1. The working principle of radar level gauge Radar level gauge uses ultra-high frequency electromagnetic waves to launch to the surface of the container to be detected by the antenna. When the electromagnetic wave hits the liquid surface, it is reflected back. The instrument detects the time difference between the launching wave and the echo. Thus, the height of the liquid surface is calculated. The better the conductivity of the measured medium or the greater the dielectric constant, the better the reflection effect of the echo signal.
The radar level gauge is mainly composed of transmitting and receiving devices, signal processors, antennas, operation panels, displays, and the like. Launch-reflection-reception is the basic principle of radar level gauge work. It is divided into time difference type and frequency difference type.
The time difference type is that the transmission frequency is fixed and the running time of the transmitted wave and the reflected wave is measured, and an intelligent signal processor is used to measure the height of the measured liquid level. The relationship between the operating time of the radar level gauge and the liquid level distance is: t=2d/c. Where C is the electromagnetic wave propagation speed, C=300,000 km/s; d is the distance between the liquid level of the measured medium and the probe, and m; t is the time from when the probe transmits the electromagnetic wave to when it receives the reflected electromagnetic wave, s.
The frequency difference equation measures the frequency difference between the transmitted wave and the reflected wave, and converts this frequency difference into an electrical signal that is proportional to the measured liquid level. The transmission frequency of this level gauge is not a fixed frequency, but an adjustable frequency.
2. Characteristics and main performance parameters of radar level gauge Radar level gauge shows excellent performance under flammable, explosive, highly corrosive, high temperature, viscous and other harsh measurement conditions, especially suitable for large scale Measurements such as vertical tanks and spherical tanks. The performance of level gauges varies from manufacturer to manufacturer. See manufacturer's data for details. The understanding of its performance is conducive to the correct use of radar level gauges.
3. Selection of Radar Level Gauge The antenna is a key component of the radar level gauge. The shape of the antenna determines the focus and sensitivity of the radar wave.
The bell mouth antenna is suitable for most occasions, and the focusing characteristics are particularly good. This type of antenna is used in many storage tanks in the field, but it is not suitable for measuring corrosive media.
The mounting flange of the rod antenna is small in size, chemically stable, easy to clean, and insensitive to the adhesion of condensed water. It is particularly suitable for measuring highly corrosive media (such as sulphur) and narrow installation tubes. .
The flanged antenna is suitable for high temperature media, corrosive media, or environments where it cannot be installed on the top.
The parabolic antenna is well-focused and is not affected by heating steam. It is particularly suitable for in-tank measurement of large vessels with heated steam, such as residual oil, asphalt, etc. Measurement range up to 40m
4, the installation of radar level gauge Try to avoid the device in the launch angle caused by false reflection, such as limit switches, temperature sensors. In particular, it is necessary to avoid the presence of obstacle devices in the nearest 1/3 cone-shaped emission area from the antenna (because the closer the obstacle device is, the stronger the spurious reflection signal is). If it cannot be avoided, it is recommended to use a refraction plate to refract too strong false reflections. This reduces the energy density of false echoes, making it easier for the sensor to filter out false signals.
Do not install above the inlet to avoid spurious reflections.
The sensor should not be installed in the center of the arch tank (otherwise the false echo received by the sensor is enhanced), nor can it be installed close to the tank wall. The distance between the inner wall of the tank and the outer wall of the installation tube should be greater than the diameter of the tank. 1/6. If the radar level gauge positioning sensor shown in Figure 1 is not installed in a place with a strong eddy current. Such as: due to stirring or a strong chemical reaction, it is recommended to use waveguide or bypass pipe installation.
If the sensor is mounted on a takeover, the antenna must extend from the take over and the flare antenna extends to take over at least 10 ram as shown in Figure 2 for a standard installation.
If the length of the bell mouth is smaller than the length of the installation tube, use an antenna extension tube, as shown in Figure 3 with the installation of the antenna extension tube.
The length and diameter of the rod antenna connection should be based on the product requirements of different manufacturers. The rod antenna must extend out of the installation tube, as shown in Figure 4 for rod antenna installation.
About the waveguide: The inner wall of the waveguide must be smooth and uniform in diameter; welding must not be carried out, no welding seam of 0.1mm can be produced in any transition section; 180Â° of angle between two rows of waveguides. (It is not 90.) The width of the waveguide groove or the maximum diameter of the waveguide is 1/10, and the deburring does not affect the measurement: the distance between the two holes is 15 cm to 50 cm. The required minimum level must be reached so that measurements can be made in the pipeline. The waveguide structure is usually shown in Figure 5.
5. Problems in the application of radar level gauges and their solutions The sealing of the instrument: The cable inlet and the cover of the instrument are not sealed tightly. As a result, rain or other liquids, dust, moisture, etc. enter the inside of the instrument, causing the circuit part of the instrument to malfunction. Therefore, seals must be made when ordering and must be installed. It may be a good idea to make a protective cover when necessary and wrap the entire instrument. Treatment of antenna crust: The hanging material with a small dielectric constant has no effect on the measurement in the dry state, and the clad material with a high dielectric constant has an influence on the measurement. It can be purged with compressed air (or clean water), and the cooled compressed air can reduce the temperature of the flange and electrical components. The scab can also be periodically cleaned with an acidic cleaning solution, but it cannot be measured during cleaning.
Practice has proved that as long as the radar level gauge is selected to meet the technical conditions and meet the installation requirements, the radar level gauge can operate for a long time and reliably.
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