How to select thermocouples of different models

Thermocouples are commonly used temperature measuring elements in temperature measurement surfaces. They directly measure temperature and convert the temperature signal into a thermoelectric electromotive force signal, which is then converted into the temperature of the measured medium through the electrical surface (secondary surface).
The appearance of various thermocouples is often very different due to needs, but their fundamental structure is generally the same. They are usually composed of primary parts such as thermal electrodes, insulation sleeve maintenance tubes, and junction boxes, and are usually used in conjunction with displaying the appearance, recording the appearance, and electronic regulators.
1. Platinum Platinum Rhodium Thermocouple (S-type) Graduation Number LB-3 Industrial Thermocouple Wire: Φ 0.5mm, laboratory use can be finer. Positive electrode: Platinum rhodium alloy wire, tempered with 90% platinum and 10% rhodium (in proportion). Negative electrode: Platinum wire. Measurement temperature: Long term: 1300 ℃, short term: 1600 ℃.
Features:
(1) Stable material function and high measurement accuracy; Can be made into standard thermocouples or reference thermocouples. Usage: Laboratory or calibration of other thermocouples.
(2) The measurement temperature is relatively high, usually used to measure high temperatures above 1000 ℃.
(3) In high-temperature restorative gases (such as gases containing Co, H2, etc.), they are prone to erosion and require the use of maintenance sleeves.
(4) The material belongs to precious metals and has a high cost. (5) Weak thermoelectric potential.
2. Platinum rhodium 30-Platinum rhodium thermocouple (Type B) with a division number of LL-2. Positive electrode: Platinum rhodium alloy (made by tempering with 70% platinum and 30% rhodium). Negative electrode: Platinum rhodium alloy (made by tempering with 94% platinum and 6% rhodium). Temperature measurement: It can reach 1600 ℃ for a long time and 1800 ℃ for a short period of time.
What are the different selection specifications for different types of thermocouples?
Features:
(1) Stable material function and high measurement accuracy.
(2) Easy to corrode in restorative gases.
(3) The low-temperature thermoelectric potential is extremely small, and the cold end temperature can be below 50 ℃ without compensation.
(4) High cost.
3. Several Thermocouples with Special Applications
(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 thermocouples were developed in the 1960s and are now a better type of high-temperature thermocouple. They can be used in vacuum inert gas or hydrogen media, but their high-temperature resistance to oxygen is poor. The domestically produced tungsten rhenium tungsten rhenium 20 thermocouple has a temperature range of 300-2000 ℃ and a division accuracy of 1%.
(3) Gold iron nickel chromium thermocouples are primarily used for low-temperature measurements and can be used in the range of 2-273K, with a sensitivity of approximately 10 μ V/℃.
(4) The palladium platinum iridium 15 thermocouple is a high output 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 a less sensitive indicator surface and is often used in the aviation industry.

(5) Iron constantan thermocouple, with a high sensitivity of the division number TK, about 53 μ V/℃, good linearity, affordable price, can be used in recovery media below 800 ℃. The primary disadvantage is that iron is highly prone to oxidation, and the use of blueing treatment can improve its rust resistance.
(6) The thermoelectric potential of the copper constantan thermocouple, with the division number MK thermocouple, is slightly higher than that of the nickel chromium nickel silicon thermocouple, about 43 μ V/℃. Good reproducibility, stability, high accuracy, and affordable price. The disadvantage is that copper is prone to oxidation and is widely used in low-temperature laboratory measurements between 20K and 473K.
4. The zirconium diboride composite ceramic thermocouple has a temperature measurement accuracy of ± 2 ℃ at 200-1900 ℃, and can be used in vacuum or oxidation environments at 1900-2200 ℃ with an error of ± 10 ℃. But it has not yet gained widespread recognition from shopping malls.