Thermocouple protection tube is mainly divided into three categories: metal tube metal protection tube high mechanical strength, good thermal performance, wide application, such as K type thermocouple protection tube. Non-metallic protection tube heat resistance of the metal protection tube is poorer, so more than 1000 ℃ above using non-metallic protection tube; ( B type thermocouple protection tube) Metal ceramic protection tube combination of metal and ceramics developed a high temperature resistance, corrosion resistance and thermal shock toughness of material, this is metal ceramic; Commonly used protection tube has the following 8 kinds of 1, 304: temperature 900 ℃, heat resistance, good corrosion resistance, good mechanical strength, suitable for use in oxidizing atmosphere, sulfur resistance, reducing resistance is poorer; 2, 316 l: temperature 930 ℃, good heat, acid, corrosion resistance, suitable for use in oxidizing atmosphere; 3, 2520: temperature 1000 ℃, generally used as heat resistant steel; 4, 3030: temperature 1100 ℃, generally used as heat resistant steel; 5, 3039: temperature 1250 ℃, usually used as a strong heat resistant steel; 6, INCOL600: use temperature of 1150 degrees, NI - CR- FE alloy, high temperature oxidation resistance, corrosion resistance is strong, but not suitable for use in sulfur environment; 7, corundum: temperature 1800 ℃, has good high temperature electrical insulation and mechanical strength, good airtight performance, small thermal expansion rate, thermal shock resistance is poor; 8, graphite: temperature 3000 ℃, good thermal conductivity and thermal shock resistance, corrosion resistance, commonly used to measure the liquid steel, hot metal temperature.