What are the common types of thermometers used in air separation equipment, and in what applications are they utilized?

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The common types of thermometers used in air separation equipment are as follows: 1) Industrial internal reference glass liquid thermometers. It is made using the principle that mercury or alcohol expands in volume when heated. The operating range is -100 to 600°C, with the commonly used range being 0 to 100°C. The advantages are simple structure, fast response, and high accuracy. The disadvantage is that it can only be measured on-site, cannot transmit data over long distances, and does not allow for automatic control. Used in small air separation units for temperature measurement of the gases at various stages of the air compressors and oxygen compressors, as well as for temperature measurement of the cooling water, and for temperature control during heating and thawing. When in use, the entire tail of the thermometer should be inserted into the medium being measured. Since thermometers are made of glass, it is necessary to prevent them from breaking as well as sudden changes in temperature. 2) Thermocouple thermometer. The temperature sensing element consists of two different wires. One end is welded together and is called the working end or hot end, which comes into contact with the object being tested. The other end is connected to the display instrument with a wire, as shown in Figure 113. A thermoelectrical potential is generated when the temperatures at the two ends differ. For a thermocouple made of a specific material, if the temperature at the cold end remains constant, the thermoelectrical potential changes as the temperature at the hot end changes; therefore, by measuring the thermoelectrical potential, it is possible to determine the temperature level. Thermocouple thermometers are used in industry to measure the surface temperatures of liquids, gases, vapors, and solids within the range of 0–1800°C, and their application is currently being extended to lower temperature ranges as well. It features a simple structure, ease of use, high accuracy, and a wide range, making it suitable for long-distance transmission for centralized monitoring and automatic control. It is commonly used in air separation units to measure the gas temperature of compressors. Nickel-chromium-nickel-silicon (grade K) is commonly used, 0–1300℃ ； Nickel-chromium-constantan (division number E), 0–800°C. It is generally used for temperature measurement and control in purifiers or dryers. 3) Thermal resistance thermometer. It consists of a thermal resistor and a display instrument that goes with it. Thermistor elements are made use of the property that the resistance value of metals (or semiconductors) changes with temperature; therefore, by measuring the change in resistance, it is possible to determine the change in temperature. Platinum resistance thermometers (with a calibration grade of Pt100) can be used in industry to measure temperatures in the range of -200 to 650°C. In addition to the advantages of thermocouples, it also features high measurement accuracy in low-temperature ranges, which is why it is more widely used in low-temperature applications. Used for low-temperature measurement in the insulation box of air separation units. 4) Gauge-type thermometer. It is designed based on the principle that the pressure of a gas (or vapor, liquid) inside a sealed container changes with temperature. When taking a temperature reading, insert the temperature probe into the medium being measured. When the temperature changes, the pressure of the gas (or vapor, liquid) inside the thermosensitive bulb changes; this pressure is transmitted to the elastic pressure gauge through capillaries, and by detecting this change in pressure, the change in temperature can be determined. It can measure temperatures in the range of -100 to 600°C. In air separation equipment, the wTZ-288 type electric contact pressure thermometer is commonly used; it has a measurement range of 0 to 100°C and is employed to measure the temperature of lubricating oil.