1) Industrial internal standard glass liquid thermometer. It is made using the principle of volume expansion when mercury or alcohol is heated. The use range is -100～600℃, and the common range is 0～100℃. The advantage is that the structure is simple, the response is fast and accurate. The disadvantage is that it can only be measured locally, cannot be transmitted over long distances, and cannot be automatically controlled. In the small air separation plant, it is used for the temperature measurement of the gas temperature of the air compressor and the oxygen compressor at all levels and the temperature measurement of the cooling water and the temperature measurement during heating and thawing.
When using, the end of the thermometer should be inserted into the measured medium. Since the thermometer is made of glass, it is necessary to prevent breakage and rapid cooling and heating.
2) Thermocouple thermometer. The temperature measuring element consists of two different wires. One end is welded, called the working end or the hot end, and is in contact with the object under test. The other end is connected to the display instrument with a wire. Since the temperature at both ends is different, thermoelectric potential will be generated. For a thermocouple composed of a certain material, if the temperature of the cold end remains unchanged, the thermoelectric potential changes with the temperature of the hot end. Therefore, the temperature can be determined by measuring the thermoelectric potential.
Thermocouple thermometers can be used in industry to measure the surface temperature of liquids, gases, vapors and solids in the range of 0 to 1800 °C, and are currently expanding to the low temperature field. It has the advantages of simple structure, convenient use, high accuracy and wide range, and is convenient for centralized detection and automatic control of long-distance transmission. It is commonly used in small air separation plants to measure the gas temperature of the compressor. Nickel-chromium-nickel-silicon (grading number K) is usually used, 0-1300℃; nickel-chromium-constantan (grading number E), 0-800℃. Generally used for temperature measurement and control of purifiers or dryers.
