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Application background: In the production processes of the petroleum and chemical industries, storage tanks for \"hazardous chemicals\" can be found everywhere, including spherical tanks, vertical tanks, and horizontal tanks. This is because the raw materials, semi-finished products, and finished products of oil and chemical enterprises are mostly liquids – gasoline, diesel, liquefied gas, liquid chlorine, liquid ammonia, and so on, an endless list. These storage tanks serve primarily to store and transport liquids during the production process, playing an important role in factories. “\"Hazardous chemicals\" refer specifically to those liquids, solids, and gases that are flammable, explosive, toxic, or highly corrosive; this article focuses only on measurement methods for liquids. In modern enterprises, temperature, pressure, liquid level, and flow rate are known as the \"four key industrial parameters,\" which require accurate measurement in real time. The importance of level measurement: The reliability and accuracy of tank level measurement directly affect the daily operations of a plant. In a plant, liquids—especially toxic and harmful ones—if they reach high levels without any sensors to promptly issue alarms and initiate control measures, can lead to spills. At minimum, this disrupts normal production; at worst, it can result in safety accidents, posing serious threats to the health and safety of employees as well as the company’s property ； If the liquid level drops too low and the sensor fails to emit an accurate alarm signal, it can cause the pump to run idle, leading to damage and eventual failure. Level measuring instruments are classified based on whether the instrument and its connectors come into contact with the liquid inside the tank. They are divided into contact type (also known as immersed type), non-contact type, and completely non-contact type (also known as isolated type). The commonly used contact-type level measurement instruments in oil and chemical industries currently include float level gauges, magnetic flap level gauges, differential pressure level gauges, servo level gauges, etc ； Non-contact level measurement instruments mainly include radar level gauges and ultrasonic level gauges ； Fully contactless level measurement instruments include external level gauges and gamma-ray level gauges. Since both contact-type and non-contact-type level measurement instruments require openings to be made in the tank during installation, it is necessary to empty the tank and stop production before installation. Welding work may also be required during installation, resulting in a halt in production; this inevitably incurs unnecessary expenses and even significant financial losses for the factory. Therefore, many oil and chemical companies now use completely contactless level measurement instruments for measuring the liquid level in their \"hazardous chemicals\" storage tanks, such as sonar-based external level gauges and ultrasonic external level switches. Below is a measurement scheme for an external level gauge that is easy to install, safe and reliable, with a high safety factor. Measurement scheme l: Instruments used – one ELL sonar-type external level gauge and one ELL ultrasonic external level switch. Functions: 1. Obtain real-time liquid level ； 2. High liquid level, very high liquid level alarms; low liquid level, very low liquid level alarms ； 3. Enter the Safety Instrumented System (SIS) to configure 3-out-of-2 or 2-out-of-2 alarm interlocks ； 4. Automatic calibration accuracy, maintenance-free. l Installation method: Online installation without stopping the operation, removing the tank contents, or making any cuts ； The level measurement probe is magnetically attached to the outside of the tank wall, while the transmitter is installed near the tank; the transmitted data is sent to the central control system. User benefits: It helps to raise the enterprise’s level of safety in production, meeting the requirements set forth in various regulations such as AQ3036-2010, An Jian Zong Guan San [2014] No. 68, Order No. 84 issued by the State Administration for Work Safety, An Jian Zong Guan San [2015] No. 113, and An Jian Zong Guan San [2016] No. 53, regarding the requirements for liquid level instruments in tanks used for storing hazardous chemicals ； The external level gauge for LELL has a \"safety integrity\" level of SIL3, meeting the requirements for instruments in SIS systems ； It is simple and fast to install, **saving project time ； l Avoid economic losses caused by parking ； l Maintenance-free, **reducing the workload of instrumentation staff.