The development history and selection principle of radar level gauge

Radar liquid level gauge is a non-contact liquid level instrument using microwave ranging. In the early days, it was mainly used for level measurement of oil tanks in marine vessels. It overcomes many disadvantages of using mechanical contact type liquid level instruments, such as: difficulty in cleaning, inconvenience in maintenance, etc. Subsequently, radar level gauges were used for level measurement in onshore storage tanks as well as for level measurement in refineries. With the continuous development of the petrochemical industry, the application range of radar level gauges is becoming more and more extensive, especially its high precision meets the requirements of material measurement. The radar level gauge has two working modes, corresponding to two measurement principles. 1. Pulsed microwave method (PTOF) This method is a 'top view' time travel measurement system. The measurement system periodically transmits microwave pulses of a certain frequency with a fixed bandwidth through the antenna, and generates reflections on the material table under test. received by the radar system. The antenna receives the reflected microwave pulse and transmits it to the electronic circuit, and the microprocessor processes the signal, identifies the echo generated by the microwave pulse in the material table, and calculates the liquid level accordingly, and calculates the distance of the measured liquid level. converted into electrical signals. 2. Frequency Modulated Continuous Wave Mode (FMCW) The microwave source of the radar level gauge in this mode is the X-band travel-controlled oscillator, and the microwave emitted by the antenna is a continuous wave whose frequency is modulated linearly. When the echo is received by the antenna , the microwave emission frequency has changed. The frequency difference between the transmitted wave and the echo is proportional to the distance from the antenna to the liquid surface, thereby calculating the liquid level height. Comparison of PTOF and FMCW (1) Working mode: For the PTOF method, the time travel of the pulse can be directly returned to the quartz oscillator which is not affected by temperature. For the FMCW method, expensive oscillator temperature stabilization must be employed, or an internal reference source must be installed, often requiring constant calibration. (2) Power supply requirements: The radar measured by the PTOF method only supplies power to the oscillating system during a small working time, and the radar level gauge using the FMCW method needs to supply power to the oscillating system for about half of the entire working process. (3) In terms of cost: the manufacturing cost of the radar level gauge of the PTOF method is relatively low, and the manufacturing cost of the radar level gauge of the FMCW method is relatively high. There is an ongoing debate as to which method to use to obtain higher accuracy. In fact, radar level gauges currently manufactured according to both measuring principles guarantee an accuracy of 1 mm or more over a fairly wide temperature range. In terms of product type, each manufacturer of radar level gauges offers both industrial control grade radars and trade metrology grade radars. Selection of radar level gauges There are many types of radar level gauges. When selecting a type, the influence of the temperature, pressure, density, viscosity and corrosiveness of the measured medium on its performance should be considered first. Therefore, when selecting the type, one should select the appropriate antenna and meter according to the characteristics of the medium under specific working conditions. Guided wave radar level gauges are recommended for tanks with small volumes and complex shapes or applications where multiple liquid interfaces need to be measured; for tanks with complex measurement environments, such as volatile media, corrosion, high pressure and high temperature etc., it is recommended to use a non-contact antenna radar liquid level gauge. Since the liquid level gauge does not contact the medium, it can avoid affecting its measurement accuracy or damage to the level gauge itself due to the physical and chemical properties of the medium. According to the purpose of medium measurement, radar level gauges with different measurement accuracy are selected. If it is only used for internal costing, the accuracy requirements do not need to be too high; if it is used for custody transfer, a high-precision radar level gauge must be selected. In order to save investment, cost-effective products can be selected on the premise that the measurement accuracy meets the requirements. Since the accuracy, routine maintenance and service life of the radar level gauge directly affect the production efficiency and economic benefits of the enterprise, excellent quality and satisfactory after-sales service are crucial for users. The selection principle is mainly considered from the following aspects, and the corresponding instrument selection specifications are specifically followed: (1) Instrument performance; (2) Medium characteristics; (3) Installation conditions; (4) Environmental conditions; (5) Economic factors. The radar level gauge is composed of an electronic control unit and an antenna, and the accuracy of the radar level gauge is slightly different in practical application and theoretical environment. The main reason is that the interference of the microwave by the tank and its internal obstacles determines the accuracy that can be obtained. The main factors are: (1) impedance jumps inside the instrument and at the connection of the antenna; (2) interference reflections from obstacles in the tank; (3) multiple reflections caused by the tank wall, tank top, and tank bottom; ( 4) Reflection interference caused by ripples on the liquid level surface. In addition, the characteristics of the liquid level medium have a certain influence on the measurement range. The liquid with a small dielectric constant has a great influence on the measurement distance of the radar level gauge, which reduces the measurement range; the liquid with a large dielectric constant has a negative impact on the radar level gauge. The measurement distance has little influence,increase the measurement range. The above is a brief introduction to the selection of radar level gauges. If you have any questions, please contact us. The installation of the liquid level gauge requires attention. According to the installation design specifications of petrochemical instruments, the installation requirements for the radar liquid (material) level gauge are as follows: (1) In the case of measuring the liquid level, it should be installed vertically downward; (2) The measurement material In the case of high position, the microwave beam should be directed to the discharge port at the bottom of the silo;
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