A thermocouple is formed by connecting two different conductors together. When the measuring and reference junctions are at different temperatures, a thermoelectric force (EMF) is generated. The measuring junction is the part of the thermocouple junction that is at the temperature being measured. The reference junction is the part of the thermocouple junction that is maintained at a known temperature, or where temperature changes are automatically compensated.
In conventional industrial applications, thermocouple elements are typically terminated at a connector; however, the reference junction is rarely located at the connector, but rather transferred to a temperature-stable controlled environment using appropriate thermocouple extension wires. Junction Types: Grounded thermocouples have the thermocouple junction physically connected (welded) to the probe sheath, allowing for excellent heat transfer-that is, heat is transferred from the outside through the probe sheath to the thermocouple junction. Grounded thermocouples are recommended for measuring the temperature of static or flowing corrosive gases and liquids, as well as some high-pressure applications. In insulated thermocouples, the thermocouple junction is separated from the probe sheath and surrounded by a soft powder. Although insulated thermocouples have a slower response time than grounded thermocouples, they provide electrical insulation.
Insulated thermocouples are recommended for corrosive environments, ideally providing complete electrical isolation of the thermocouple from the surrounding environment through sheath shielding. Exposed junction thermocouples allow the tip of the junction to extend into the surrounding environment. This type provides the best response time but is limited to non-corrosive, non-hazardous, and non-pressurized applications. Response time is expressed as a time constant, defined as the time required for the sensor to change 63.2% between the initial and final values in the controlled environment. Exposed junction thermocouples have the fastest response time, and the smaller the probe sheath diameter, the faster the response time, but the lower the maximum allowable measurement temperature. Extension Wires: Thermocouple extension wires are a pair of wires with the same thermoelectric characteristics as the thermocouple to which they are connected. When properly connected, the extension wires transfer the reference junction from the thermocouple to the other end of the wire, which is usually located in a controlled environment. When selecting a thermocouple, the following factors should be considered:
1. The temperature range to be measured;
2. The required response time;
3. The type of connection;
4. The chemical corrosion resistance of the thermocouple or sheath material;
5. Abrasion or vibration resistance;
6. Installation and limitation requirements, etc.
