Radar level gauges are measuring instruments based on the time-of-flight principle. Radar waves travel at the speed of light, and the travel time can be converted into a level signal by electronic components. The probe emits high-frequency pulses that propagate along the cable. When the pulse encounters the material surface, it is reflected back and received by the receiver inside the instrument, converting the distance signal into a level signal.
The measurement principle of radar level gauges is the same as that of radar liquid level gauges and radar water level gauges, except that they measure different media and are used in different environments.
Installation considerations:
(1) When measuring liquid materials, the sensor axis should be perpendicular to the medium surface; when measuring solid materials, the sensor should be tilted at a certain angle due to the angle of repose of the solid medium.
(2) Avoid devices that cause false reflections within the emission angle. Especially avoid obstructions within the closest 1/3 of the conical emission area to the antenna (because the closer the obstruction, the stronger the false reflection signal). If unavoidable, it is recommended to use a refractor to refract the excessively strong false reflection signal. This reduces the energy density of the false echo, making it easier for the sensor to filter out the false signal. (3) Avoid installing the sensor near the feed inlet to prevent false reflections.
(4) Do not install the sensor in the center of the arched tank (otherwise, false echoes received by the sensor will be amplified), nor install it too close to the tank wall. The optimal installation position is at 1/2 of the container radius.
(5) Avoid installation in areas with strong eddies. For example, due to stirring or strong chemical reactions, it is recommended to use a waveguide or bypass tube for measurement.
(6) If the sensor is installed on a connecting pipe, the antenna must extend beyond the connecting pipe. For flared antennas, extend at least 10mm beyond the connecting pipe. For rod antennas, the maximum connecting pipe length is 100 or 250mm. The minimum connecting pipe diameter is 250mm. Increasing the connecting pipe diameter can reduce interference echoes generated by the connecting pipe.
(7) Regarding waveguide antennas: The inner wall of the waveguide must be smooth, and the bottom opening of the waveguide must reach the required minimum liquid level to allow for measurement within the pipe. The sensor type label must be aligned with the axis of the waveguide opening. If the dielectric constant being measured is less than 4, a reflector plate needs to be installed at the end of the waveguide, or the end of the waveguide needs to be bent to refract the reflected echo from the bottom of the container.
Problems and Solutions in Application: In some operating conditions, radar level gauges have encountered problems due to improper sensor installation and other factors. Solutions to some common problems are provided below for your reference.
1. Solutions for Probe Scaling and Frequent Failures: The first solution is to raise the probe installation position. However, sometimes installation conditions prevent this. In such cases, the level measurement value should be interlocked with the pump in the tank to solve the problem: reduce the maximum level setting by about 0.5m. When the level reaches this maximum value, the feed pump can be stopped or the discharge pump can be started.
2. The Influence of Foam on Measurement: Dry and wet foam can reflect radar waves back and have no effect on measurement; neutral foam absorbs and diffuses radar waves, thus severely affecting the reflection of the echo or even eliminating the echo altogether. When the medium surface is a thick, viscous foam, the measurement error is large or it becomes impossible to measure. In such conditions, radar level gauges are not advantageous, which is a limitation of their application.
3. Treatment of antenna scale: Scale with a very low dielectric constant has no effect on measurement in a dry state, while scale with a very high dielectric constant does affect measurement. It can be purged with compressed air (or rinsed with clean water), and the cooled compressed air can lower the temperature of the flange and electrical components. Alkaline scale can also be cleaned with acidic cleaning solutions, but level measurement cannot be performed during cleaning.
