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Thermocouple application and principle
Thermocouples are among the most widely used temperature sensing devices in industrial applications. Their popularity stems from several key advantages. First, they offer high measurement accuracy because they come into direct contact with the object being measured, eliminating the influence of intermediate media. Second, thermocouples have a broad temperature range, typically operating between -50°C and +1600°C. Some specialized types can even measure as low as -269°C (such as gold-iron-nickel-chromium) or as high as +2800°C (like tungsten-bismuth). Third, their simple structure makes them easy to use. Composed of two different conductive wires, they are not limited by size or configuration, and are often enclosed in a protective sheath for durability and convenience.
The basic principle behind thermocouple operation involves joining two dissimilar conductors or semiconductors, A and B, to form a closed loop. As shown in Figure 2-1-1, when there is a temperature difference between the two junctions (the hot and cold ends), an electromotive force is generated, resulting in a current flow within the loop. This phenomenon, known as the thermoelectric effect, is the foundation upon which thermocouples function.
Thermocouples can be categorized into standard and non-standard types. Standard thermocouples are defined by national standards that specify the relationship between thermoelectric potential and temperature, allowable error, and standardized calibration tables. They are compatible with display instruments. Non-standard thermocouples, on the other hand, have more limited application ranges and do not follow uniform indexing tables, making them suitable for special or specific situations.
Since January 1, 1988, thermocouples and resistance temperature detectors (RTDs) have been produced according to IEC international standards. In China, seven standardized types—S, B, E, K, R, J, and T—are commonly used.
The structure of a thermocouple must ensure reliable and stable performance. Key requirements include: firm welding of the two hot electrodes, proper insulation between them to prevent short circuits, secure and convenient connections between the compensation wire and the free end, and a protective sleeve that isolates the hot electrode from harmful environments.
Cold junction compensation is essential due to the high cost of thermocouple materials, especially those made from precious metals, and the long distance between the measurement point and the instrument. To reduce material usage and costs, compensation wires are often used to extend the cold junction to a controlled environment where temperature fluctuations are minimal. However, it's important to note that compensation wires only extend the hot electrode and do not eliminate the effects of temperature changes at the cold junction. Additional correction methods are necessary when the cold junction temperature is not exactly 0°C.
When using compensation wires, it is crucial to match the correct model, ensure proper polarity, and keep the temperature at the connection point below 100°C. These precautions help maintain the accuracy and reliability of temperature measurements.