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Difference between ultrasonic level gauge and radar level gauge

Time: 2012-09-24 14:52:36 source: Author:

We generally call the sound wave whose frequency is more than 20kHz as ultrasonic wave. Ultrasonic wave is a kind of mechanical wave, that is, the propagation process of mechanical vibration in elastic medium. Its characteristics are high frequency, short wavelength, small diffraction phenomenon, and good directivity, which can be directional propagation of rays. The attenuation of ultrasonic wave in liquid and solid is very small, so it has strong penetrability. Especially in the solid which is opaque to light, the ultrasonic can penetrate tens of meters in length, and there will be significant reflection when it encounters impurities or interfaces. This feature is used to measure material level by ultrasonic wave.
In the ultrasonic testing technology, no matter what kind of ultrasonic instrument, it must convert the electric energy into ultrasonic wave, and then receive it back to transform it into electrical signal. The device to complete this function is called ultrasonic transducer, also known as probe. The ultrasonic transducer is placed on the top of the measured liquid and emits the ultrasonic wave downward. The ultrasonic wave passes through the air medium and is reflected back when it meets the water surface. It is received and converted into electrical signal by the transducer. After the electronic detection part detects this signal, it turns into a liquid level signal for display and output.

According to the propagation principle of ultrasonic wave in the medium, if the medium pressure, temperature, density, humidity and other conditions are constant, then the ultrasonic propagation speed in the medium is a constant. Therefore, when the time required for the ultrasonic wave from transmitting to receiving when encountering the reflection of liquid surface is measured, the distance through which the ultrasonic wave passes can be converted, that is, the data of liquid level can be obtained.

There is a blind area in ultrasonic, so the distance between the sensor installation position and the measured liquid must be calculated.
The radar level gauge adopts the mode of transmitting, reflecting and receiving. The antenna of radar level gauge emits electromagnetic wave, which runs at the speed of light. These waves are reflected by the surface of the measured object and then received by the antenna. The time from transmitting to receiving electromagnetic waves is directly proportional to the distance to the liquid surface. The relationship is as follows:
D=CT/2
Where D is the distance from radar level gauge to liquid level
C -- speed of light
T -- electromagnetic wave operation time

The radar liquid level gauge records the time of pulse wave, and the transmission speed of electromagnetic wave is constant, then the distance from liquid level to radar antenna can be calculated, so as to know the liquid level of liquid level.
In practical application, there are two modes of radar level gauge, i.e. FM continuous wave type and pulse wave type. The liquid level meter using FMCW technology has high power consumption, and it has to adopt four wire system, and the electronic circuit is complex. The liquid level gauge with radar pulse wave technology has low power consumption and can be powered by two-wire 24 V DC, which is easy to realize intrinsic safety, high accuracy and wide application range.
Ultrasonic wave uses sound wave, radar uses electromagnetic wave, this is the big difference. Moreover, the penetrability and directivity of ultrasound are much stronger than electromagnetic wave, which is why ultrasonic detection is more popular now.

Differences in main applications:
Ultrasonic and radar are mainly different in measurement principle, which leads to their different application occasions. Radar is based on the dielectric constant of the measured material, while ultrasound is based on the density of the measured material. Therefore, the measurement effect of radar for materials with low permittivity should be discounted. Ultrasonic wave is generally recommended for solid materials. At the same time, radar emits electromagnetic waves, which do not need a medium, while ultrasonic wave is sound wave, which is a kind of mechanical wave and needs a medium. In addition, the components of wave emission are different. For example, ultrasonic wave is transmitted through the vibration of piezoelectric materials, so it can not be used in high pressure or negative pressure occasions, and it is generally only used in atmospheric pressure vessels. Radar can be used in high-pressure process tanks. The transmitting angle of radar is larger than that of ultrasonic wave. The non-contact radar is not recommended in small containers or slender containers, but guided wave radar is generally recommended. The latter is the problem of accuracy. Of course, the accuracy of radar is definitely higher than that of ultrasonic wave. The high-precision radar must be used on the storage tank instead of ultrasonic wave. As for the price, ultrasound is generally lower than radar. Of course, the price of some ultrasonic waves with large range is also very high, such as the range of 6-70 meters. At this time, the radar can not reach it, so we can only choose ultrasonic wave!
The transmission of sound waves requires a medium, so it cannot be transmitted in a vacuum. Therefore, the limitations of ultrasound in practical application are still very large, compared with radar, there are many deficiencies. First of all, the ultrasonic level meter has a temperature limit, the temperature at the probe can not exceed 80 degrees, and the acoustic velocity is greatly affected by temperature. Secondly, the ultrasonic level gauge is greatly affected by the pressure, which is generally within 0.3MPa, because the sound wave is generated by vibration, and the acoustic components will be affected when the pressure is too high. Third, when there is a lot of fog or dust in the measurement environment, it will not be able to measure well. All these restrict the application of ultrasonic level meter. In contrast, radar is electromagnetic wave, which is not affected by vacuum degree, and has a wide application range for medium temperature and pressure. With the emergence of high frequency radar, its application range is more extensive. Therefore, in the level measurement, radar is a very good choice.
However, both radar and ultrasonic liquid level meter must pay attention to the installation position and blind area in the installation process. For example, when it is installed on the tank body, it should not be installed at the feeding port or near the ladder. The distance from the tank wall should be 300 to 500 mm to prevent echo interference. When there is stirring and the liquid level fluctuates greatly, the proper installation method should also be selected. In short, there is nothing perfect.

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