Thermocouple Principle and Structure
Thermocouples are one of the most commonly used temperature sensing elements in industry. Their advantages are:
① High measurement accuracy. Because the thermocouple is in direct contact with the object being measured, it is unaffected by intermediate media.
② Wide measurement range. Commonly used thermocouples can continuously measure from -50℃ to +1600℃. Some special thermocouples can measure as low as -269℃ (e.g., gold-iron-nickel-chromium) and as high as +2800℃ (e.g., tungsten-rhenium).
③ Simple structure and easy to use. Thermocouples are usually composed of two different metal wires and are not limited by size or shape. They have a protective sheath, making them very convenient to use.

1. Basic Principle of Thermocouple Temperature Measurement Two conductors or semiconductors, A and B, of different materials are welded together to form a closed circuit. When a temperature difference exists between the two junctions 1 and 2 of conductors A and B, an electromotive force is generated between them, thus forming a current in the circuit. This phenomenon is called the thermoelectric effect. Thermocouples utilize this effect to function.

2. Types and Structures of Thermocouples
(1) Types of Thermocouples
Common thermocouples can be divided into two main categories: standard thermocouples and non-standard thermocouples. Standard thermocouples are those whose thermoelectric potential-temperature relationship, allowable error, and standardized calibration table are specified by national standards. They also have matching display instruments available for selection. Non-standard thermocouples are less widely used or in larger quantities than standard thermocouples, and generally do not have standardized calibration tables. They are mainly used for measurements in certain special situations. Standardized Thermocouples

(2) Thermocouple Structure
To ensure reliable and stable operation of the thermocouple, its structure requires the following:
① The welding of the two thermoelectrodes forming the thermocouple must be firm;
② The two thermoelectrodes should be well insulated from each other to prevent short circuits;
③ The connection between the compensating wire and the free end of the thermocouple should be convenient and reliable;
④ The protective sheath should ensure sufficient isolation between the thermoelectrodes and harmful media.

3. Temperature Compensation for Thermocouple Cold Junction
Because thermocouple materials are generally expensive (especially when precious metals are used), and the distance between the temperature measuring point and the instrument is often considerable, to save on thermocouple material and reduce costs, compensating wires are typically used to extend the cold junction (free end) of the thermocouple to the more temperature-stable control room and connect it to the instrument terminals. It must be pointed out that the function of the thermocouple compensating wire is only to extend the thermocouple electrodes, moving the cold junction to the instrument terminals in the control room; it does not eliminate the influence of cold junction temperature changes on temperature measurement and therefore does not provide compensation. Therefore, other correction methods are needed to compensate for the influence of cold junction temperature t0 ≠ 0℃ on temperature measurement.

When using thermocouple compensating wires, it is essential to ensure that the model is compatible, the polarity is correct, and the temperature at the connection point between the compensating wire and the thermocouple does not exceed 100℃.

For example: Type S thermocouple) Platinum-Rhodium 10-Platinum Thermocouple
The Platinum-Rhodium 10-Platinum Thermocouple (Type S thermocouple) is a precious metal thermocouple. The thermocouple wire diameter is specified as 0.5 mm, with an allowable deviation of -0.02 mm. Its positive electrode (SP) has a nominal chemical composition of platinum-rhodium alloy, containing 10% rhodium and 90% platinum, while the negative electrode (SN) is pure platinum; hence, it is commonly known as a single platinum-rhodium thermocouple. The maximum long-term operating temperature of this thermocouple is 1300℃, and the maximum short-term operating temperature is 1600℃.

The S-type thermocouple boasts the highest accuracy, best stability, wide temperature measurement range, and long service life among thermocouple series. It possesses excellent physical and chemical properties, good thermoelectric potential stability, and good oxidation resistance at high temperatures, making it suitable for oxidizing and inert atmospheres. Due to its excellent overall performance, the S-type thermocouple, conforming to the International Temperature Scale (ITS-90), has long been used as an interpolation instrument for the ITS-90. Although ITS-90 stipulates that it will no longer be used as an interpolation instrument for the ITS-90, the International Advisory Committee on Temperature (CCT) considers that the S-type thermocouple can still be used to approximate the ITS-90.

The disadvantages of S-type thermocouples are their relatively small thermoelectric potential and thermoelectric potential rate, low sensitivity, decreased mechanical strength at high temperatures, high sensitivity to contamination, and high cost of precious metal materials, resulting in a large initial investment.