Thermocouple is the most widely used temperature device in temperature measurement. Its main characteristics are wide temperature measurement range, relatively stable performance, simple structure, good dynamic response, and the ability to transmit 4-20mA electrical signals remotely, which is convenient for automatic control and concentration. control. The principle of thermocouple temperature measurement is based on the thermoelectric effect. Two different conductors or semiconductors are connected into a closed loop. When the temperature at the two junctions is different, thermoelectric potential will be generated in the loop. This phenomenon is called pyroelectric effect, also known as Seebeck effect.
Simple judgment method of thermocouple polarity
When the thermocouple has no polarity indication, the easiest way is to use a magnet to judge, except for the detection with a multimeter, but it can only judge the commonly used types K, J and E, and the others are not clear. Because the K-type thermocouple material is positive electrode-nickel chromium, negative electrode-nickel silicon, so the negative electrode has strong magnetic permeability, so the K-type negative electrode can be attracted by a magnet, but the positive electrode cannot. The material of type J is that the positive electrode is iron and the negative electrode is copper-nickel, so the positive electrode can be attracted by a magnet, but the negative electrode cannot. The material of type E is positive electrode-nickel chromium, negative electrode-copper nickel, so its positive and negative electrodes can not be attracted by magnets.
Judgment and distinguishing method of thermocouple positive and negative polarity
In the process of using thermocouples, we often encounter an embarrassing thing, that is, we cannot distinguish between the positive and negative of the thermocouple. Don't look at the small problems that make you blush. A few days ago, when you were working in a field, you met a company leader to check your work.
One person happened to receive the thermocouple, but it was because of the positive and negative poles. The boss asked him, which one is positive and which one is negative? He thought for a long time and couldn't speak. Finally, the boss could not answer how he detected the positive and negative poles of the thermocouple. Since some manufacturers’ products are not affixed with detailed labels when they leave the factory, some field operators cannot determine which one is positive and which is negative, so some field operators cannot determine which one is positive and which It is the negative electrode.
1. Look at the color of most manufacturers when producing thermocouples, or they are responsible for marking the color, generally one is green, one is gray, positive is green, and then gray, in a few cases Below, there is a compensation line, its color is red, positive, and another is based on different materials and different colors.
2. When distinguishing the magnetic force, we can place a small magnet around the magnetic line of force, and then judge the magnitude of the magnetic force. Because the negative electrode contains more nickel, the magnetic force of the negative electrode will be greater, and the magnetic force of the positive electrode will be smaller. This is a very practical method.
3. The hardness of the wire can be slightly folded. If the wire is hard, the end of the wire is positive, and the positive electrode is usually harder.
4. Use a multimeter to measure MV. If the display is positive during the test, the positive end of the bar is the positive end.
In addition, the thermocouple has a positive electrode and a negative electrode, and the compensation wire has a positive electrode and a negative electrode. First, make sure the connection and configuration are correct. During operation, common faults include short circuit, open circuit, poor contact (judged by a multimeter) and deterioration (identified by surface color).
During inspection, the thermocouple should be separated from the secondary instrument. Thermal resistance is nothing but short circuit and open circuit. In operation, if you suspect a short circuit, just remove the resistance end and check the display instrument, such as maximum value, thermal resistance short circuit, zero loop, conductor short circuit, etc. When the normal connection and configuration are ensured, the meter value display is low or unstable, and the protection pipeline is likely to enter the water.
Display the largest, thermal resistance display the smallest short circuit.