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Thermocouple protection sleeve material types and characteristics
Dec 24, 2018

Thermocouple protection sleeve material types and characteristics

Thermocouples have become the most widely used temperature sensing element in the fields of industrial production and scientific research. The performance of the thermocouple protection sleeve material affects the long-term stability and service life of the thermocouple. Thermocouples operating at high temperatures have stricter requirements on their casing materials.

Thermocouple protection sleeve material types and characteristics


With the advancement of technology, temperature measurement technology has developed rapidly, and has formed a trend from point to line, from line to surface; from the inside and outside; from wired to wireless. Among the contact and non-contact products, a large number of contact thermocouples have been rapidly developed in the development of modern technology. Thermocouples have become the most widely used temperature sensing components in industrial production and scientific research because of their stable performance, accuracy and reliability, simple structure, convenient use, wide temperature range and long-distance signal transmission. Thermocouple protection sleeves play an important role in their temperature measurement. The performance of the casing material affects the long-term stability and service life of the thermocouple. Especially for thermocouples operating at high temperatures, there is a higher requirement for their materials. For example, continuous measurement of molten steel, thermocouple protection casing not only to withstand high temperatures, but also by the corrosion and impact of hot steel. When measuring temperature in a circulating fluidized bed boiler, there are not only high temperature oxidation and high temperature corrosion, but also high temperature erosion wear of the fluid powder. Such a harsh working environment is a severe test for protecting the casing material, and it also poses a problem for engineers and technicians - how to economically and practically solve the failure problem of the thermocouple protection sleeve in a high temperature environment.

Thermocouple protection sleeve material types and characteristics

ⅱ Performance requirements

The thermocouple protection sleeve is applied under high temperature conditions. In addition to the air tightness, mechanical strength, stability and thermal conductivity required by ordinary thermocouples, the material also requires:

(1) High temperature resistance: It is used at the upper limit of thermocouple temperature measurement, without deterioration and deformation, and has good oxidation resistance at high temperature.

(2) Thermal strength: does not soften at the temperature of the thermocouple.

(3) Corrosion resistance: When the thermocouple protection sleeve must be immersed in molten metal, glass, molten salt and corrosive gas, it should have

Corrosion resistance.

(4) A certain resistance to temperature drastic changes, comprehensive performance against thermal shock, high thermal conductivity, and low thermal expansion.

ⅲ Material classification

1 metal material

The materials are characterized by high mechanical strength, good toughness and good slag resistance; but the high temperature resistance is not good enough and is susceptible to corrosion. 

Thermocouple protection sleeve material types and characteristics


2 non-metallic materials

The characteristics of this kind of material are: high temperature resistance, oxidation resistance, but not impact resistance, relatively brittle and brittle, thermal conductivity is not ideal, and thermal response sensitivity is poor. Therefore, it is generally required that the pipe wall is relatively thin, thus causing a series of problems. 


The hot blast stove in the steel industry is mainly used to provide hot air to the smelting process. The dome temperature measuring thermocouple protection sleeve runs for a long time at 1200~1280°C, the temperature is above 1300°C, and the high-speed airflow vibrates during operation. Particles of sulfur, phosphorus and other compounds produced during combustion; temperature-measuring protective sleeves of gasifiers in the field of coal chemical industry operate at temperatures above 1400 °C for a long time. These industrial environments place extremely stringent requirements on the high-temperature performance of protective tube materials. .

SiC high temperature ceramics have good temperature resistance, thermal conductivity, high strength, high hardness, low density, small thermal expansion coefficient and good oxidation resistance and wear resistance, making it a temperature protection protective sleeve above 1400 °C. . However, ordinary SiC has high porosity, high brittleness and the like due to high porosity, and it is easy to cause brittle fracture during use, which causes contamination of the thermocouple wire and seriously affects the reliability of the thermocouple. Therefore, the SiC material used as the temperature measuring protective sleeve should have a series of advantages such as high temperature resistance, oxidation resistance, high strength, high hardness, erosion resistance, corrosion resistance and temperature resistance, in order to ensure that the thermocouple is ensured at 1400. Long-term stable use in high temperature environment around °C, meeting the industrial production needs of metallurgy and coal chemical industry.

3 composite materials


A cermet is a composite material consisting of a metal or alloy and another or several ceramics by powder metallurgy. Its characteristics are integrated with the wear resistance, high temperature resistance, oxidation resistance of ceramic materials, and high strength of metal materials, but the thermal shock performance is poor. Generally suitable for use in liquid metals, such as molten iron, molten steel, non-ferrous metal melts, etc. In addition, the non-metallic protective sleeve materials commonly used in Table 2 are used in some protective sleeves made of high-temperature wear-resistant materials. They are suitable for temperature measuring thermocouples, ball mills and fans in boiling furnaces, circulating fluidized bed boilers, cement rotary kiln exhaust chambers. Uses such as grinding and coal mills for export and pipeline temperature measurement. 

Thermocouple protection sleeve material types and characteristics


Composite coating

Surface coating technology can impart a variety of special properties such as wear resistance, high temperature resistance, corrosion resistance and oxidation resistance. It is a new surface technology that improves product quality, saves energy and resources, and obtains significant economic benefits. The use of material surface coating technology to solve the problem of high temperature corrosion and wear of workpieces is a popular trend in the world. This type of technology takes advantage of surface technology. Instead of mixing different materials to make composite materials, it combines different materials to achieve their own characteristics and achieve excellent performance.

The above content introduces the types and characteristics of thermocouple protection sleeve materials from three aspects: background, performance requirements and material classification.