Measurement principle of radar level gauge and common troubleshooting method

1. Overview There are many methods of level measurement. For different working conditions and media, different measurement principles can be used. The air blowing method, the static pressure type, the floating ball type, the heavy hammer type, the ultrasonic wave and other commonly used objects Position measuring instruments have their own characteristics and application scope. 2. Principle and technical performance Radar wave is a special form of electromagnetic wave, and radar level meter utilizes the special performance of electromagnetic wave to carry out level detection. The physical properties of electromagnetic waves are similar to those of visible light, and the propagation speed is equivalent to the speed of light. Its frequency is 300MHz-3000GHz. Electromagnetic waves can penetrate interference sources such as space steam and dust, and are easily reflected when encountering obstacles. The better the conductivity of the measured medium or the larger the dielectric constant, the better the reflection effect of the echo signal. The higher the frequency of the radar wave, the smaller the launch angle, the greater the energy (magnetic flux or field strength) per unit area, the smaller the attenuation of the wave, and the better the measurement effect of the radar level gauge. 1. The basic principle of radar level gauge The radar level gauge is composed of: it is mainly composed of transmitting and receiving device, signal processor, antenna, operation panel, display, fault alarm and other parts. Transmit-reflect-receive is the basic principle of radar level gauge work. The antenna of the radar sensor transmits the radar signal of z-5.8GHz in the form of a beam. The reflected signal is still received by the antenna, and the running time of the radar pulse signal from transmission to reception is proportional to the distance from the sensor to the surface of the medium and the level of the material. Namely: h=H-vt/2 where h is the material level; H is the height of the tank; v is the speed of the radar wave; t is the interval time from the radar wave to the reception; When measuring the liquid level, the axis of the sensor is vertical to the surface of the medium; when measuring solid materials, since the solid medium will have a stacking angle, the sensor should be inclined at a certain angle. (2) Try to avoid devices that cause false reflections within the emission angle. In particular, avoid obstructing devices in the 1/3 cone emission area close to the antenna z (because the closer the obstructing device is, the stronger the false reflection signal). If it is unavoidable, it is recommended to use a refraction plate to refract the strong false reflection signal away. This can reduce the energy density of false echoes, making it easier for the sensor to filter out false signals. (3) Avoid the feeding port to avoid false reflections. (4) The sensor should not be installed in the center of the arched tank (otherwise the false echo received by the sensor will be enhanced), and it should not be installed very close to the tank wall. The best installation position is 1/2 of the radius of the container. (5) Avoid installing in places with strong eddy currents. Such as: due to stirring or strong chemical reaction, etc., it is recommended to use a still-pipe or bypass tube for measurement. (6) If the sensor is installed on the nozzle, the antenna must protrude from the nozzle. The horn antenna protrudes from the tube at least 10mm. Rod antenna takes over lengths up to 100 or 250mm. The diameter of the nozzle is 250mm smaller. The method of increasing the diameter of the nozzle can be taken to reduce the interference echo generated by the nozzle. (7) Regarding the still-pipe antenna: the inner wall of the still-pipe must be smooth, and the still-pipe with the opening below must reach the required low liquid level, so that the measurement can be carried out in the pipeline. The sensor type plate should be aligned with the axis of the still-pipe opening. If the measured dielectric constant is less than 4, a reflector should be installed at the end of the still-pipe, or the end of the still-pipe should be bent into a camber to refract the reflected echo at the bottom of the container. 4. Problems existing in application and solutions Everyone for reference. 1. The solution to probe scarring and frequent failures is to increase the installation position of the probe, but sometimes the installation conditions are limited and cannot be improved, the method of interlocking the level measurement value with the pump of the tank should be adopted to solve the problem. This problem: reduce the set value of the z-high material level by about 0.5m. When the material level reaches the z-high value, the feed pump can be stopped or the discharge pump can be turned on. 2. The corresponding improvement method of the radar level gauge is submerged The way to solve this problem is to change the radar level gauge to still-pipe measurement. The still-pipe radar level gauge is still installed at the original opening, and the still-pipe is about 0.2m higher than the exhaust pipe. In this way, even if the slurry overflows from the exhaust pipe, the material will not be damaged. The position meter antenna is submerged by the slurry, and the interference of the eddy current of the stirrer and the emergence of a large amount of steam from the probe are avoided, which reduces the damage to the probe. good measurement results. Using the still-pipe measurement method can improve the measurement conditions of the meter and improve the measurement performance of the meter, which has high promotion and application value. 3. Regarding the influence of foam on the measurement: dry foam and wet foam can reflect the radar wave back and have no effect on the measurement; neutral foam will absorb and diffuse the radar wave,Therefore, the reflection of the echo is seriously affected and there is even no echo. When the surface of the medium is thick and thick foam, the measurement error is large or cannot be measured. In this case, the radar level gauge has no advantage, which is the limitation of its application. 4. For the treatment of antenna scarring: the hanging material with a small dielectric constant has no effect on the measurement in the dry state, while the hanging material with a high dielectric constant has an impact on the measurement. It can be purged with compressed air (or flushed with clean water), and the cooled compressed air can reduce the temperature of flanges and electrical components. Alkaline scabs can also be cleaned with an acid cleaning solution, but level measurement is not possible during cleaning
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