We generally refer to sound waves with frequencies exceeding 20kHz as ultrasonic waves. Ultrasonic waves are a type of mechanical wave, which is a propagation process of mechanical vibration in elastic media. Their characteristics are high frequency, short wavelength, small diffraction phenomenon, and good directionality, which can become rays and propagate in a directional manner. Ultrasonic waves have very little attenuation in liquids and solids, therefore they have strong penetration ability, especially in opaque solids. Ultrasonic waves can penetrate a length of several tens of meters and have significant reflections when encountering impurities or interfaces. Ultrasonic wave level measurement utilizes this feature.
In ultrasonic testing technology, regardless of the type of ultrasonic instrument, it is necessary to convert electrical energy into ultrasonic waves and emit them, then receive them and convert them into electrical signals. The device that completes this function is called an ultrasonic transducer, also known as a probe. Place the ultrasonic transducer above the measured liquid and emit ultrasonic waves downwards. The ultrasonic waves pass through the air medium and are reflected back when encountering the water surface. They are then received by the transducer and converted into electrical signals. The electronic detection part detects this signal and converts it into a liquid level signal for display and output.
According to the principle of ultrasonic propagation in a medium, if the conditions such as pressure, temperature, density, and humidity of the medium are constant, the propagation speed of ultrasonic waves in that medium is constant. Therefore, when the time required for the ultrasonic wave to be reflected and received upon encountering the liquid surface is measured, the distance traveled by the ultrasonic wave can be converted to obtain the data of the liquid level.
Ultrasound has blind spots, and the distance between the reserved sensor installation position and the measured liquid must be calculated during installation.
The radar level gauge adopts a working mode of emission reflection reception. The antenna of the radar level gauge emits electromagnetic waves, which travel at the speed of light. These waves are reflected on the surface of the tested object and then received by the antenna. The time from emission to reception of electromagnetic waves is proportional to the distance to the liquid surface, and the relationship is as follows:
D=CT/2
In the formula, D – distance from radar level gauge to liquid level
C – Speed of light
T – Electromagnetic wave operating time
The radar level gauge records the time that the pulse wave has experienced, while the transmission speed of the electromagnetic wave is constant, so the distance from the liquid level to the radar antenna can be calculated, thereby determining the liquid level of the liquid level.
In practical application, there are two types of radar level gauges, namely frequency modulated continuous wave and pulse wave. The liquid level gauge using frequency modulation continuous wave technology has high power consumption and requires a four wire system, resulting in complex electronic circuits. The liquid level gauge using radar pulse wave technology has low power consumption, can be powered by a two wire 24V DC, is easy to achieve intrinsic safety, high accuracy, and a wider range of applications.
Ultrasound uses sound waves, while radar uses electromagnetic waves, which is the biggest difference. Moreover, the penetration ability and directionality of ultrasound are much stronger than electromagnetic waves, which is why ultrasound detection is now more popular.
Differences in main application scenarios:
The difference in measurement principles between ultrasound and radar mainly leads to their different application scenarios. Radar is based on the dielectric constant of the measured substance, while ultrasound is based on the density of the measured substance. So the measurement effect of radar for materials with low dielectric constant needs to be compromised, and ultrasonic waves are generally recommended for solid materials. At the same time, radar emits electromagnetic waves without the need for a propagation medium, while ultrasonic waves are sound waves, mechanical waves that require a propagation medium. In addition, the emission method of waves is different for different components. For example, ultrasound is emitted through the vibration of piezoelectric materials, so it cannot be used in situations with high pressure or negative pressure. Generally, it is only used in atmospheric containers. And radar can be used in high-pressure process tanks. The emission angle of radar is larger than that of ultrasound, and non-contact radar is not recommended for small or slender containers. Generally, guided wave radar is recommended. Finally, there is the issue of accuracy. Of course, the accuracy of radar is definitely higher than that of ultrasound, and high-precision radar is definitely used on storage tanks instead of ultrasound. As for the price, ultrasound is generally lower than radar. Of course, some ultrasonic waves with a large range are also very expensive, such as a range of 6-70 meters. At this time, radar cannot reach it, so ultrasound can only be chosen!
The transmission of sound waves requires a medium, so they cannot propagate in a vacuum. So the limitations of ultrasound in practical applications are still significant, and compared to radar, it has many shortcomings. Firstly, the ultrasonic level meter has a temperature limit, generally the temperature at the probe cannot exceed 80 degrees, and the speed of sound waves is greatly affected by temperature. Secondly, ultrasonic level meters are greatly affected by pressure, usually within 0.3 MPa, because sound waves rely on vibration to be emitted, and the sound producing components will be affected when the pressure is too high. Thirdly, when there is a large amount of fog or dust in the measurement environment, it will not be possible to measure well. All of these limitations limit the application of ultrasonic level meters. Compared to it, radar is an electromagnetic wave that is not affected by vacuum, and has a wide range of applications for medium temperature and pressure. With the emergence of high-frequency radar, its application range has become more extensive, so radar is a very good choice in level measurement.
However, whether it is a radar or an ultrasonic level gauge, attention must be paid to the installation position and blind spots during the installation process. For example, when installing on the tank, do not install it at the feed inlet or near the ladder. The distance from the tank wall should be 300 to 500mm to prevent echo interference. When there is stirring and large liquid level fluctuations, it is also necessary to choose a suitable installation method. In short, there is nothing perfect.
1. Radar measurement range is much larger than ultrasound.
2. Radar has horn type, pole type, and cable type, which can be applied to more complex working conditions compared to ultrasound.
3. Ultrasonic accuracy is not as good as radar.
4. The relative price of radar is relatively high.
5.When using radar, the dielectric constant of the medium should be considered.
6. Ultrasonic cannot be used in vacuum, high steam content or foam on liquid surface.
Post time: 15-12-23