The basic laws of thermocouple
s are not three, but four: 1. The law of homogeneous conductor consists of a closed loop formed by welding the two ends of the same homogeneous material (conductor or semiconductor). No matter how the conductor cross section and how the temperature is distributed, it will not produce The contact potential and the temperature difference potential cancel each other out, and the total potential in the loop is low. 2. The law of the middle conductor. Connect the middle conductor (third conductor) in the thermocouple circuit. As long as the temperature at both ends of the middle conductor is the same, the introduction of the middle conductor has no effect on the total electric potential of the thermocouple circuit. This is the law of the middle conductor. 3. The law of intermediate temperature The thermoelectric potential between the two junctions (temperatures T and T0) of the thermocouple circuit is equal to the algebraic sum of the thermoelectric potential at the temperature of T and Tn and the thermoelectric potential at the temperature of Tn and T0. Tn is called the intermediate temperature. 4. Reference electrode law This law is only studied and paid attention to by professionals. Generally, people in production and use do not know it well. The simple explanation is: use high-purity platinum wire as standard electrode, assuming that the nickel-chromium-nickel-chromium thermocouple is positive The negative electrode is paired with the standard electrode, and their values u200bu200badd up to the value of this nickel-chromium-nickel-chromium. Extended information: When there are two different conductors or semiconductors A and B forming a loop, and their two ends are connected to each other, as long as the temperature at the two junctions is different, the temperature at one end is T, which is called the working end or the hot end, and the other end The temperature is T0, which is called the free end (also called the reference end) or cold end. An electromotive force will be generated in the circuit. The direction and magnitude of the electromotive force are related to the conductor material and the temperature of the two junctions. Thermocouple cold junction compensation calculation method: 1. From millivolt to temperature: measure the cold junction temperature, convert it to the corresponding millivolt value, and add the millivolt value of the thermocouple to calculate the temperature; 2. From temperature to millivolt: Measure the actual temperature and the cold junction temperature, respectively convert them to millivolts, and then subtract the millivolts to get the temperature. In the industrial production process, temperature is one of the important parameters that need to be measured and controlled. In temperature measurement, the application of thermocouples is extremely wide. It has many advantages such as simple structure, convenient manufacturing, wide measurement range, high precision, low inertia and easy remote transmission of output signals. In addition, because the thermocouple is an active sensor, no external power supply is needed for measurement, and it is very convenient to use, so it is often used to measure the temperature of gas or liquid in furnaces and pipes and the surface temperature of solids.
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