The application of radar technology in non-contact liquid level measurement has developed rapidly in recent years. The transducer in ultrasonic level gauge is the eye, while the tuner and antenna in radar level gauge are the eyes, and echo processing is the brain of level gauge. Radar level gauge inherits the echo processing technology of ultrasonic level gauge. ?

The electromagnetic wave emitted by radar level gauge will be reflected when it hits the measured medium, and the quality of the reflected echo reflects the application effect of the level gauge. Echo quality is defined as the minimum echo amplitude (echo amplitude in the worst condition) compared with the maximum noise amplitude (amplitude of false echo, multipath reflected wave, etc.). ). The greater the echo quality value, the better the application effect of the level gauge.

The echo intensity is mainly affected by the following factors?

The more stable the dielectric constant of the propagation medium is, the more favorable it is for propagation. Radar wave is an electromagnetic wave, which is not affected by the stability of propagation medium, but only related to its dielectric constant. This is a major difference between radar technology and ultrasonic technology. ?

The flatter the surface of the measured medium, the greater its dielectric constant, which is more conducive to echo reflection. ?

Therefore, when considering the field working conditions, we should pay special attention to these two aspects: (1) the air dielectric constant distribution between the antenna and the measured medium; (2) Surface state and dielectric constant of the measured medium. ?

Radar level gauge has the advantages of being unaffected by air fluctuation, small attenuation with distance and strong penetration. ?

Limitations of radar: 1) Dielectric constant affects the performance of radar. Theoretically, radar attenuation in vacuum is very small. When there are radar attenuation substances in the air, such as dust powder (graphite, ferroalloy, etc.). ) High dielectric and high water vapor will affect the measuring distance and effect. 2) Volatile gas of the measured medium will gather on the antenna, and water vapor will converge on the antenna. At this time, radar wave transmission will be affected, and radar waves cannot be transmitted in severe cases. 3) The dielectric constant of the measured medium should not be too small. 4) Although the temperature and pressure have little influence on the radar, the radar antenna is made of materials, and the range that the radar can adapt to temperature and pressure is related to the materials used and the sealing structure. ?

At present, radar level gauge has become the mainstream product in the market, while low-frequency radar level gauge is gradually eliminated in the main application fields despite its relatively low price. From the application of ultrasonic level gauge, it can be seen that in order to obtain good echo, the working frequency of the transducer is about 40KHz, the wavelength is about 9mm, and the opening angle of the transmitted wave is 7-8. The higher the working frequency, the smaller the opening angle, but the smaller the range. Compared with ultrasonic wave, radar liquid level gauge should work at 26GHz, and its wavelength is 1 1mm, so as to obtain the echo with the above effect. When a horn with a diameter of 100mm is used, the emitted wave with an opening angle of 7-8 can be obtained. If the working frequency of radar is 6GHz, the working frequency equivalent to ultrasonic wave is 10KHz. However, the ultrasonic material level meter with the working frequency of 10kHz is not ideal in material level measurement, especially not suitable for solid material level measurement. Compared with low-frequency radar, high-frequency radar has the following advantages:

1) high frequency radar level meter (mainly refers to 26GHz and 24GHz) has the advantages of high energy, small beam angle (generally, the beam angle of φ 95 horn antenna is 8o, while the diameter of φ 246 horn antenna of 6GHz low frequency pulse radar is 15o), small antenna size and high accuracy. ?

2) The wavelength of 2)26GHz radar is 1 1mm, and that of 6 GHz radar is 50 mm.. When measuring bulk material level with radar, radar wave reflection mainly comes from diffuse reflection of material surface. The intensity of diffuse reflection is directly proportional to the material size and inversely proportional to the wavelength, while the diameter of most bulk materials is much smaller than 50 mm, which is why 26GHz radar is the best choice for bulk material level measurement at present. ?

3) In some small tank applications with small diameter and low height, the antenna length of 6GHz radar (300-400mm) invisibly increases the blind area (about 600mm). Due to the poor directivity (large opening angle) of 6GHz radar, multipath reflection will occur in small tanks. 26GHz radar has high frequency, short antenna and good directivity, which overcomes the shortcomings of 6GHz radar and is suitable for small tank measurement. ?

4) Due to the harsh outdoor environment, with the passage of time, the radar antenna will accumulate dirt, water vapor, etc. Moreover, the antenna of 26GHz radar is small, and adding radome can greatly improve the influence of dirt and water vapor; 6GHz radar antenna is large, so it is difficult to add radome. And the instrument is heavy and difficult to clean. ?

5) Because of the good directivity of the 26GHz radar, many bad working conditions can be measured only by isolating the radar from the container. ?

At present, the price of 26GHz radar level gauge is equivalent to that of 6GHz radar level gauge, which further promotes the application of 26GHz radar level gauge. ? It can be predicted that the market share of 6GHz radar level gauge will be greatly reduced. With the development of technology, we are looking forward to the emergence of radar level gauge with higher frequency (such as 90GHz), smaller opening angle (such as 2 and 3) and smaller volume. We will make unremitting efforts in this field to make radar level measurement to the extreme. ?

Guided Wave radar level meter: A Supplement to Non-contact radar level meter?

The level gauge of guided wave radar emits a pulsed electromagnetic field with the radius of 100mm centered on the guided wave cable, which propagates forward along the cable and returns when it meets the medium. Guided wave radar not only has the characteristics of non-contact radar, but also has good directivity and penetration at low frequency (500M- 1GHz). The disadvantages are obvious, especially in solid measurement, which is inconvenient for debugging and maintenance, and cables are often worn or even broken. Some special applications can be realized by using the low frequency penetration of guided wave radar level gauge. Such as: oil-water interface; And the end reflection of the cable is used to measure the powder level of the powder with very small dielectric constant (dust removal powder bin). ?

Guided wave radar level gauge has more advantages than pulse (non-contact) radar level gauge at high temperature and high pressure. Pulse radar antenna is made of stainless steel and PTFE. The maximum operating temperature of PTFE is 200℃ and the maximum pressure is 4MP. When the guided wave radar is made of stainless steel and ceramics, the maximum service temperature is 400℃ and the maximum pressure is 40MP. ?

Attention:? Dielectric constant, the position of the discharge port, if the dielectric constant is low and the dust concentration is high, it is recommended to replace the heavy hammer or use the 3D material level scanner (mainly for the products with high dust working conditions to break the bottleneck of material level measurement and control)!