How to choose different types of thermocouples?

Thermocouple is a commonly used temperature measuring element in temperature measuring instruments. It directly measures temperature and converts the temperature signal into a thermoelectric potential signal, which is then converted into the temperature of the measured medium through electrical instruments (secondary instruments). The appearance of various thermocouples often varies greatly due to their needs, but their basic structures are roughly the same. They are usually composed of main parts such as hot electrodes, insulation sleeves, and junction boxes, and are usually used in conjunction with display instruments, recording instruments, and electronic regulators. 1. Platinum Platinum Rhodium Thermocouple (S-type) Scale LB-3 Industrial Thermocouple Wire: Φ0.5mm, It can be finer for laboratory use. Positive electrode: Platinum rhodium alloy wire, made by smelting 90% platinum and 10% rhodium (weight ratio). Negative electrode: platinum wire. Temperature measurement: Long term: 1300 ℃, short-term: 1600 ℃. Characteristics: (1) Stable material properties and high measurement accuracy; Can be made into standard thermocouples or reference thermocouples. Purpose: Laboratory or verification of other thermocouples. (2) The measurement temperature is relatively high, generally used to measure high temperatures above 1000 ℃. (3) In high-temperature reducing gases (such as gases containing Co, H2, etc.), they are prone to corrosion and require protective sleeves. (4) The material belongs to precious metals and the cost is relatively high. (5) The thermoelectric potential is relatively weak. 2. Platinum rhodium 30- Platinum rhodium thermocouple (Type B) with graduation number LL-2 positive electrode: Platinum rhodium alloy (made by smelting 70% platinum and 30% rhodium). Negative electrode: Platinum rhodium alloy (made by smelting 94% platinum and 6% rhodium). Temperature measurement: Long term can reach 1600 ℃, short-term can reach 1800 ℃.

Characteristics: (1) Stable material properties and high measurement accuracy. (2) Easily corroded in reducing gases. (3) The low-temperature thermoelectric potential is extremely small, and the cold end temperature below 50 ℃ can be left uncompensated. (4) High cost. 3. Several specialized thermocouples (1), iridium and iridium alloy thermocouples such as iridium 50 rhodium iridium 10 ruthenium thermocouples, can measure high temperatures up to 2100 ℃ in an oxidizing atmosphere. (2) Tungsten rhenium thermocouple was developed in the 1960s and is currently one of the better high-temperature thermocouples. It can be used in vacuum inert gas media or hydrogen media, but has poor high-temperature oxygen resistance. The domestically produced tungsten rhenium tungsten rhenium 20 thermocouple has a temperature range of 300-2000 ℃ and a calibration accuracy of 1%. (3) The gold iron nickel chromium thermocouple is mainly used for low-temperature measurement and can be used in the range of 2-273K, with a sensitivity of about 10 μ V/℃. (4) The palladium platinum iridium 15 thermocouple is a high-performance thermocouple with a thermoelectric potential of 47.255mV at 1398 ℃, which is three times higher than the thermoelectric potential of the platinum platinum rhodium 10 thermocouple at the same temperature. Therefore, it can be used with less sensitive indicator instruments and is commonly used in the aviation industry. (5) The iron constantan thermocouple has a high sensitivity with a division mark TK of about 53 μ V/℃, good linearity, and is inexpensive. It can be used in reducing media below 800 ℃. The main disadvantage is that iron is prone to oxidation, and the use of bluing treatment can improve its corrosion resistance. (6) The thermoelectric potential of copper constantan thermocouple with division mark MK is slightly higher than that of nickel chromium nickel silicon thermocouple, about 43 μ V/℃. Good reproducibility, good stability, high accuracy, and affordable price. The disadvantage is that copper is prone to oxidation and is widely used in low-temperature laboratory measurements ranging from 20K to 473K. 4. Zirconium diboride composite ceramic thermocouple has a temperature measurement accuracy of ± 2 ℃ between 200-1900 ℃, and can be used in vacuum or oxidation environments with an error of ± 10 ℃ between 1900-2200 ℃. But it has not yet gained widespread recognition in the market.