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The chemical name for liquid LV is liquid LV; it is a yellow-green liquid with a boiling point of -34.6°C and a melting point of -103°C. It vaporizes into a gas at normal pressure, and inhalation can cause severe poisoning. It has a strong irritating effect and is corrosive; when mixed with other flammable gases in sunlight, it can cause combustion and explosion. Chlorine is a highly reactive substance that can react with most elements (or compounds). Due to these characteristics of liquid LV, liquid LV storage tanks must meet requirements for corrosion resistance, safety, reliability, easy installation, and ease of maintenance when it comes to level measurement. This makes the monitoring of the liquid level in LV storage tanks a common challenge in the LV alkali industry. By combining the actual use of level gauges for measuring the liquid level in LV storage tanks containing contrast liquids, this article provides practical guidance for selecting such level gauges in the LV alkali industry under existing technical conditions. In level measurement, the following types of level gauges are mainly used. (1) Buoyant level gauge. Such as float (buoy, floating disc, float) level gauges, and steel belt level gauges. This type of level gauge offers high measurement accuracy, but its structure and installation are complex, it contains many mechanical components, and it requires a lot of routine maintenance. (2) Pressure-type level gauge. Transmitters such as differential pressure or pressure transmitters are not suitable for measuring the level in pressure vessels; moreover, they require a lot of maintenance, and their cost is high if the medium is corrosive. (3) Ultrasonic level gauges, radar level gauges. Although these level gauges are technologically advanced and some of them come in high-precision versions, they cannot be used in pressure vessels that store light hydrocarbons, liquid ammonia, liquid LV, etc., because the gaseous medium has electrical conductivity and contains a large number of liquid droplets, which give it a strong ability to absorb and reflect electromagnetic waves and sound waves, thereby preventing the pressure vessel from functioning properly. The practical application of different level gauges in measuring the liquid level in LV tanks is the only criterion for testing truth. Each type of level gauge has its advantages and disadvantages, and their performance under the same operating conditions varies from one gauge to another. A LV alkali company in Tangshan has carried out extensive practical applications in the level measurement of liquid LV storage tanks, and would like to share with everyone the experience gained from using different types of level gauges for such measurements in that factory’s liquid LV storage tanks. 1. Application of radioactive level gauges in measuring the level in LV liquid storage tanks. In the 1980s, a LV alkali company in Tangshan used cobalt-60 radioactive level gauges to perform high-limit measurement and alarm functions for the level in its LV liquid storage tanks. The specific application is as follows: a Co-60 radiation source and a receiving device are installed on each side of the high liquid level point in the liquid LV storage tank. A radioactive level gauge generates different levels of radiation when the storage tank is full or empty; based on these changes in radiation level, the gauge determines whether the liquid level in the tank has exceeded the set limit and issues corresponding digital alarm signals. The cobalt-60 radioactive level gauge is installed outside the storage tank and does not come into contact with the liquid; there is no risk of chlorine gas leakage. Its position measurement is accurate and reliable. However, this type of gauge cannot provide continuous monitoring of the liquid level, and the cobalt-60 radiation source can pose a risk to maintenance personnel, so it is not suitable for use in production environments. 2. Application of top-mounted float level gauges in measuring the level of LV storage tanks. In the 1980s, a LV alkali company in Tangshan once used top-mounted float level gauges to measure the level of its LV storage tanks. A top-mounted float level gauge is inserted into the liquid storage tank from the top. As the liquid level in the tank changes, the float moves up and down along the measurement tube in accordance with the changes in the liquid surface. The magnet inside the corrosion-resistant float continuously attracts reed relay switches located at different positions, allowing an indicator to show the level of the liquid. Top-mounted float level gauges perform well in the initial stages of use, but over time their disadvantages become apparent: these gauges need to come into contact with liquid fluorine during measurement, which can lead to fluorine gas leaks ； The on-site environment is highly corrosive; the measurement rods of reed relay devices often suffer from desoldering due to corrosion, resulting in a very high failure rate. This leads to substantial maintenance efforts and disrupts normal production ； The acidic sludge present in liquid LV adheres to the float tube, causing the float to move unevenly or even get stuck, as a result of which the level gauge float cannot move properly in response to changes in liquid level. This leads to inaccurate level readings that affect the normal operation of the manufacturing process; therefore, it is not suitable for measuring the level in LV storage tanks. 3. Application of capacitive level gauges in measuring the level in LV liquid storage tanks. A LV alkali company in Tangshan switched to using capacitive level gauges for measuring the level in its LV liquid storage tanks in the late 1990s. A capacitive level gauge is inserted from the top of the LV liquid storage tank; the change in capacitance of the gauge is proportional to the change in the liquid level within the tank. This principle is utilized to achieve continuous measurement of the liquid level in the LV storage tank. Capacitive level gauges can meet the requirement for continuous measurement of the liquid level in LV storage tanks, but they have the following disadvantages: the measuring electrodes of such gauges need to be inserted into the tank, and chlorine leakage can occur during installation or removal, posing a risk to human safety ； After operating for a period of time, the acid sludge carried by the liquid LV will adhere between the inner and outer electrodes of the capacitive level gauge, causing changes in capacitance and resulting in significant measurement errors ； Due to the high toxicity of chlorine, faulty instruments cannot be removed and repaired at will, resulting in those instruments remaining unrepaired for a long time. For the above reasons, capacitive level gauges were gradually phased out after being used for a period of time. 4. Application of side-mounted magnetic flap level gauges in LV storage tanks. In 2003, a LV alkali company in Tangshan tried using imported side-mounted magnetic flap level gauges. These gauges consist of a measuring tube that is connected to the storage tank; a magnetic float is placed inside this tube. Outside the tube, a dual-color ceramic roller assembly serves as an indicator. As the liquid level in the tank changes, the magnetic float moves up and down, causing the magnetic rollers to rotate, and the change in color of these rollers indicates the liquid level. The side-mounted magnetic flap level gauge performs well in the initial stages of use; as the indicator is completely separated from the medium, it overcomes the effects caused by corrosion ； The magnetic float is sealed inside the measurement tube, eliminating the risk of chlorine leakage and ensuring safety ； The side-mounted magnetic flap level gauge provides accurate and reliable measurement and indication, has a very low failure rate, and has little impact on production. The disadvantage is ice formation on the wall of the measuring tube; moreover, during prolonged operation, the acidic sludge carried by the liquid LV can cause the magnetic float to get stuck or jammed, resulting in measurement errors that severely affect the manufacturing process. Therefore, it cannot be fully utilized for measuring the liquid level in LV storage tanks. 5. Application of external level gauges in measuring the level in LV storage tanks: In 2005, a LV alkali company in Tangshan began to use external level gauges to measure the level in one of its storage tanks on a trial basis. A. Working principle of the external level gauge: The external level gauge is an intelligent field transmitter-type instrument. Its explosion-proof main unit is installed near the liquid storage tank, while its sensing head is placed in close contact with the outer wall of the container. By detecting the slight mechanical vibrations on the container wall, these vibrations are converted into electrical signals that are sent to the explosion-proof main unit of the external level gauge. After processing these signals, they are transformed into digital signals which are then sent to the CPU. The instrument host filters and identifies the various vibration wave patterns detected by the sensing head, removes signals that are not related to changes in liquid level, and uses specialized software to perform complex calculations on the waveforms of these signals. Artificial intelligence algorithms are then employed to analyze them, thereby determining the liquid level height. Therefore, an external level gauge measures the liquid level inside a tank continuously and accurately from outside the tank, without any contact with the liquid or gas inside, thus achieving true isolated measurement. B. Advantages of external level gauges External level gauges have the following advantages: ① They can be used in the most demanding environments. Liquids capable of measuring any pressure ； The liquid with the most severe measurable toxicity ； Capable of measuring the most corrosive liquids ； It can measure liquids that are absolutely sterile or of extremely high purity. ②Safe to use. When measuring liquids that are toxic, corrosive, under pressure, flammable, explosive, volatile, or prone to leakage, since both the measuring probe and the instrument are located outside the container, operators do not come into contact with the liquids and gases inside the tank during installation, repair, and maintenance tasks, ensuring high safety. Leakage will never occur, even when the instrument is damaged or under maintenance. ③Meets environmental protection requirements. It neither leaks liquids nor gases, nor does it pollute the environment; it is a green, environmentally friendly instrument. ④It is the most convenient and cost-effective for installation and maintenance. Since this level gauge requires no openings in the container, no flanges, and no connecting pipes, its installation and maintenance are very convenient. External level gauges do not require manual calibration during installation and use, which makes them very convenient. ⑤High reliability and long service life. Since there are no mechanical moving parts in the level gauge’s sensing head or within the instrument, and it is tightly sealed to be isolated from the outside environment, it does not suffer from wear or corrosion; it is therefore very durable and reliable, with minimal maintenance requirements. ⑥Accurate measurement. The external level gauge can be automatically calibrated continuously to ensure the highest measurement accuracy. In addition, external level gauges are divided into temperature-compensated types, with a measurement accuracy of 1% ； Self-calibrating type, with a measurement accuracy of 0.2%. Among them, self-calibrating level gauges do not require manual calibration. C. Disadvantages of external-type level gauges: ① External-type level gauges still have some shortcomings; they possess a measurement dead zone, and within about 200 mm of the bottom of the container, such gauges can only provide a low-level alarm without being able to indicate the exact level value. Since the level measurement and monitoring of the liquid LV storage tank in this company are primarily aimed at ensuring accurate monitoring at high levels, this defect does not affect normal operation. After analysis and comparison, it is concluded that from the perspective of safe production and maintenance, external-type level gauges are currently the most suitable instruments for measuring the liquid level in LV storage tanks. After being used by a LV alkali company in Tangshan for nearly a year, these gauges have demonstrated good performance. To date, Dinghua Electronics’ external level gauges have successfully assisted hundreds of LV alkali manufacturers in monitoring liquid levels.