The upper limit of temperature measurement is as high as 1350 ° C. in the process of temperature measurement, the thermocouple should not only bear high temperature, but also accept the corrosion and impact of high temperature molten iron. In order to maintain the accuracy of temperature measurement, it is necessary to take protective measures for the thermocouple.

The thermowell is in direct contact with molten iron to accept the corrosion and high temperature of molten iron. It is required to have high temperature resistance, corrosion resistance, good air tightness and certain high temperature strength. At the same time, the thermowell does not affect the measurement accuracy, the thermal response time is as short as possible, and the thermal conductivity is better than% 67681. In addition, the temperature measurement process is from liquid to solid, the temperature range is large, accompanied by phase change and volume change, which requires that the protective casing should also have the performance of small thermal expansion coefficient and resistance to temperature change.

Currently, there are three kinds of high temperature thermocouple protective sleeve materials: metal protective sleeve, non-metal protective sleeve and composite protective sleeve. Composite protective sleeve includes cermet and composite coating. These three types of protective casings have different characteristics and applications, as follows:

① Metal protective sleeve has high mechanical strength, good initial property and slag resistance, but it is easy to be oxidized and corroded at high temperature. It is generally used when the temperature is less than 1000 ° C;

② The non-metallic material protection sleeve has high temperature resistance, oxidation resistance and good air tightness, but it is not impact resistant, easy to break and poor thermal conductivity. However, it can be used in a wide range of temperatures, and is mostly used in melts above 1000 ° C;

③ The composite protective sleeve is wear-resistant, high temperature resistant and anti-oxidation, but its thermal shock performance is poor. Generally, it is only used in liquid state.

Considering the measurement medium temperature range, corrosion resistance, air tightness, thermal shock resistance, cost performance and other conditions, the protection sleeve of non-metallic material is selected. The commonly used non-metallic protective sleeves include quartz, corundum, silicon carbide, silicon nitride and alumina.

Among the common non-metallic materials, quartz has better comprehensive performance than other non-metallic materials, and its melting point is 1705 ° C, which can fully meet the needs of temperature measurement. Therefore, the material of thermocouple protective sleeve is quartz glass protective tube. The thermal response time of the assembled thermocouple is only related to the diameter of the protective sleeve. When the diameter of the protective tube is within a certain range, the larger the diameter of the protective tube is, the higher the upper limit of the allowable temperature measurement is, and the better the impact resistance is.