How to maintain the six temperature measurement instruments in a chemical plant?

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How to maintain the six temperature measurement instruments in a chemical plant? Depending on the temperature-sensitive substance used and the temperature range measured, temperature instruments include kerosene thermometers, alcohol thermometers, mercury thermometers, gas thermometers, resistance thermometers, differential temperature thermometers, radiation thermometers, photometric thermometers, and so on. If the centrifugal pump is the “heart” of a chemical plant, then temperature measuring instruments are the indicator of its normal operation. Xiao 7 summarized the six common types of instruments used for temperature measurement in chemical plants, as well as troubleshooting and maintenance. /uploads/allimg/160728/1-160HQ0322D20.jpg Depending on the substance used for temperature measurement and the range of temperatures measured, there are kerosene thermometers, alcohol thermometers, mercury thermometers, gas thermometers, resistance thermometers, differential thermometers, radiation thermometers, optical thermometers, and so on. Classification of temperature measuring instruments based on measurement principle: contact measurement and non-contact measurement. The contact measurement method involves the temperature-sensitive element coming into direct contact with the medium being measured. It ensures thorough heat exchange between the medium under test and the temperature-sensing element, so that both reach the same temperature, thereby achieving the purpose of measurement. Non-contact measurement methods utilize the principle of thermal radiation of materials, with the temperature-sensitive element not coming into contact with the medium being measured. Heat exchange is achieved through radiation and convection to achieve the purpose of measurement. Principles and characteristics of various types of thermometers: /uploads/allimg/160728/1-160HQ03315433.jpg Commonly used temperature measuring instruments in industry: /uploads/allimg/160728/1-160HQ03350562.jpg 1 Glass tube liquid thermometers: /uploads/allimg/160728/1-160HQ034162P.jpg/uploads/allimg/160728/1-160HQ0343L05.jpg They consist of three parts: a glass bulb filled with liquid, a capillary tube, and a scale. Disadvantages: cannot record automatically, lacks remote transmission capability, is fragile, and has a certain delay. The most commonly used is the mercury thermometer ； Main cause of error: Permanent zero shift ； Temporary deformation of the bulbous part ； Pressure change ； The scale is inaccurate ； The reading method is incorrect ； thermal hysteresis effect ； Special reasons for errors in alcohol thermometers ； Special reasons for errors in the maximum temperature gauge. 2 Bimetallic thermometers/uploads/allimg/160728/1-160HQ0353b18.jpg The sensing element is made by welding two metal sheets with different coefficients of linear expansion together; one end of it is fixed, while the other end (referred to as the free end) is connected to the pointer through a transmission mechanism. When the temperature changes, the linear expansion coefficients of the two metal sheets differ, causing the free ends to bend toward the side with the lower linear expansion coefficient. The higher the temperature, the greater the difference in the length of expansion, and thus the greater the bending angle. Common types: Axial bimetallic thermometers: The dial is connected vertically to the protective tube. Radial bimetallic thermometer: The dial is connected parallel to the protective tube. 135° orientation bimetallic thermometer: the dial is connected to the protective tube at a 135° angle. Universal bimetallic thermometer: The angle between the dial and the protective tube can be adjusted arbitrarily. Fault maintenance: During use and maintenance, try to avoid hitting the protective tube, and never allow it to bend or deform. If linear error occurs, the issue of inaccurate temperature reading can be resolved by adjusting the knob behind the thermometer; the thermometer can only be used after it has been calibrated and found to be accurate. 3 Pressure-type thermometer /uploads/allimg/160728/1-160HQ0363X53.jpg It consists of a bulb, a capillary tube, and a Bourdon tube. It measures temperature by detecting the changes in pressure that occur when a liquid, gas, or vapor inside a sealed container is heated; the temperature is then determined based on these pressure changes. When the heating element is heated, the working fluid expands, causing the pressure to rise. This pressure is transmitted through the capillary tube into the spring tube, resulting in its deformation. The displacement at the free end of the spring tube drives the pointer via a transmission system to indicate the temperature. Fault maintenance: When the thermosensitive bulb is heated, the working substance expands, causing the pressure to rise. This pressure is transmitted through the capillary tube to the Bourdon tube, which then deforms; the free end of this tube moves, and the transmission system uses this movement to drive the pointer to indicate the temperature. 4 Thermal Resistance Thermometer /uploads/allimg/160728/1-160HQ03I0523.jpg Structure of thermal resistance thermometers: These thermometers measure temperature based on the relationship between the resistance of a material and its temperature. They produce strong output signals, offer high accuracy and good stability. However, their component structure is usually relatively large, resulting in poor dynamic response; hence they are not suitable for use in areas with limited space or where there are rapid temperature changes. A metal thermistor thermometer consists of a resistive element, an insulating sleeve, and a terminal box; the most commonly used materials for thermistors are platinum and copper. /uploads/allimg/160728/1-160HQ03P1Y2.jpg The resistance wire is wound around insulating cores such as mica, quartz, ceramics, and plastic, fixed in place, and then covered with a protective sleeve. /uploads/allimg/160728/1-160HQ04245296.jpg Structure of armored thermoresistors. Fault maintenance: Common faults include open circuits and short circuits in industrial thermoresistors. A break occurs because the thermistor wire is too thin. Open circuits and short circuits are easy to identify; one can use the “×1Ω” setting on a multimeter. If the measured resistance is less than R0, there may be a short circuit somewhere ； If the multimeter reads infinity, it can be determined that the resistor is open-circuited. A short circuit in a resistor is generally easy to resolve; as long as it does not affect the length and thickness of the resistance wire, the shorted area can be dried out and the insulation can be strengthened. To repair a broken resistor, it is necessary to change the length of the resistance wire, which in turn affects the resistance value; therefore, it is better to replace it with a new resistor. If welding is used for repairs, the resistor must be tested to ensure it functions properly before it can be used. Common faults and solutions in the operation of thermistor temperature measurement systems /uploads/allimg/160728/1-160HQ043459C.jpg5 Thermocouple thermometers /uploads/allimg/160728/1-160HQ0440T10.jpg A sensor that can convert temperature into voltage – the thermocouple. The principle behind a thermocouple is that two metals of different materials are welded together; when there is a temperature difference between the reference terminal and the measuring terminal, a thermoelectric potential is generated. By utilizing the linear relationship between this thermoelectric potential and temperature, it is possible to measure the temperature. Thermocouple thermometers: thin-film thermocouples, heat-shielded thermocouples, high-temperature wear-resistant thermocouples, fast miniature disposable thermocouples /uploads/allimg/160728/1-160HQ0450ED.jpg/uploads/allimg/160728/1-160HQ045534Y.jpg/uploads/allimg/160728/1-160HQ0461c34.jpg Encased thermocouples: used for measuring small objects; they have a slender design and are flexible ； /uploads/allimg/160728/1-160HQ04U55X.jpg Thin-film thermocouples are used for measuring the temperature of nozzles in rockets and aircraft; they have a thin structure and are suitable for temperature ranges from -200 to 300°C. Fault maintenance: /uploads/allimg/160728/1-160HQ052024A.jpg /uploads/allimg/160728/1-160HQ05R2F2.jpg/uploads/allimg/160728/1-160HQ1000XD.jpg 6 Radiation thermometers /uploads/allimg/160728/1-160HQ10211J7.jpg rely on the relationship between thermal radiation and temperature, with the blackbody radiation law providing a quantitative description of this relationship. It can be divided into total radiation pyrometers, brightness pyrometers, and colorimetric pyrometers. Example of an optical pyrometer in a brightness-type thermometer: /uploads/allimg/160728/1-160HQ10249592.jpg. The principle of the filament-type optical pyrometer involves comparing the monochromatic radiance of the object being measured with the brightness of a temperature lamp whose current can be adjusted. A millivolt meter is used to measure the voltage drop across the filament at different brightness levels, with the results expressed on a temperature scale. As a standard for comparing brightness, the relationship between the filament brightness of tungsten lamps, the heating current, and temperature is known; the brightness can be used to represent the temperature through the current. /uploads/allimg/160728/1-160HQ10323143.jpg Fiber optic thermometers – fiber optics, abbreviated as fibers, transmit large amounts of information at high speeds and with high reliability. They offer advantages such as immunity to electromagnetic interference, excellent insulation properties, safety in explosive environments, low loss, wide transmission bandwidth, large capacity, small diameter, light weight, flexibility, and corrosion resistance, which makes them suitable for use in signal detection applications. The most commonly used type of fiber optic is glass fiber, which is made from quartz glass filaments that are even thinner than human hair. It consists of a light-guiding core and a cladding surrounding it; the cladding is often covered with a protective layer made of plastic or rubber, etc. /uploads/allimg/160728/1-160HQ10404503.jpg Liquid crystal fiber thermometers have liquid crystal chips placed on the fiber’s end face. Three types of liquid crystals are mixed in appropriate proportions, and as the temperature ranges from 10 to 45°C, the color changes from green to red. This sensor measures temperature by utilizing the principle that the reflectivity of light changes with color. /uploads/allimg/160728/1-160HQ10440526.jpg Fluorescent fiber thermometer: based on the photoluminescence effect, a powder-like fluorescent substance is shaped into a wafer format and placed at the end of the sensor as the temperature sensing element. Application features: Can be used in almost all temperature measurement applications. For example, it is used for temperature measurement in various industrial furnaces and heat treatment ovens, as well as during induction heating processes. It is particularly useful for measuring temperatures in processes such as the rolling of high-speed wire rods and seamless steel tubes in the steel industry, as well as in the continuous casting and hot rolling of non-ferrous metals, without interfering with the temperature field of the object being measured. It will not be corroded or poisoned by the object under test ； There is no need to be at the same temperature as the object being measured, and there is no limit on the upper range of measurements ； No need to reach thermal equilibrium with the object under test; excellent dynamic performance ； However, the measurement accuracy is greatly affected by the environment and the properties of the object. 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