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A petrochemical company in Shandong has a crude oil backwash tank that is 4 meters high and 1.5 meters in diameter; the tank contains 100 vertically arranged filter rods, with a temperature range of 170–200 degrees℃； Purpose: On-site crude oil filter, mainly used to remove impurities such as sediment and small stones with larger particles contained in crude oil ； Purpose: To protect the proper operation of the pump and ensure the quality of crude oil. Not only is the structure inside the tank complex, but the discharge speed is also high; the process requires discharging the contents every 8 hours, and the discharge process itself takes less than 1 minute. The user originally used an imported radio frequency admittance level switch, which malfunctioned during operation. Both replacement and maintenance required shutting down the system, resulting in high maintenance costs and significant workload. The crude oil backwash tank was circulated every 8 hours; after the level switch failed, the liquid level in the tank had to be estimated based on time, which posed considerable safety risks. Other level measurement instruments were hardly suitable for use under such conditions. Users have been looking for suitable products to replace them. Due to the simple installation and operation of Dinghua Electronics’ external level switches, as well as their immunity to the effects of medium viscosity, temperature changes, and material deposition on the walls of the container, the users decided to try out the ELL external level switches. ▏The ELL variable-frequency ultrasonic level switch is easy to install and operate, meeting the requirements for on-site use. It is a level measuring instrument that allows for measurement without drilling holes, cleaning the tank, using fire, or shutting down production; it operates in a completely isolated manner. All that is required is to attach a pair of probes to the area being measured, which **reduces installation costs and the amount of work needed. ▏The ELL variable-frequency ultrasonic level switch is not affected by the viscosity or purity of the liquid, making it suitable for measuring in complex media. This switch utilizes the attenuation of ultrasonic waves as they propagate through the tank walls, and it detects the remaining energy after propagation. Due to the different transmission rates of ultrasonic energy in gases and liquids, it is possible to easily detect when the liquid level reaches the desired height and trigger an alarm; it also outputs a digital signal to control the inlet and outlet of the liquid. Since the signal propagates inside the tank wall, the viscosity and purity of the liquid being measured do not affect the operation of the frequency-variable ultrasonic level switch. ▏The ELL variable-frequency ultrasonic level switch features a unique \"variable-frequency ultrasonic technology\" that is less affected by temperature changes and material accumulation on the tank walls, enabling accurate level measurement. This switch also boasts a proprietary \"variable-frequency ultrasonic technology\" that makes it highly intelligent; it can automatically compensate for changes in temperature and material accumulation, thereby reducing the rate of false alarms and enhancing the stability of the instrument. This is also a key reason why the ELL variable-frequency ultrasonic level switch offers higher stability compared to products from other manufacturers. ▏The unique product design of the ELL variable-frequency ultrasonic level switch enables it to meet the requirements of high-temperature measurements. For such applications, a \"super-high temperature\" probe as well as a special imported coupling agent have been developed specifically for users, allowing the ELL variable-frequency ultrasonic level switch to better adapt to temperature changes and provide accurate measurements. After signing the trial contract, the technical engineers began to arrange the tests. Given the complexity of these conditions, the user involved many relevant personnel in the testing process as well as in the evaluation of the measurement results. In total, there were 5 tests for liquid intake and discharge; by comparing historical data, accurate measurements could be obtained, and the alarm outputs remained stable. On-site installation is extremely simple: a pair of Ø80 probes are installed 1 meter apart at a height level corresponding to 80% of the tank’s full capacity for crude oil backwashing, and these probes are connected to an instrument transmitter. When the level of crude oil in the tank exceeds this height, the transmitter sends a digital signal to the control system, which then controls the pump. 1. Automated and digital safety production management has been implemented. Without any manual intervention, the crude oil backwash tank automatically controls the inflow and outflow of liquid; data is recorded within the system to calculate the oil output per unit of time. 2. Low cost. **It reduces the workload of operators and saves on labor costs ； Installation and debugging are simple and convenient, with no need for any modification costs. 3. Easy and simple to expand. If more accurate measurement of the liquid level inside the tank is required later, it is sufficient to add measurement points to the tank and install high-temperature external liquid level probes. Through prolonged monitoring by the process department, it has been confirmed that the instruments maintain stable performance, fully meeting the requirements for level measurement under such extreme operating conditions, and continuously providing the control room with updated data on liquid levels and equipment status. Since the PFDavg (T1=1Y) (failure rate under safe conditions) of the ELL variable-frequency ultrasonic level switch is ≤6.04×10-⁵ ； With an SFF (Safety Failure Fraction) of ≤99.35%, the functional safety integrity level is SIL3, allowing it to be used directly in SIS systems (Safety Instrumented Systems). When the oil level in the tank is too high, an alarm is triggered and the interlock stops the liquid feed pump from operating. With the continuous development and improvement of external level measurement technologies, external level gauges are being used in an increasing number of complex operating conditions. Their notable advantages of enabling level measurement without drilling holes, starting fires, emptying tanks, or shutting down production provide users with fast solutions for level measurement, while also saving significant amounts of labor, resources, and time.