High-temperature sensors with a ceramic protective sheath THA-022, THK-022, THJK-022 are designed to measure temperatures up to 1200 °C. The sensors allow measuring the temperature of gaseous and chemically non-aggressive media to the protective fitting material. Thermocouple operating principle The operating principle of the thermocouple is based on the thermoelectric Seebeck effect. A thermocouple has two joined ends: one end is placed in the measured medium and is called the measuring or hot junction, and the other end at the device connection input is called the cold junction. Methods of manufacturing the hot junction are shown in Figure 1. When there is a temperature difference between the hot and cold junctions, a thermo-EMF appears in the circuit whose value is supplied as an input signal. Figure 1 - Methods of manufacturing the hot junction The measuring junction of the high-temperature sensors is placed inside a protective fitting. Thermocouple junctions are manufactured in different configurations. The junctions can be grounded or ungrounded, and two-element thermocouples can be isolated or non-isolated. Variants of hot junctions are shown in Figure 2. Figure 2 - Configurations of hot junctions The thermocouple junction is grounded if the junction is connected to the shell of the primary converter. Grounded junctions have better thermal conductivity which increases response speed. However, grounding makes thermocouple circuits more susceptible to electrical interference that can distort the voltage signal if the measuring instrument does not have electrical isolation. All high-quality measuring converters and I/O boards provide electrical isolation as standard. A grounded junction is also more prone to contamination by chemical impurities over time. An open junction thermoelectric converter has the hot junction protruding from the sealed end of the sheath, providing high response speed. Sealing prevents moisture or other contaminants from entering inside the sheath. Such thermocouples are usually used only in non-aggressive gases, e.g. in air ducts. Ungrounded junctions occur when thermocouple elements do not merge with the sheath of the primary converter but are surrounded by insulating powder. Features of using high-temperature sensors THA-022, THK-022, THJK-022: Has a terminal head that allows convenient cable connection to the sensor and protects the connection point. Has a fixed fitting that allows reliable sensor mounting. Short-term operation at 1200 °C is possible. The model is not high-temperature along its entire length; therefore, only the ceramic part should be in the hot zone. Since part of the protective fitting is ceramic, impacts on the thermoelectric converter should be avoided during installation and operation. Not used for temperature measurement of liquids and in aggressive gases such as acid or alkali vapors. High-temperature sensors with ceramic protective sheath THA-022, THK-022, THJK-022 can be manufactured without a fitting — THA-021, THK-021, THJK-021.
