A thermocouple is formed by connecting two different conductors together, and when the measurement and reference connection points are at different temperatures, they generate the so-called thermoelectric magnetic force (EMF). The purpose of the connection point is to measure the thermocouple connection point that is located at the measured temperature. The reference connection point is a thermocouple connection point that is maintained at a known temperature or can automatically compensate for temperature changes.

In conventional industrial applications, thermocouple elements are generally terminated at the junction; But the reference connection point is rarely located on the joint, and instead uses appropriate thermocouple extension wires to transfer to a controlled environment with relatively stable temperature. The connection point type of shell type thermocouple is physically connected (welded) to the probe wall, which can achieve good heat transfer - that is, heat is transferred from the outside to the thermocouple connection point through the probe wall. It is recommended to use shell type thermocouples to measure the temperature of static or flowing corrosive gases and liquids, as well as some high-pressure applications. In insulated thermocouples, the thermocouple connection point is separated from the probe wall and surrounded by a soft powder. Although the response speed of insulated thermocouples is slower than that of shell type thermocouples, they can provide electrical insulation. It is recommended to use insulated thermocouples to measure corrosive environments, which can ideally be insulated from the surrounding environment by shielding the thermocouple with a sheath. Exposed thermocouple allows the top of the connection point to penetrate deep into the surrounding environment, providing the best response time, but is limited to use in non corrosive, non hazardous, and non pressurized applications. The response time is represented by a time constant, which is defined as the time required for the sensor to change 63.2% between the initial and final values in the controlled environment. The exposed thermocouple has the fastest response speed, and the smaller the diameter of the probe sheath, the faster the response speed, but its maximum allowable measurement temperature is also lower. Extension wire thermocouple extension wire is a pair of wires with the same temperature electromagnetic frequency characteristics as the thermocouple connected to it. When the connection is appropriate, the extension wire will transfer the reference connection point from the thermocouple to the other end of the wire, which is usually located in the controlled environment.