Teach you to distinguish the positive and negative poles of thermocouples

Thermocouples often encounter a problem in the application process, that is, the positive and negative of thermocouples cannot be distinguished. Don't look at this small question, which can make you blush. When working on a site a few days ago, I encountered a problem. The leader of the company came down to check the work, and there happened to be an old man who took over the thermocouple, but he was stumped because of the question of the positive and negative electrodes. The boss asked him which one is positive or negative? He thought about it for a long time, but couldn't tell. In the end, what method did the boss use to check the positive and negative poles of the thermocouple, he still couldn't answer, because some manufacturers' products did not have specific labels when they left the factory, so some on-site operators could not figure out which one was the positive pole and which one was the positive pole. Negative pole, I will tell a friend a question today to avoid confusion. Let me talk about it first: Ni-Cr-Ni-Si Thermocouple Let me talk about the relevant parameters of Ni-Cr-Ni-Si thermocouple: Ni-Cr-Ni-Si Thermocouple ( K-type thermocouple) is the most expensive metal thermocouple in the temperature measurement area above 500 ℃, and its consumption is the sum of other metal thermocouples. K-type thermocouples have the advantages of good linearity, large thermoelectromotive force, high sensitivity, good stability and uniformity, strong anti-oxidation function and low price, and can be used in oxidative and lazy atmospheres. Type K thermocouples cannot be directly used in sulfur, reductive or reductive, oxidative alternating atmospheres and vacuums at high temperatures, nor are they recommended for weak oxidizing atmospheres. Let's talk about the chemical composition of the nickel-chromium-nickel-silicon thermocouple: the positive electrode of the nickel-chromium-nickel-silicon (nickel-aluminum) thermocouple (the graduation number is K) is a nickel-chromium alloy (KP) containing 10% chromium, and the negative electrode is a silicon-containing 3% nickel-silicon alloy (KN). , according to this characteristic, the positive and negative poles of the thermocouple can be easily identified with a magnet. It is characterized by wide operating temperature range, stable performance at high temperature, approximately linear relationship between thermoelectromotive force and temperature, and low price. Therefore, it is currently the most widely used thermocouple, nickel-chromium-nickel-silicon thermocouple, K type Thermocouples are suitable for continuous use in oxidative and lazy atmospheres. The short-term use temperature is 1300°C, and the long-term use temperature is 1100°C. After selection, the high-quality K-type thermocouple can be used as a standard to be used for base metal thermocouples such as nickel-chromium-nickel-silicon and other base metal thermocouples. The elements such as metal yttrium and magnesium are added to the north and south poles of this thermocouple, and the anti-oxidation function can be further improved. Progress, the maximum operating temperature can reach 1300 ℃. In order to give full play to the advantages of low price of cheap metals, in the same temperature measurement occasion, several thermocouples can be installed, and the characteristics of high sensitivity and approximately linear thermoelectric characteristics can be used to achieve the purpose of accurate measurement. Type K thermocouples are resistant to oxidation. Strong base metal thermocouple. It is not suitable for bare wires to be used in vacuum, carbon-containing, sulfur-containing atmospheres and alternate atmospheres of oxidation and recovery. When the partial pressure of oxygen is low, the chromium in the nickel-chromium electrode will be preferentially oxidized (also called green corrosion), which will greatly change the thermoelectromotive force. However, the influence of metal gas on it is more difficult. Therefore, metal thermocouple protection tubes are mostly used.

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