​Detailed description of the similarities and differences between ultrasonic level meters and radar level meters

The radar in radar level meter is the abbreviation of Radio Detection and Raging. Because it emits microwaves to the target to calculate the level distance, it is also called microwave level meter. Ultrasonic level meter is a non-contact measuring instrument like radar level meter, but it calculates the level distance by emitting ultrasonic pulses to the target to be measured. It is a microprocessor-controlled digital level meter. Is there any difference between ultrasonic level meter and radar level meter? Of course, let's start with electromagnetic waves.

The band of electromagnetic waves is generally in the range of 3kHz~3000GHz, and the band is relatively wide, while microwaves refer to electromagnetic waves with a frequency of 300MHz~300CHz. In level measurement instruments, the frequency band used by microwaves is approximately between 4~30GHz, among which the frequencies of 5.8GHz, 10GHz, 24GHz and 5.8GHz belong to C-band microwaves; the frequency of 10GHz belongs to X-band microwaves; and the frequency of 24GHz belongs to K-band microwaves.

Unlike electromagnetic waves, sound waves are mechanical waves with a frequency range of 20Hz~20kHz. If the vibration frequency of sound waves is higher than 20kHz or lower than 20kHz, it is indistinguishable to the human ear, so sound waves with a frequency higher than 20kHz are called "ultrasound waves".

Electromagnetic waves and sound waves not only have different frequencies, but also different principles. For sound waves, their generation must rely on the vibration of matter and cannot propagate in a vacuum; electromagnetic waves are generated by the oscillation of electrons. Since they are matter themselves, they do not need a medium to propagate and can propagate in a vacuum. The common point between electromagnetic waves and sound waves is that when they encounter different media, they will undergo refraction, reflection, diffraction, scattering and absorption. Ultrasonic level meters and radar level meters use this phenomenon to measure the level distance.

The working theory of ultrasonic level meter actually originates from sonar technology. Earlier ultrasonic level meters used liquid to conduct sound, and installed ultrasonic transducers, i.e. probes, at the bottom of the tank, so that ultrasonic waves were emitted from the bottom, propagated through the liquid medium to its surface, and then reflected back to the transducer. Although this measurement method of installing ultrasonic level meters at the bottom of the tank is feasible, it is not very practical, because the speed at which ultrasonic waves propagate in different measured media is also different, and the measurement data will inevitably have deviations; and the method of installing probes at the bottom of the tank is also difficult to install to a great extent. Therefore, humans realized the convenience of ultrasonic level meters using air as a sound-conducting medium, and began to gradually install them on the top of the tank, greatly improving their working efficiency.

The sound wave signal of the ultrasonic level meter is reflected on the interface of different acoustic resistivity (acoustic resistivity = material density x sound velocity). Due to the different densities of air and materials, their acoustic resistivity is also different. However, since ultrasonic waves are mechanical waves, when propagating in the air, their wavelength is less than 17mm, and the propagation speed is affected by temperature (for example, when the temperature is 0℃, the sound velocity is 331.6m/s; when the temperature is 20℃, the sound velocity is 344m/s). Therefore, for an ultrasonic level meter to become a widely used mature product, it must have temperature compensation technology.

Jiwei Uson-11 ultrasonic level meter is an economical level measuring instrument, equipped with internationally advanced temperature compensation technology, and uses microprocessor program control, intelligent signal processing technology and special echo processing method to effectively avoid false echoes. In addition, the protection level of the whole machine is as high as IP66, and the measurement results are accurate and reliable.

Compared with ultrasonic level meters, microwave signals of radar level meters are reflected at interfaces with different dielectric constants. Dielectric constant is a coefficient representing the insulation capability characteristics, generally expressed as ε, which changes with temperature and the frequency of electromagnetic waves propagating in the medium. Generally, the larger the dielectric constant, the stronger the ability to bind charges; the smaller the dielectric constant, the better the insulation.

The microwave signal of the radar level meter propagates at the speed of light, and the speed is almost unaffected by the characteristics of the medium. The propagation attenuation is also very small, about 0.2dB/km. However, the strength of the echo signal is closely related to the reflection of the measured liquid surface. It can be said that the reflectivity on the measured liquid surface is related to the area and shape of the measured material, and mainly depends on the relative dielectric constant (εr) of the material. If the relative dielectric constant is high, the reflectivity is also high, and the echo is strong; if the relative dielectric constant is low, the material will absorb part of the microwave energy and the echo is weak. Most of the materials measured by radar level on the market today have a relative dielectric constant εr>4, but for materials with low dielectric constants, it is necessary to add a waveguide to enhance the echo signal, or only high-end radars with a measurement limit of εr>2 can be used.