type thermocouples, it has been difficult to promote their use in China; aside from being used for temperature measurement in imported equipment, they are rarely employed for such purposes domestically. Between 1967 and 1971, three research institutions – the UK’s NPL, the US’s NBS, and Canada’s NRC – carried out a collaborative study. The results showed that R-type thermocouples had better stability and reproducibility than S-type thermocouples; no such research has been conducted in China to date. The disadvantages of R-type thermocouples include a low thermoelectric potential, low sensitivity, reduced mechanical strength at high temperatures, high sensitivity to contamination, and expensive noble metal materials, resulting in high initial investment costs. (B-type thermocouple) Platinum-rhodium 30-Platinum-rhodium 6 thermocouple. The platinum-rhodium 30-Platinum-rhodium 6 thermocouple (B-type thermocouple) is a noble metal thermocouple. The diameter of the wire is specified as 0.5 mm, with an allowable deviation of -0.015 mm. The nominal chemical composition of its positive electrode (BP) is a platinum-rhodium alloy containing 30% rhodium and 70% platinum, while the negative electrode (BN) is also a platinum-rhodium alloy containing 6% rhodium; hence it is commonly referred to as a dual-platinum-rhodium thermocouple. The maximum continuous operating temperature for this thermocouple is 1600°C, while the maximum short-term operating temperature is 1800°C. Type B thermocouples possess the highest accuracy, best stability, a wide temperature measurement range, a long service life, and a high upper temperature limit among all thermocouple types. It is suitable for use in oxidative and inert atmospheres, and can also be used briefly in a vacuum, but it is not suitable for use in reducing atmospheres or atmospheres containing metal or non-metal vapors. A significant advantage of Type B thermocouples is that no compensation wire is required, as the thermoelectromotive force remains below 3 μV in the range of 0–50°C. The disadvantages of Type B thermocouples include a low thermoelectric potential, low sensitivity, reduced mechanical strength at high temperatures, high sensitivity to contamination, and expensive precious metal materials, resulting in high initial investment costs. (K-type thermocouple) Nickel-chromium-nickel-silicon thermocouples. The nickel-chromium-nickel-silicon thermocouple (K-type thermocouple) is the most widely used type of inexpensive metal thermocouple at present, with its usage accounting for more than all other types of thermocouples combined. The nominal chemical composition of the cathode (KP) is Ni:Cr = 90:10, while that of the anode (KN) is Ni:Si = 97:3. Their operating temperature range is -200~1300°C. K-type thermocouples have advantages such as good linearity, a large thermoelectromotive force, high sensitivity, good stability and uniformity, strong oxidation resistance, and low cost; they can be used in oxidizing inert atmospheres. It is widely adopted by users. K-type thermocouples cannot be used directly at high temperatures in environments containing sulfur, or in atmospheres with alternating reduction and oxidation conditions, nor in vacuum; they are also not recommended for use in weakly oxidizing atmospheres. (N-type thermocouple) Nichrome-silicon-Nickel-silicon thermocouple. The nichrome-silicon-Nickel-silicon thermocouple (N-type thermocouple) is a low-metallic thermocouple and represents a newly standardized type of thermocouple at the international level. It was developed in the early 1970s by laboratories of the Australian Department of Defense. This thermocouple overcomes two major shortcomings of K-type thermocouples: the instability of the electromotive force in the 300–500°C range, which is caused by the short-range ordering of the crystal lattice in nickel-chromium alloys ； At around 800°C, the thermoelectromotive force becomes unstable due to preferential oxidation of the nickel-chromium alloy. The nominal chemical composition of the positive electrode (NP) is Ni:Cr:Si=84.4:14.2:1.4, while that of the negative electrode (NN) is Ni:Si:Mg=95.5:4.4:0.1. The operating temperature range is -200~1300°C. N-type thermocouples possess advantages such as good linearity, a large thermoelectromotive force, high sensitivity, good stability and uniformity, strong oxidation resistance, low cost, and immunity to short-range ordering. Their overall performance is superior to that of K-type thermocouples, making them a thermocouple with great potential for development. N-type thermocouples cannot be used directly at high temperatures in sulfur-containing environments, in atmospheres with reducing or alternating reducing/oxidizing conditions, or in vacuum; they are also not recommended for use in weakly oxidizing atmospheres. (The Type E thermocouple) Nickel-chromium-copper-nickel thermocouples. The nickel-chromium-copper-nickel thermocouple (Type E thermocouple), also known as the nickel-chromium-constantan thermocouple, is another type of metal thermocouple. The positive terminal (EP) is made of a nickel-chromium 10 alloy with a chemical composition identical to that of KP; the negative terminal (EN) is made of a copper-nickel alloy with a nominal chemical composition of 55% copper, 45% nickel, along with small amounts of elements such as manganese, cobalt, and iron. The operating temperature range of this thermocouple is -200~900°C. Type E thermocouples have the highest thermoelectromotive force and sensitivity among all thermocouples, making them suitable for use in thermopiles to measure minor temperature changes. It is not very sensitive to corrosion in high-humidity atmospheres, making it suitable for use in environments with high humidity. E thermocouples also have advantages such as good stability, superior oxidation resistance compared to copper-constantan and iron-constantan thermocouples, and low cost; they can be used in both oxidative and inert atmospheres, which has led to their widespread adoption by users. Type E thermocouples cannot be used directly at high temperatures in sulfur-containing, reducing atmospheres, as their thermoelectric potential exhibits poor uniformity. (J-type thermocouple) Iron-copper-nickel thermocouple. The iron-copper-nickel thermocouple (J-type thermocouple), also known as the iron-constantan thermocouple, is another type of thermocouple made from inexpensive base metals. Its positive electrode (JP) has a nominal chemical composition of pure iron, while its negative electrode (JN) is a copper-nickel alloy, often informally referred to as beryllium copper; its nominal chemical composition consists of 55% copper and 45% nickel, along with small but important amounts of elements such as manganese, cobalt, and iron. Although it is called beryllium copper, it differs from nickel-chromium beryllium copper and copper beryllium copper, so EN and TN cannot be used as substitutes for it. The temperature range covered by iron-constantan thermocouples is from -200 to 1200°C, but the commonly used temperature range is 0 to 750°C. J-type thermocouples offer advantages such as good linearity, a large thermoelectromotive force, high sensitivity, good stability and uniformity, as well as low cost, which is why they are widely used by users. J-type thermocouples can be used in vacuum, oxidizing, reducing, and inert atmospheres. However, the positive iron electrode oxidizes rapidly at high temperatures, which limits the operating temperature; moreover, they cannot be used directly and unprotected in sulfiding atmospheres at high temperatures. (T-type thermocouple) Copper-copper-nickel thermocouple. The copper-copper-nickel thermocouple (T-type thermocouple), also known as the copper-constantan thermocouple, is another type of inexpensive metal thermocouple that is excellent for measuring low temperatures. Its positive electrode (TP) is pure copper, while the negative electrode (TN) is a copper-nickel alloy, typically Beryllium Copper. It is compatible with Beryllium Copper EN of the nickel-chromium-Beryllium Copper type, but not compatible with Beryllium Copper JN of the iron-Beryllium Copper type; although both are called Beryllium Copper. The temperature measurement range for copper-copper-nickel thermocouples is from -200 to 350°C. T-type thermocouples have advantages such as good linearity, a large thermoelectromotive force, high sensitivity, good stability and uniformity, as well as low cost. They exhibit even better stability in the temperature range of -200 to 0°C, with an annual stability of less than ±3μV; after calibration at low temperatures, they can be used as second-class standards for the transfer of low-temperature measurement values. The positive copper of T-type thermocouples has poor oxidation resistance at high temperatures, thus limiting the upper temperature limit for their use.