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Radar level meter and ultrasonic level meter are two important instruments for measuring level, which are widely used in continuous level measurement of various tanks, silos and pools in chemical plants, food processing plants, building material plants, raw material plants and other factories to monitor industrial processes. Because of their similar functions, they are often replaced by each other. In fact, the boundary between the two is not clear in the application. In order to help users better distinguish between the two, this paper introduces the different principles and choices of radar level transmitter and ultrasonic level meter as follows.
A. The different principles of two kinds of level meter
1、Radar level meter
According to the working mode, radar level meter is mainly divided into two types: pulse type and continuous frequency modulation type.
Pulse type radar level meter adopts the principle of microwave (emission → reflection → reception), the electromagnetic wave signal emitted by the antenna, and produce reflection on the surface of the measured material, the reflected echo signal is received by the radar level meter, and then calculate the travel time from emission to reception.
The working principle of continuous frequency modulation radar level meter is different from pulse type, the electromagnetic wave signal is reflected by the liquid surface and received by the same antenna, then based on the technical principle of fast Fourier (FFT) transform, the signals of different frequencies in the time domain are transformed into the frequency spectrum in the frequency domain, and the distance from the antenna to the material surface is calculated according to the frequency difference between the emitted signal and the return signal proportional to the distance to the surface of the medium.
2、Ultrasonic level meter
Similar to the pulsed radar level meter, the ultrasonic level indicator also uses the reflection principle of echo to measure the height of the level, the only difference between the two is that the radar level measurement uses electromagnetic waves, without the propagation medium, but the ultrasonic level meter uses mechanical waves, which must be propagated with the help of certain media. The propagation speed is a constant
Second, the selection of two kinds of level meter applications
Ultrasonic level meter is generally used for level measurement under more stable conditions because the propagation speed of its sound waves is closely related to the temperature and pressure of the propagating medium and the characteristics of the measured medium; while the radar level meter selection should take into account the dielectric constant of the medium, the height of the silo, the material form and stability and other factors, so as to determine its microwave frequency, beam angle and antenna type.
(1) Dielectric constant
As the attenuation coefficient of electromagnetic waves is inversely proportional to the square root of the dielectric constant of the medium, therefore, the larger the dielectric constant of the measured medium, the less the attenuation of electromagnetic waves, the stronger the reflected signal received by the level meter, and in this case, the level measurement is more suitable for radar level sensor.
(2) Measuring range
Compared with the radar level meter, the measurement range of ultrasonic level meter is shorter, generally not more than 15 m. Therefore, in large or super-large storage tanks, towers and tanks, boilers and other equipment applications, the radar level meter is more suitable.
(3) Product characteristics
As temperature and pressure have a greater impact on the measurement of ultrasonic level sensor, it generally cannot be used in high pressure or negative pressure, so ultrasonic level meter is mostly used in atmospheric pressure vessels. The radar level meter is not affected by temperature and pressure, and can be used in high temperature and high pressure applications. This is because mechanical waves are susceptible to the influence of the propagation medium, and the energy attenuation is relatively large, and the performance is more obvious in gaseous or inhomogeneous media.
Electromagnetic wave propagation is stronger than ultrasound, so the range of the radar level indicator is also larger than ultrasound, especially now using high frequency and continuous frequency modulation technology, making its range further increase.
Radar level meter on the environment and the medium itself to produce a stronger ability to distinguish the disturbance, but also better able to eliminate interference, so that it can better ensure the measurement accuracy. It is also because the radar level meter's measurement range is wider, higher accuracy, the measurement technology and instrument structure is more high-end, complex, so the price of the radar level meter is more expensive than the ultrasonic level meter, many users for cost considerations, more use of ultrasonic level meter instead of radar level meter.
(4) Material form and stability
When the material is solid or powder state, due to refraction, diffuse scattering and other influences, so that the effective echo reduced, clutter interference increased, in this case, the choice of ultrasonic level transmitter is not recommended. However, when the measurement surface will fluctuate, the choice of radar level indicator also has the problem of effective echo detection difficulties.
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