Thermocouple Overview | Thermocouple Working Principle | Basic Structure of Thermocouple | Common Thermocouple Wire Materials and Their Properties
As one of the most widely used temperature sensors in industrial temperature measurementthermocouples, together withthermal resistors, account for about 60% of the total temperature sensors. Thermocouples are usually used in conjunction with display instruments to directly measure the surface temperature of liquids, vapors, gaseous media, and solids in various production processes within the range of -40 to 1800°C.
Thermocouple Working Principle:
When two conductors of different compositions (called thermocouple wires or thermoelectrodes) are joined at both ends to form a loop, an electromotive force is generated in the loop if the temperatures at the junctions are different. This phenomenon is called the thermoelectric effect, and the electromotive force is called thermoelectric potential. Thermocouples use this principle for temperature measurement. The end directly used to measure the temperature of the medium is called the working end (also called the measuring end), and the other end is called the cold end (also called the compensating end). The cold end is connected to a display instrument or supporting instrument, which indicates the thermoelectric potential generated by the thermocouple.
A thermocouple is actually an energy converter that converts thermal energy into electrical energy, using the generated thermoelectric potential to measure temperature. Regarding the thermoelectric potential of a thermocouple, the following points should be noted:
(1) The thermoelectric potential of a thermocouple is a function of the difference in temperature at the two ends, not a function of the temperature difference between the two ends.
(2) When the thermocouple material is uniform, the magnitude of the thermoelectric potential generated is independent of the length and diameter of the thermocouple, and only depends on the composition of the thermocouple material and the temperature difference between the two ends.
(3) Once the material composition of the two thermocouple wires is determined, the magnitude of the thermoelectric potential only depends on the temperature difference of the thermocouple. If the temperature at the cold end is kept constant, the thermoelectric potential is then a single-valued function of the temperature at the working end.
Basic Structure of Thermocouple:
The basic structure of a thermocouple used for industrial temperature measurement includes thermocouple wires, insulating tubes, protective tubes, and terminal boxes.
1. Common Thermocouple Wire Materials and Their Properties
1. Platinum Rhodium 10 - Platinum Thermocouple (Index number S, also called single platinum rhodium thermocouple)
The positive electrode of this thermocouple is a platinum rhodium alloy containing 10% rhodium, and the negative electrode is pure platinum. Its characteristics are:
(1) Stable thermoelectric performance, strong oxidation resistance, suitable for continuous use in oxidizing atmospheres, with a long-term service temperature up to 1300°C. Above 1400°C, even in air, pure platinum wire will recrystallize, causing grain coarsening and fracture.
(2) High accuracy; it has the highest accuracy level among all thermocouples and is usually used as a standard or for measuring higher temperatures.
(3) Wide range of applications, good uniformity and interchangeability.
(4) Main disadvantages: small differential thermoelectric potential, resulting in low sensitivity; relatively expensive, low mechanical strength, not suitable for use in reducing atmospheres or conditions with metal vapor.
2. Platinum Rhodium 13 - Platinum Thermocouple (Index number R, also called single platinum rhodium thermocouple)
The positive electrode of this thermocouple is a platinum rhodium alloy containing 13% rhodium, and the negative electrode is pure platinum. Compared with type S, its electromotive force rate is about 15% higher, and other properties are almost the same. This type of thermocouple is the most widely used high-temperature thermocouple in Japanese industry, but it is less used in China.
3. Platinum Rhodium 30 - Platinum Rhodium 6 Thermocouple (Index number B, also called double platinum rhodium thermocouple)
The positive electrode of this thermocouple is a platinum rhodium alloy containing 30% rhodium, and the negative electrode is a platinum rhodium alloy containing 6% rhodium. At room temperature, its thermoelectric potential is very small, so compensating wires are generally not needed during measurement, and the influence of cold end temperature changes can be ignored. The long-term service temperature is 1600°C, and the short-term service temperature is 1800°C. Due to the small thermoelectric potential, a display instrument with higher sensitivity is required. Type B thermocouples are suitable for use in oxidizing or neutral atmospheres, and can also be used for short periods in vacuum atmospheres. Even in reducing atmospheres, their lifespan is 10 to 20 times that of type R or S. Since both electrodes are made of platinum rhodium alloy, they do not have the disadvantages of the negative electrode of platinum rhodium-platinum thermocouples, have little tendency for large crystallization at high temperatures, and have greater mechanical strength. Moreover, due to less absorption of impurities or migration of rhodium, the change in thermoelectric potential after long-term use is not severe. The disadvantage is high cost (compared to single platinum rhodium).
