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Tianjin Dagu Chemical Co., Ltd., formerly known as Tianjin Dagu Chemical Plant, was established in 1939. It is a large-scale chlor-alkali enterprise that focuses on salt electrolysis as its foundation and aims to develop the marine chemical and petrochemical industries; it produces a variety of products such as caustic soda, polyvinyl chloride resin, propylene oxide, polyethers, liquid chlorine, and synthetic hydrochloric acid. The company is located in the Binhai New Area of Tianjin, China, covering an area of 2.82 million square meters. It boasts great development prospects and is adjacent to Tianjin Xingang, Tianjin Economic-Technological Development Zone, Tianjin Port Free Trade Zone, as well as the Jingjin Tang, Tangjin, and Jinbin highways and Binhai International Airport, all of which provide the company with ample raw materials and fuels for the development of its marine chemical and petrochemical production activities. Its unique geographical location and convenient transportation facilities provide ideal conditions and an environment for the company to engage in domestic and international trade. As a company that has obtained ISO9002 quality system certification, Tianjin Dagu Chemical Co., Ltd. has implemented a series of reforms and innovations in order to achieve and maintain an excellent standard of product quality. Technologically, production facilities for ion-exchange membrane caustic soda, propylene oxide, and PVC produced by the ethylene oxychlorination method, all of which are at the international advanced level, have been introduced one after another. In the production of its key products such as caustic soda, polyvinyl chloride resin, propylene oxide, and polyethers, microcomputer-based automatic control is employed; the four main parameters in the company’s production processes—temperature, pressure, flow rate, and liquid level—are monitored in real time and managed centrally. Since enterprises have many areas where \"hazardous chemicals\" are stored and used, these hazardous chemicals mainly include vinyl chloride, liquefied gas, propylene, phenol, ABS oligomers, butadiene, VCM, and VCM monomers. High standards are required in terms of safety and environmental protection; therefore, measuring instruments must operate without any faults, and even in the event of a fault, they must not pose risks such as combustion, explosion, or leakage. This imposes even higher safety requirements on such measuring instruments. In the past, we have used magnetic flap level gauges, differential pressure level gauges, and imported radar level gauges from certain brands for our tanks. Due to the presence of deposits in some fluids and impurities in others, especially the problem that VCM monomer is volatile and prone to self-polymerization, level measurement has always been a challenge. With magnetic flap level gauges and differential pressure level gauges, the connection parts of the pipelines get blocked after being in use for a while, requiring cleaning, maintenance, and re-calibration ； The emission port of the radar level gauge is covered by volatiles and self-polymerizing substances; before maintenance, it is necessary to empty the tank and replace the contents, a process that is very complex and requires significant effort, thus having an adverse impact on the company’s daily operations. In 2000, we heard about a new type of measurement method for measuring liquid levels in isolation; a new kind of level gauge was introduced – the ELL sonar external level gauge. Its installation, calibration, and maintenance do not require emptying the tank, using fire, or making openings in the tank. Due to its easy installation – all that is needed is to attach a sensor to the bottom outside the tank and connect it to a transmitter, which will then display the liquid level – and since debugging does not affect production, we decided to give it a try. In March 2001, we asked Dinghua Electronics, the inventor and manufacturer of this new type of gauge, to send a unit for testing. We chose a vinyl chloride (VCM) storage tank, where it is difficult to measure the liquid level, as the medium in such tanks is volatile and tends to polymerize, with sediment forming at the bottom of the tank. An ELL sonar-type external level gauge model AE was installed on this tank. After using it for 3 months, the gauge has operated stably, provided accurate level readings, and offered reliable remote transmission – fully meeting our requirements for level measurement. Since 2002, we have successively installed ELL sonar-based external level gauges manufactured by Xi’an Dinghua on all our vinyl chloride horizontal tanks, as well as on the liquefied gas storage tanks; in total, ten such gauges were installed. They have been in use for 15 years now, with virtually no failures occurring. There were a few minor issues over that period, but Dinghua’s customer service team responded very quickly, arriving at the site whenever needed to resolve all problems, thus providing a reliable guarantee for our safe operations. The regular follow-up visits from Xi’an Dinghua’s customer service have made us place great trust in their instruments. In 2010, we expanded our facilities by constructing 9 new storage tanks, all of which were equipped with external level gauges from Xi’an Dinghua. In particular, their instruments helped us solve the problem of measuring liquid levels in the ABS oligomer storage tanks. Currently, ELL sonar-type external level gauges from Dinghua Electronics have become the standard instruments for measuring liquid levels in tanks storing hazardous chemicals.