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The practical application of PCB thermoelectric phenomenon is of course the use of thermocouples to measure temperature. The complex relationship between electron energy and scattering makes the thermoelectric potentials of different metals different from each other. Since the thermocouple is such a device, the difference in thermoelectric potential between its two electrodes is an indication of the temperature difference between the hot and cold ends of the thermocouple. If the thermoelectric potential of all metals and alloys are different, it is impossible to use Thermocouple to measure temperature. This potential difference is called the Scebeek effect. For a pair of conductors A and B of different materials, one junction is maintained at temperature T1, and the two free ends are maintained at a lower temperature To. The contact point and the free end are both located in an area of uniform temperature, and both conductors experience the same temperature gradient. In order to be able to measure the thermoelectric potential difference between the free ends A and B, a pair of conductors C of the same material are respectively connected to the conductors A and B at the temperature to and connected to a detector with a temperature of T1. Obviously, the Seebeck effect is by no means a phenomenon at the connection point, but a phenomenon related to the temperature gradient. In order to correctly understand the performance of thermocouples, this point cannot be overemphasized.
The application range of PCB thermocouple temperature measurement is very wide, and the problems encountered are also diverse. Therefore, this chapter can only cover some important aspects of thermocouple temperature measurement. Thermocouple is still one of the main methods of temperature measurement in many industries, especially in steelmaking and petrochemical industries. However, with the development of electronics, the application of resistance thermometers in the PCB industry has become more and more extensive, and PCB thermocouples are no longer the only and most important industrial thermometers.
Compared with the thermocouple (resistance measurement and thermoelectric potential measurement), the advantage of resistance thermometer is the fundamental difference in the working principle of the two kinds of PCB components. The resistance thermometer indicates the temperature of the area where the resistance element is located, and it has nothing to do with the lead and the temperature gradient along the lead. However, the thermocouple measures the temperature difference between the cold end and the hot end by measuring the potential difference between the two electrodes of the cold end. For an ideal thermocouple, the potential difference is only related to the temperature difference between the two ends. However, for an actual thermocouple, a certain non-uniformity of the thermocouple wire at the temperature gradient will also cause a change in the potential difference, which is still a factor that limits the accuracy of the thermocouple.
Seven kinds of international thermocouples using PCB thermal design, the so-called "standardized thermocouples", such as the nominal composition of each electrode, the general trade name of each alloy, and the letter code of the thermocouple. These letter codes were originally introduced by the Instrument Society of American (Instrument Society of American), but they are now widely used all over the world. These letter codes can be used as various types.
