1.Selection Based on Maximum Operating Temperature (Core Criterion)
This is the primary selection standard, as the maximum temperature range of thermocouples varies significantly by material.
Type K Thermocouple (Nickel-Chromium/Nickel-Silicon): The most commonly used type, with a temperature range of -200℃~1300℃. It offers high cost-effectiveness and is suitable for most medium-to-low temperature tube furnaces (≤1200℃) in air atmospheres.
Type S Thermocouple (Platinum-Rhodium 10/Platinum): Made of precious metals, it has a temperature range of 0℃~1600℃. It is suitable for high-temperature oxidizing atmospheres (e.g., air) between 1200℃~1600℃, featuring high accuracy but a high price.
Type B Thermocouple (Platinum-Rhodium 30/Platinum-Rhodium 6): It has the highest maximum temperature limit (0℃~1800℃) and is specifically designed for ultra-high temperature oxidizing atmospheres between 1600℃~1800℃. It has poor accuracy at low temperatures and needs to be used in high-temperature environments.
2. Selection Based on Furnace Atmosphere (Key Supplement)
The atmosphere can corrode the thermocouple, and an incorrect choice will lead to failure.
Oxidizing/Neutral Atmospheres (e.g., air, nitrogen, argon): The above-mentioned Type K, S, and B thermocouples can be used directly without special protection.
Reducing Atmospheres (e.g., hydrogen, carbon monoxide): Precious metal thermocouples (Type S and B) cannot be used (as they will be "poisoned" and fail). A Type K thermocouple with a corundum protective tube or a dedicated Type N thermocouple should be selected.