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In the mid-1960s, radar level gauges began to be produced and used abroad. It is a non-contact liquid level measuring instrument that adopts microwave measurement technology and is mainly used for liquid level measurement in marine oil tanks, overcoming many disadvantages of using mechanical contact-type liquid level instruments. Subsequently, radar level gauges were used for liquid level measurement in onshore storage tanks and in refining units. With the development of the petrochemical industry, its application scope has become increasingly wide, especially its relatively high accuracy meeting the requirements of material measurement.

Radar level gauges have two working modes: pulsed microwave mode (PTOF) and frequency-modulated continuous wave mode (FMCW), corresponding to different measurement principles respectively. PTOF is a "top-down" time-of-flight measurement system that emits microwave pulses of a fixed frequency through the antenna, and the measurement system calculates the liquid level based on the reflected microwave pulses; FMCW transmits a continuous wave with a linearly modulated frequency through the antenna and calculates the liquid level height based on the frequency difference between the transmitted wave and the reflected wave.

During the development process, radar level gauges have been continuously improved and perfected. For example, more advanced technologies are adopted to improve measurement accuracy, additional functions are added to meet different application requirements, and costs are reduced through optimized design. Meanwhile, with the development of industrial automation and intelligence, radar level gauges have gradually been integrated with other systems to achieve more efficient production and management.

In recent years, radar level gauges have been widely applied globally. In industries such as chemical, petroleum, pharmaceutical, and food, radar level gauges are used for liquid level measurement in equipment such as storage tanks, reactors, and pipelines to ensure the safety and stability of the production process. Besides, in fields such as water conservancy, environmental protection, and municipal administration, radar level gauges are also used for monitoring and controlling water levels and liquid levels.

Our Fuzhou CHINASIMBA Electronics Co., Ltd. (CHINASIMBA), established in 2004, is the developer and publisher of the world's first terahertz 120GHz radar level meter. In the past 20 years, we have remained true to our original intention and focused on designing and manufacturing industrial automation products.

CHINASIMBA is a cultivator of radar level measurement technology in China. It constantly introduces new ideas and innovations in measurement technology fields such as pulsed radar, Doppler radar, FMCW frequency-modulated wave radar, 120GHz terahertz 3D level scanner, metrology-grade magnetostrictive liquid level gauge, etc., providing technical support for solving application problems for industrial-level customers all over the world. Over the past two decades, we have provided radar OEM products for 30 industrial enterprises and various civil radar products for 150 customers worldwide.

The working principle of millimeter-wave radar for liquid level measurement is based on the emission and reception of high-frequency electromagnetic waves and advanced signal processing technology. The radar system first emits millimeter-wave signals to the liquid surface to be measured through the transmitting antenna. These signals will be reflected when encountering the liquid surface. The reflected electromagnetic wave signals are captured by the receiving antenna and converted into electrical signals for processing. The signal processing unit will amplify, filter, and demodulate the received signals to eliminate noise and interference and improve the signal-to-noise ratio of the signals. By analyzing the flight time of the signal and combining the propagation speed of electromagnetic waves in the air, the distance between the radar antenna and the liquid surface can be accurately calculated, thereby obtaining the height of the liquid level. This non-contact measurement method not only has high precision and high sensitivity but also has the advantages of no wear, no maintenance, and is suitable for liquid level monitoring in various complex environments.

Figure 1

B - Measurement blind zone; E - Measurement range setting; F - Full tank height (high and low level setting); D - Distance from the probe to the surface of the medium; L - Actual measured liquid level. The running time of the radar signal from emission to reception is directly proportional to the distance D from the antenna to the surface of the medium, that is: D = v * t / 2. In the formula, t is the time interval from the emission to the reception of the pulse, and v is the wave propagation speed. As the empty tank distance E is known, the actual material level distance L can be calculated as: L = F - D. In the formula, F is the actual measurement working range.

Figure 2

FMCW (Frequency Modulated Continuous Wave) is a technique that uses high-frequency continuous waves with frequency varying in a triangular wave pattern over time. FMCW technology and pulsed radar technology are two techniques used in high-precision radar ranging. The basic principle is that the transmitted wave is a high-frequency continuous wave, and its frequency changes according to the triangular wave pattern over time. The frequency of the echo received by the radar is the same as the transmission frequency change pattern, both following a triangular wave pattern, but with a slight time difference. This tiny time difference can be used to calculate the target distance.
The name FMCW（Frequency Modulated Continuous Wave） fully explains the working principle. Starting from t0 as the initial time, the transmitter circuit increases the radiated frequency in a linear manner over time. (The amplitude of the signal is constant, and after time T, the radiation is periodically repeated). In the example shown in Figure 2, the frequency varies between 77GHz and 80GHz. Most of the signal is reflected by the surface, some by interfering objects, and finally reflected back from the bottom of the tank, with time delays of t1, t2, and t3, respectively. These delays are not measured directly by the system. The amplitude of the received signal is proportional to the degree of reflection; its frequency varies between 77GHz and 80GHz and remains constant. The transmitted and received signals coexist in the antenna. To make precise measurements, it is necessary for the transmitted frequency to change linearly over time. The principle of signal processing is shown in Figure 3, where a mixer is used to combine the transmitted and received signals, producing a signal containing the sum and difference frequencies of the incoming signal. A low-pass filter only allows the very low-frequency differential frequency to pass. The spectrum of the differential frequency provides the echo signal itself, as the frequency is proportional to the delay time (travel times), while the amplitude is proportional to the strength of the echo signal.
 

Figure 3

Overview of Radar Level Gauge Characteristics

Radar level gauge, as an outstanding measurement device in complex industrial environments, has won the favor of many industry users with its excellent performance and wide applicability. Here are several notable characteristics of radar level gauges:
(1) Continuously stable and high-precision measurement
Based on the electromagnetic wave principle, radar level gauge performs non-contact measurement, and its measurement process is hardly affected by external environmental interference. Therefore, it can provide accurate and rapid measurement results regardless of the medium, including toxic, corrosive media, or solid, liquid, dusty, and slurry media. At the same time, the probe exhibits high stability to changes in temperature, pressure, and gas, ensuring the reliability of measurement accuracy even under extreme conditions such as 450°C high temperature or 50bar high pressure.

(2) Excellent interference suppression capability
In complex industrial environments, factors such as joints, feeding or discharging noise within the beam range may interfere with measurement results. However, radar level gauges are equipped with advanced fuzzy logic control functions that can effectively suppress these interfering echoes, ensuring the accuracy and reliability of measurement data.

(3) High precision, safety, and environmental friendliness
Radar level gauges provide accurate and safe measurements under vacuum or pressurized conditions, meeting the needs of various working environments. At the same time, their materials have excellent chemical and mechanical stability, not only high reliability but also environmentally friendly, with materials that can be recycled to reduce the impact on the environment.

(4) Exceptional durability and reliability
Radar level gauges utilize microwave technology, which is not affected by external interference and does not directly contact the measured medium. Therefore, it is suitable for almost all types of measurement occasions, such as vacuum measurement, liquid level measurement, or material level measurement. The use of advanced materials enables it to operate stably for a long time under complex chemical and physical conditions, providing users with accurate and reliable analog or digital material level signals.

(5) Easy maintenance and operation
To facilitate user use and maintenance, radar level gauges are equipped with fault alarm and self-diagnosis functions. When the equipment malfunctions, the operation display module will provide an error code, and users can easily analyze and troubleshoot the fault based on the error code. In addition, the maintenance and calibration processes have been greatly simplified, reducing users' maintenance costs and time.

(6) Wide application range
Radar level gauges can measure almost all types of media, regardless of the media type, container shape, or working environment. Whether it is a spherical tank, horizontal tank, cylindrical tank, or cylindrical cone tank; whether it is a storage tank, buffer tank, microwave tube, or bypass pipe; whether it is liquid, particles, or slurry, radar level gauges can provide accurate liquid level measurement data. This wide applicability makes it a leader in the field of industrial measurement.

Installation of Radar Level Gauge
The accurate measurement of a radar level gauge relies on the reflected signal of the electromagnetic wave. If the liquid surface cannot reflect the electromagnetic wave back to the radar antenna or if there are interfering objects within the signal range that reflect interfering waves to the radar level gauge at the selected installation location, the radar level gauge will not accurately reflect the actual liquid level. Therefore, choosing an appropriate installation location for the radar level gauge is crucial, and the following points should be noted during installation:
(1) The axis of the radar level gauge antenna should be perpendicular to the reflecting surface of the liquid level.
(2) Objects such as stirring valves, adhering substances on the tank wall, and steps within the signal range of the radar level gauge can produce interfering reflected waves that affect the liquid level measurement. It is necessary to select an appropriate installation position during installation to avoid interference from these factors.
(3) For horn-type radar level gauges, the horn mouth should extend beyond the inner surface of the installation hole by a certain distance (>10mm). The antenna of the rod-type liquid level gauge should protrude from the installation hole, and the length of the installation hole should not exceed 100mm. For circular or elliptical containers, it should be installed at a distance of 1/2R (R is the radius of the container) from the center. It should not be installed at the center of the top of the circular or elliptical container, otherwise, the radar waves will be reflected multiple times on the container wall and converge at the center of the container top, forming a strong interfering wave that will affect accurate measurement.

Figure 4

◆     It is recommended that the distance from the inner wall of the tank to the outer wall of the installation tube should be less than or equal to L/4 of the tank diameter.
◆     The best installation position is ①, with a minimum distance of 300mm from the tank wall, and a recommended installation distance >= 500mm.
◆     It should not be installed above the feed inlet ②.
◆     It should not be installed at the center position ③. If installed at the center, multiple false echoes will be generated, causing interference and leading to signal loss.
◆     If the distance between the instrument and the tank wall cannot be maintained, the medium adhering to the tank wall will cause false echo storage.

 

 

Description

Process radar level transmitters operate at microwave frequencies between 24GHz and about 120GHz. Manufacturers have chosen frequencies for different reasons ranging from licensing considerations, availability of microwave components and perceived technical advantages.

There are arguments extolling the virtues of high frequency radar, low frequency radar or every frequency radar in between. In reality, no single frequency is ideally suited for all of the radar level measurement applications.

If we compare 26GHz radar with 80GHz/120GHz radar, we can see the relevant benefits of high frequency and low frequency radar:

■ Antenna size - beam angle

The higher the frequency of a radar level transmitter, the more focused the beam angle for the equivalent size antenna.
With lens antennas, this allows smaller nozzles to be used with a more focused beam angle.

For example, a G1-½" (40mm) lens antenna radar at 80 GHz has approximately the same beam angle as a4" (100 mm) horn antenna at 26 GHz.

However, this is not the complete picture. Antenna gain is dependent on the square of the diameter of the antenna as well as being inversely proportional to the square of the wavelength. Antenna gain is proportional to:

Diemater 2 /wavelength 2

Antenna gain also depends on the aperture efficiency of the antenna. Therefore, the beam angle of a small lens antenna at a high frequency is not necessarily as efficient as the equivalent beam angle of a larger, lower frequency radar. A 4" horn antenna radar at 26 GHz gives excellent beam focusing.
For a full description of antenna gain and beam angle at different frequencies, please read 'cSIMBA's Application Note on Radar Antennas'.

■ Antenna focusing and false echoes

An 80GHz beam angle is more focused but, in some ways, it has to be. The wavelength of an 80GHz radar is only 3.75mm compared with a wavelength of 11.5mm for a 26GHz radar. The short wavelength of the 80 GHz radar means that it will reflect off many small objects that may be effectively ignored by the 26GHz radar. Without the focusing of the beam, the high frequency radar would have to cope with more false echoes than an equivalent lower frequency radar.

■ Agitated liquids and solid measuring

High frequency radars are susceptible to signal scatter from agitated surfaces. This is due to the signal wavelength in comparison to the size of the surface disturbance. The high frequency radar will receive considerably less signal than an equivalent 26GHz radar when the liquid surface is agitated. The lower frequency radars are less affected by agitated surfaces. It is important that, whatever the frequency, the radar echo processing software can cope with very small amplitude echo signals. Note: Normally pulse radar has an advantage in this area no matter what the frequency.

■ Condensation and build up

High frequency radar level transmitters are more susceptible to condensation and product build up on the antenna. There is more signal attenuation at the higher frequencies, such as 80GHz. Also, the same level of coating or condensation on a smaller antenna lens naturally has a greater effect on the performance. ANL-8260AG2 lens antenna with 26GHz frequency is virtually unaffected by condensation, it is more forgiving of product build up.

■ Steam, dust and foam

Lower frequencies such as 26GHz are not adversely affected by high levels of dust or steam. These frequencies have been very successful in applications ranging from cement, fly ash and blast furnace levels to steam boiler level measurement.

ANL-8260AG2 26GHz pulse radar

In steamy and dusty environments, higher frequency radar will suffer from increased signal attenuation.

ANL-9127 with 80mm lens / 120GHz

Note: Normally, for a radar of higher emitting frequency, using a larger lens antenna has an advantage in this area no matter what the steam.

■ Foam

The effect of foam on radar signals is a grey area. It depends a great deal on the type of foam including the foam density, dielectric constant and conductivity. However, low frequencies such as 26GHz cope with low density foam better than higher frequencies such as 80GHz. For example, an 80GHz radar signal will be totally attenuated by a very thin detergent foam on a water surface. A 26GHz radar signal will see through this type of foam and continue to see the liquid surface as the foam thickness increases to 150 mm or even 250 mm.

ANL-9080 with 50mm lens

Note: For such thick foam measurement applications, an 80 GHz radar with a small lens (50 mm) is not an optimal product choice, which often leads to instability and level jumps. It is recommended to use an 80 GHz radar with a lens antenna at least 80 mm in diameter, which has advantages in this regard.

The thickness of foam will cause a small measurement error because the microwaves slow down slightly as they pass through the foam. When foam is present, it is important to ask us with as much information as possible on the application.

■ Minimum distance

Higher frequency radars have a reduced minimum distance (near blind) when compared with the lower frequencies. When measuring in small containers and still tubes, 80GHz/120 GHz may be a preferred choice.

■ Summary of the effects of radar frequency

1. Better focusing at higher emitting frequency means higher antenna gain (directivity), less false echoes and reduced antenna size.

2. Reduced signal strength caused by signal damping (Signal fluttering) at higher emitting frequency caused by condensation, build-up and steam and dust.

3. Higher damping caused by agitated medium surface (wave movement, material cones with solids, signal scattered).

References

‘To suggest that any one type of level transmitter technology could be regarded as 'universal' would be unrealistic and potentially irresponsible due to the variation and complexity of available applications when liquids, powders and solids are all considered. However, the rate at which radar based level transmitters have established themselves over the last couple of years would tend to suggest that this technology is closer to that definition that any principle has ever been.’ --- By Mel Henry

Overview

Frequency is a fundamental property of any radar as it has direct effects on measurement performance. It is important to remember that different frequencies are not equally suitable for all applications. Indeed, radars using different frequencies are required to solve different problems.

This paper will first describe the fundamental physical properties of different frequencies and thereafter explain what practical effects these properties have in some common, real-life level measurement applications. To this end, this paper differentiates between non-contacting radar transmitters using low microwave frequency (6/10GHz), mid frequency (26GHz), high frequency (80GHz) and terahertz frequency(120GHz).

Frequency and wavelength impact

At the fundamental level, radar level transmitter instruments emit microwaves to measure distance. Microwaves are commonly defined as electromagnetic radiation with wavelengths (λ) between 300mm and 3mm. The wavelength is inversely proportional to the microwave frequency (ƒ), i.e. shorter wavelength = high frequency. In the case of microwaves λ=300 mm corresponds to ƒ=1GHz and λ=3mm corresponds to ƒ=100 GHz. The properties of low, mid and high frequency level radars are summarized in below. These different physical properties have direct impact on the suitability of each frequency for different level measuring applications and conditions.

First and foremost, high frequency microwave signals suffer more attenuation (i.e. they are absorbed to a higher degree) when propagating through a medium, resulting in weaker signal return. For a simplified analogy, think of when you hear loud music played by your neighbor: low frequency sound (i.e. bass) will travel long distances and be heard clearly even through walls. High frequency sounds (i.e. treble) however are quickly absorbed and do not carry over long distances or through objects. When it comes to level measurement this means that high frequency radars are more likely to have problems with condensation, vapor, foam, build-up on the antenna, and dust. Low and mid frequency signals with wavelengths in the range of 50 mm to 10 mm are less affected by these kinds of challenges and more likely to pass through them unaffected.

Another important effect of the frequency is that it impacts the antenna beam width and beam angle, i.e. how focused the microwave propagation is. The beam angle and beam width are determined by the antenna design in combination with the microwave frequency. High frequency signals can achieve small beam angles with small antennas. Equally, small beam angles can be achieved with low frequency radars, but this requires larger antennas. The benefit of a small beam angle in level measurement is that it can make it easier to avoid hitting installations in the tank.

However, a narrow beam width can also be a disadvantage. For example, if there is an obstruction directly below the radar a narrow beam will be completely blocked, but a radar with a wider beam will be only partially blocked and still able to measure the product level.

A practical limitation established from experience is that the beam angle should if possible, not be smaller than about 3°, unless it is used for the measurement of straight pipes and the liquid level is statically flat, i.e. in oil tanks. A narrower beam makes installation sensitive to misalignment of the antenna. Consider the extreme case: if a radar has a beam like a laser it is almost impossible to align it with the plumb line on a real-life tank. Consequently, the reflected beam will miss the antenna and the signal will be lost.

Finally, waves and ripples on a liquid’s surface are common in industrial applications and may cause problems for radar level measurement. Instead of reflecting back upwards towards the antenna, microwaves hitting a turbulent surface may scatter and disperse. Thus, a lot of the signal strength (as much as 90 percent) can be lost and give the radar problems with obtaining an accurate and reliable level measurement. Microwaves remain unaffected by surface irregularities such as turbulence if the wavelength is larger than the ripple size. For example, the signal return of high frequency radars will be affected and scattered by ripples down to 3.8 mm in size, whereas mid-range frequencies will remain unaffected by turbulence up to 2.5 times as large and be reflected as if from a flat surface.

Low, mid, high and terahertz frequencies: strengths and weaknesses

It is clear then that the frequency has a major impact on the type of application a radar is best suited to. The following is a guide to the challenges that exist within some common level measurement applications and the implications of frequency choice.

Application suitability

For dirty and contaminating applications

During operating of a radar level transmitter, dirt and contaminants can build up on the antenna lens over time, which can affect the strength and direction of the radar signal. With radar level gauges operating at low and medium frequencies transceiver, the echo signal is less sensitive to this contamination and can pass through the build-up more or less unaffected. For radar signals operating at high frequencies, more energy is absorbed by the dirt covering the antenna lens, and the direction of the beam may also be shifted. A deposit of uneven thickness covering part of the antenna can redirect the beam by about 1.5°. For radars with narrow beam angles, this can cause serious problems because the return echo is not directed at the antenna, resulting in a loss of signal strength. As a result, low- and medium-frequency technologies are better suited for dirty and polluting applications.

For tanks with condensation and/or vapor applications

Condensation and vapor are sometimes a challenge for radar level measurement. Water reflects microwaves much more strongly than most industrial liquids. Condensation and vapor can therefore cause the reflection from the product surface to be obscured by 'noise' from water droplets. This is more problematic for high frequency signals because their shorter wavelengths also reflect strongly from very small particles like steam and aerosols. Low and mid frequency technology is therefore a better choice for applications with steam and condensation. It should be noted, however, that for condensation the design of the antenna is also of critical importance. Antennas with flat, horizontal surfaces should always be avoided.

Applications with turbulence, waves and ripples

In general, low and mid frequencies perform best in applications with turbulence, waves and ripples. Small ripples on the liquid surface is especially detrimental to high frequency measurements. The short wavelength means that the signal reflection will be scattered also by small surface movements causing loss of returning signal strength. Longer wavelengths are reflected as if from a flat surface and are therefore better suited to this type of application.

Applications with foam

Just like dirt and condensation, a layer of foam on top of the liquid will absorb the radar signal and make accurate measurement more difficult. Foam can have very different properties depending on which product it comes from, but once more, lower frequency generally provides better measurement reliability and accuracy. For dense and thick foam (e.g. from beer, molasses and latex) 6 or 10 GHz works best. For lighter foam, 26 GHz performs very well. High frequency technology should be avoided in applications with foam.

For Bulk liquid storage tanks application

In very large tanks, for example those used for bulk liquid storage at tank terminals, the size and placement of nozzles are typically not a restraint when it comes to choose of radar device. Obstructions and disturbing objects in the tank are usually not an issue either. Due to the vessel size, waves and ripples are often present on the liquid surface. Condensation is also common. As previously explained, this causes problems for high frequency technology.

Many bulk storage tanks are floating roof tanks where measurement is performed through still-pipes. Low frequency radars are preferred as they are less sensitive to build-up on the pipe wall, slots, and pipes that are not completely straight. High frequency radars have difficulties in such situations.

Furthermore, bulk storage tanks often have significant roof movements due to sunlight and shade, wind, and tank bulging. This causes problems for high frequency radars because their narrow beam width makes them very sensitive to tilting, if the axis moves from the vertical plumb line. Tilting can result in the reflected signal 'missing' the antenna opening. It also makes installation of high frequency radars challenging as they must be installed absolutely level in order to perform correctly. Low frequency technology is therefore the most appropriate choice for this kind of application.

For Small to medium size vessels application

This kind of vessel, typically 1~20 m tall, is among the most common in process industry. They are used for a broad range of tasks: from intermediate storage to blending, separation, and as reactor vessels. Process connections are usually 2–4-in. and conditions inside the tank are often difficult with one or several challenges such as condensation, contamination, turbulence, foam, etc. Mid frequency technology is a good choice in this kind of tank due to its versatility – it combines small antennas with good reliability in difficult conditions. Low frequency radars may be less suitable due to the small nozzles and high frequency technology is less able to cope with the tough process conditions.

For Small tanks/buckets

Summary

Radar level measurement has come a long way since its introduction 40+ years ago and the technology will continue to develop and improve. The recently introduced radars using the high frequency range may have benefits in level measurement applications for very small tanks with small process connections and very short measuring ranges.

However, the fundamental suitability of 6~11 and 24~29 GHz frequencies for reliable mm-accuracy level measurement cannot be overlooked. High frequency technology will usually perform well in less challenging process conditions, but fundamental microwave physics shows us it is less suitable when the going gets tough.

In contrast, low and mid frequency technology was developed specifically to meet the toughest level measurement challenges in the most demanding industrial applications. And the reason these instruments have been such an incredible success is because they provide reliable and accurate measurements in almost all applications.