In non-contact level measurement, the application of radar technology has developed rapidly in recent years. 发出的电磁波碰到被测介质被反射，反射回波的质量反映了物位计应用效果。 The electromagnetic wave emitted by the radar level gauge hits the measured medium and is reflected. 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 under the worst conditions) than the maximum noise amplitude (the amplitude of false echoes, multipath reflections, etc.). The larger the echo quality value, the better the application of the level gauge.
The echo strength is mainly affected by the following factors: The more stable the dielectric constant of the propagation medium, the more favorable it is for propagation. Radar waves are electromagnetic waves, and electromagnetic waves are not affected by the stability of the propagation medium during the propagation process, and are only related to their dielectric constant. This is a major difference between radar technology and ultrasonic technology.
The flatter the surface of the measured medium, the larger the dielectric constant is, the better it is for echo reflection.
Therefore, when considering site conditions, special attention should be paid to these two aspects: (1) the distribution of the dielectric constant of the air from the antenna to the measured medium and (2) the surface state of the measured medium and its dielectric constant.
The advantages of radar level gauges are that they are not affected by air fluctuations, have small attenuation with distance, and have strong penetrating power.
Limitations of radar: 1) The dielectric constant is the factor that affects the performance of radar. Theoretically, the radar attenuation is very small in a vacuum. When there is radar attenuation material in the air, for example: dust powder (graphite, iron alloy, etc.) containing high dielectric ), The water vapor is very large, and the measurement distance and effect are affected. 2) The volatile gas of the measured medium will accumulate on the antenna, and water vapor will condense on the antenna. At this time, the radar wave emission will be affected. In severe cases, the radar wave cannot be emitted. 3) The dielectric constant of the measured medium cannot be too small. 4) Although the temperature and pressure have little effect on the radar, the radar antenna is made of materials. The range of use of the radar to adapt to temperature and pressure depends on the materials and sealing structure used.
Radar level gauges have now become mainstream products on the market. Although low-frequency radar level gauges have the advantage of relatively low prices, they are gradually being phased out in the main application areas. It is known from the application of the ultrasonic level meter that to obtain a better echo, the working frequency of the transducer is about 40KHz and the wavelength is about 9mm. At this time, the opening angle of the transmitted wave is 7 ° -8 °. The higher the operating frequency, the smaller the opening angle, but the smaller the range. In analogy with ultrasound, the radar level gauge needs to obtain an echo with the above-mentioned effect, and its operating frequency should be 26 GHz. At this time, its wavelength is 11 mm. When using a horn with a diameter of 100mm, a 7 ° -8 ° open wave can be obtained. If the radar operating frequency is 6GHz, it is equivalent to the ultrasonic operating frequency of 10KHz. The ultrasonic level meter with a working frequency of 10kHz is not ideal in the level measurement, and is not particularly suitable for the measurement of solid level. Compared with low-frequency radar, high-frequency radar has the following advantages:
(主要指26GHz和24GHz)具有能量高，波束角小(一般Φ95的喇叭天线的波束角为8o，而6GHz低频脉冲雷达的喇叭天线直径为Φ246时，波束角为15o)，天线尺寸小，精度高等优点。 1) High-frequency radar level gauges (mainly referring to 26GHz and 24GHz) have high energy and small beam angle (generally, the beam angle of a horn antenna of Φ95 is 8o, and when the diameter of a horn antenna of a 6GHz low-frequency pulse radar is Φ246, the beam angle is 15o), small antenna size and high accuracy.
2) The wavelength of the 26GHz radar is 11mm, and the wavelength of the 6GHz radar is 50mm. When the radar measures the bulk material level, the radar wave reflection mainly comes from the diffuse reflection on the material surface. The intensity of the diffuse reflection is proportional to the material size and inversely proportional to the wavelength. The material diameter is far less than 50mm, which is why the current 26GHz radar is the best choice for bulk material level measurement.
3) In some small tank applications with small diameter and short height, the 6GHz radar antenna length (300-400mm) virtually increases the blind area (about 600mm). Due to the poor directivity of the 6GHz radar (large opening angle), it will occur in the small tank. Multipath reflection; 26GHz radar has high frequency, short antenna and good directivity. It overcomes the shortcomings of 6GHz radar and is suitable for small tank measurement.
4) Due to the harsh environment on the site, over time, the radar antenna will accumulate dirt, water vapor, etc. The 26GHz radar antenna is small, and adding a radome can greatly improve the impact of dirt and water vapor. The large 6GHz radar antenna makes it difficult to add a radome. And the instrument is heavy and difficult to clean.
5) Due to the good directivity of the 26GHz radar, many harsh conditions can be measured by simply isolating the radar outside the container.
At present, the price of the 26GHz radar level meter has been comparable to the price of the 6GHz radar level meter, which has further promoted the application of the 26GHz radar level meter. It can be predicted that the market share of the 6GHz radar level gauge will be greatly reduced. With the advancement of technology, we expect the launch of radar level gauges with higher frequencies (such as: 90GHz), smaller opening angles (such as: 2 °, 3 °), and smaller volumes. We will make unremitting efforts in this field to achieve the ultimate radar level measurement.
The guided wave radar level gauge transmits a pulsed electromagnetic field, with the guided cable as the center 100mm in radius, propagates along the cable, and returns when it encounters the medium. In addition to the characteristics of non-contact radar, guided wave radar has good directivity and low frequency (500M-1GHz) and good penetrability. The disadvantages are obvious. Especially in solid measurement, debugging and maintenance are not convenient, and they often wear out or even break cables. Low frequency penetration of guided wave radar level gauges can be used to achieve some special applications. Such as: oil-water interface; and the use of reflection at the end of the cable to measure powder level (dust removal powder bin) with very small dielectric constant.
与脉冲(非接触)雷达物位计相比更具优势。 Under conditions of high temperature and high pressure, guided wave radar level gauges have more advantages than pulsed (non-contact) radar level gauges. The 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 ceramic, the maximum operating temperature is 400 ° and the maximum pressure is 40MP.