The role of the thermocouple protection tube

Thermocouples are the most widely used temperature sensors. The long-term stability of thermocouples, life and other performance indicators are closely related with the protection tube. Thermocouple failure rate depends directly on the level of protection tube material properties.
With the increasing degree of automation, temperature measurements related to all areas of the national economy, such as industry and agriculture, national defense, scientific research, medical, health, environmental protection, meteorology and space sectors. Thermocouple its stable performance, accurate and reliable, simple structure, easy to use, wide temperature range, the signal can be remote, etc., in areas such as industrial production and scientific research has become the most widely used temperature sensing element. The long-term stability of thermocouples, life and other performance indicators are closely related with the protection tube. According to statistics, the thermocouple measurements in the normal range, the failure rate depends directly on the level of protection tube material performance is excellent. Thus, in actual use the correct choice of the protective tube, the thermocouple is to extend the service life of the premise.
Action
Thermocouple protection tubes primarily for two purposes, first, to prevent mechanical damage suffered; second is to protect the thermocouple from the measured medium chemical corrosion, oxidizing or reducing atmosphere free from the harmful effects of the thermocouple as at close of its most good atmosphere. Therefore thermocouple must be protected, so that is not bad atmosphere against.
Thermocouples used in the following media must be isolated:
1. When a change in contact with the thermocouple thermocouple metal chemical composition (solid, liquid, gas)
 2. Thermocouple materials will corrode furnace gas and other vapors (sulfur and its compounds are particularly harmful).
 3. Thermocouple materials can corrode various electrolytes.
Thermocouple protection tube is usually a single protection tube, in some special cases, can be installed to protect two concentric tubes. Close to the thermocouple protection tubes called main, outer protective tube called supplementary protection tube. Usually alumina tube-based protection tube, silicon carbide tube combination auxiliary protective tube sets constitute protective tube to get a good overall performance, such as with anti-flame cutting, thermal shock, or to prevent mechanical damage and so on.