Radar Technology
In 1904 a patent was filed for a unit detecting reflected radio waves from objects, but it was not until the early thirties that prototypes with various functions appeared in the US, British and German navies, under acronyms such as RADAR (Radio Detecting and Ranging).
Why use radar for Tank Gauging?
The main advantages are:
- Radar waves are not affected by the atmosphere above the product in the tank.
- The only part located within the tank is the antenna.
- No moving parts - High reliability.
- High accuracy.
The non-contact measurement of TankRadar ® level gauges gives you trouble-free and maintenance-free operation and provides you with a cost effective solution to your level gauging applications.
Radar compared with other technologies
Radar is extremely well suited for level gauging. One reason for this is that the velocity of radar waves is largely unaffected by temperature, pressure and chemical composition, making radar well suited for 0.01% class accuracy. This is in sharp contrast to acoustical waves, for which velocity can change a few dozen per cent due to chemical composition and temperature. The rather long radar wavelength (12-52 mm for different systems) makes the antennas fairly insensitive to layers of oil, splashing droplets which are much smaller than the wavelength etc. This is again in sharp contrast to the third classical non-contact method using light or infrared (having a wavelength below 0.01 mm).
One basic difference between light waves and radar waves is that the much longer wavelength of the radar waves allows them to penetrate even the densest fog and rain, and, of course, operate during darkness.
Safety
Safety has been a major theme in radar development at Rosemount Tank Radar AB from the very beginning, resulting in extremely high reliability. Automatic function monitoring and overfill protection are key features. One safety issue sometimes asked about is whether the radar can have any effect on someone entering the tank. Even right in front of the antenna, the power level is about one ten thousandth of what the authorities in most countries consider to be the safe limit for continuous exposure. Another comparison can be made with the ship’s surveillance radar, which has around a million times higher power, and a mobile phone, which has ten thousand times higher microwave power. One simple reason for this is that the intrinsically safe feeding only allows very low power to be fed out on the deck.
Radar Unit
Even if wave propagation conditions are the most important reasons to use radar, the radar unit itself is very important for creating conditions for accuracy, reliability and low cost. The radar unit consists of transmitter, antenna, receiver and a more or less complex electronic unit performing signal processing, communication, power supply and often display function and auxiliary measurements (temperature etc). The antenna is located in the tank environment, and its size is critical for its function. A bigger antenna will give better sensitivity and a narrower antenna beam, which may be necessary in order to pass between steel beams etc. in a deep tank. The construction of transmitter and receiver are important for making the system sensitive (able to pick up faint echoes), for allowing high accuracy and low power consumption. Power consumption is important for allowing use in an explosive environment (gasoline etc.) and for allowing connection to various bus systems (Fieldbus, Profibus, CAN etc.).
Radar Transmitting Methods
All distance measuring radar units have a transmitter that generates signals over a certain frequency spectrum. Accuracy can be better (or worse), but the resolution (ability to distinguish two closely separated echoes) is closely tied to the width of the frequency spectrum used. The oldest and best-known radar system (since W.W.II) uses short pulses, which in terms of spectrum is a way of spreading the power over a certain bandwidth.
Pulsed systems can have very low power consumption, but as a result they usually have low sensitivity and are thereby inadequate for highest accuracy. For many applications sensitivity is the most important system parameter, as it decides whether or not measurements can be taken under bad conditions. For radar level gauging, the mostly used method is FMCW (Frequency Modulated Continuous Wave), which typically makes the frequency spread by means of a linear sweep of a signal having constant amplitude. Custody transfer approved FMCW systems have been used since the late eighties, and the FMCW method can provide peak performance in both accuracy and sensitivity.
Radar History
In the late forties radar altimeters for sensing the height of an aircraft began to be used (and this application is a sort of early predecessor to radar level gauging). At that time all radar equipment required bulky and expensive units including several kV lethal voltages.
In the mid sixties, semiconductor technology took a big leap forward, allowing real radar frequencies in the order of 10.000 MHz to be generated by semiconductor components. At Saab Aerospace, the new potential for building small lightweight units was immediately exploited for various defense systems installed on fighter airplanes, such as altimeters.
Saab was a fairly diversified company at that time, and had one department working on electronics for use on merchant ships, such as engine monitoring systems. There was huge growth in the building of tanker ships, and Saab’s market connections to shipyards indicated a great need for far more reliable and accurate level measuring systems for use in the cargo tanks, compared to existing systems. The coexistence of technological skill and market connections within Saab resulted in studies and development aiming at a radar level gauge for liquids tanks. In comparison with aircraft altimeters, the far higher accuracy requirement, the far lower cost target and especially the mandatory requirement of “electrical intrinsic safety” (formal guarantee for not being able to cause explosions) constituted a formidable task. Finally, in 1973, a patent was filed for a somewhat new radar configuration that fulfilled these requirements.
The first radar level gauging system was installed on this ship, the Norwegian crude oil tanker M/S Havdrott, and it is still in operation.
The first production unit was supplied by Saab in 1975. That was also the year that witnessed the disappearance of the shipbuilding market for tankers for some years to come, following what is known as “the first oil crisis”. In spite of that, the number of installed units grew by at least 100% per year until 1983, when the company Saab Marine Electronics AB was formed under Saab to exploit this invention. The reason for this fast growth despite the high price was that radar proved to be the only working solution to the somewhat severe environmental conditions in cargo tanks at sea, combined with increased customer demands for more reliable equipment.
In 1985, two new systems were launched from Saab Marine Electronics, each of them optimized for the two important markets of that time. The TRM-system (TankRadar Marine) was optimized for ships, while the other TRL (TankRadar Land), was adapted for measurements on oil refineries, with increased accuracy and a number of other features. The TRL system was the first of its kind, and was also the first radar level gauge to get a custody transfer approval for accuracy. In order to utilize proven technology and production quality, many parts were the same in both TRL and TRM, but there were also a number of very significant differences:
- Flameproof enclosure for intrinsic safety in refineries
- Signal transmission with an internal bus
- Connection of temperature sensors to the radar unit (necessary for high precision).
- The most important radar technology innovation, patented in 1984, was a system which made it possible to measure level in existing still-pipes.
The marine heritage in the TRL concept included some drawbacks in the new application, and in 1991 a completely new system called TRL/2 was launched, designed to be optimised for high precision measurements at refineries and storage plants. Besides being much lighter and more accurate, it had a number of additional functions. In terms of radar technology, there were four major improvements:
- Microwave feeding between the electronics and the antenna was performed in a new way (patented in 1987) allowing various types of antennas to be used on the same electronic unit by utilizing waveguide polarization in a particular way. One achievement is to use circular polarization, allowing the echo from the tank wall to be suppressed when the gauge is located close to the wall.
- The use of a special waveguide mode allowing existing still-pipes to be used for high precision measurements.
- Pipe measurement in LPG tanks.
- Signal processing improvements. Manufacturing of microwave electronics gradually became much more of an industrial standard, and costs were reduced significantly due to the rapid development in available microprocessors.
In contrast to TRL, the TRL/2 was gradually extended to a full inventory system by hardware and software integration.
The TRL/2 - optimised for high precision measurements at refineries and storage plants.
A new high-precision model named Rex was launched in 1999. It was based on TRL/2 but utilized newer processor technology and a great number of improvements in mechanics, signal processing, communication and user interface.
The TankRadar® Rex - measures levels to a precision of ± 0.5 mm and can emulate level gauges from other manufacturers.
Marine Radar Technology
The TankRadar ® gauges have been developed over a period of more than 25 years. A number of factors affect the performance of a radar level. Our engineers have developed Rosemount TankRadar ® gauges that give our customers excellent performance under the most diverse conditions.
There were many reasons why radar technology for level gauging was first adopted on tanker ships, despite being a new, unproven and expensive method at that time. Firstly, the tank environment is harsh, both in terms of the various chemicals (even in crude oil) and in mechanical terms, from the waves in high seas as well as the cleaning guns. In that environment the new technology soon proved its superior ability to measure where other systems failed. The float gauges (fairly common in the past) were literally wiped out when new regulations during the late seventies required cleaning guns to be used during discharging. The primitive pressure gauges used at that time were not reliable enough when inert gas made it impossible to open the tanks and look in as a back-up method. Radar technology soon proved to be fairly reliable, partly due to the absence of moving parts, as well as to its location well above the cargo, with the sensitive parts completely outside the harsh tank environment. The gauges for the Rosemount SUM-21 system, the only level radar system in the years 1975-1985, for instance, had an experienced mean time between failure of 65 years, and later models improved that figure. With the antenna being the only part exposed to the atmosphere inside the tank, it was possible to have it made of acid-resistant steel and PTFE only, in order to allow it to withstand a wide range of cargoes.
The first ever radar level gauging system installation was carried out on this ship
- M/S Havdrott - with Rosemount SUM21 radar level gauging system
The antenna is the most critical part for use in marine systems. The tanks are not simply empty space: pipes, ladders, steel beams and other obstacles can cause reflections that disturb the measurement. The surface of a cargo of oil has a rather weak reflective power for radar waves: about 3% of the power is reflected, as compared with a metal surface, and consequently it is far from easy to make the strength of the disturbing echoes much weaker than the echo desired from the oil surface. To achieve that, the antenna is an important tool for making the radar beam narrow enough to pass between typical tank constructions, thereby avoiding false echoes. With the antenna diameter 39 cm SUM-21 and its follower TRM, the antenna beam could get the right beam-width for avoiding echoes from pipes in beams even in deep ships of 25-30 m. For smaller ships, e.g. 10-12 m deep, an antenna with a smaller diameter can sometimes be used with a wider antenna beam. Regardless of the type of antenna, the width of the antenna beam is closely related to its diameter as counted in wavelengths. For level gauges in industrial tanks, higher frequencies can sometimes be used, giving a narrower beam. In marine tanks, however, where the antenna is occasionally washed over by the cargo when at high seas, the higher frequencies would be attenuated too high, due to the layers of wet cargo on the antenna. Large-diameter antennas are typically parabolic antennas, but for diameters under 20 cm various kinds of conical horn antennas are used. In the highly condensing tank environment, antennas that include a horizontal surface are known to be subject to problems when condensation creates large droplets on surfaces such as these. Interesting enough, the antenna beam-width cannot be too narrow either, as the reflection from the surface will move slightly over the surface, depending on trim and list conditions (reflection is always perpendicular to the surface). The optimum antenna diameter is therefore fairly large, and depends to a certain extent on the size of the ship.
The second generation TankRadar® Marine - the TRM.
An increasing number of tanker ships carry liquid gas. There is little electrical difference between liquid gas and a liquid (mainly a question of a certain interval of temperature, pressure and chemical composition) but from time to time the liquid gas will boil, temporarily making the echo much weaker. In order to guarantee accurate measurements under all such conditions, including typical installation methods, liquid gas is measured through a pipe guiding the radar waves, and because liquid gas is very clean it does not produce any deposits, which otherwise could easily ruin the accuracy of the measurement. In some liquid gas tanks (those under pressure), the typical property of waves, i.e. that they depend very little on atmospheric conditions, will not apply fully, and it will be necessary to correct the value by several hundredths of a percent or even up to one percent, depending on pressure etc. that can be measured.
An LNG (liquified natural gas) carrier and the Rosemount TankRadar® for LNG applications.
All marine systems to date use some kind of FMCW technique, meaning that the microwave frequency is swept over the frequency band required for the distance resolution desired. The radar echo from the tank is quite complex, and normally contains a number of disturbances in addition to the real echo from the surface. The echoes arrive over slightly different times (15 m corresponds to 0.1 microseconds) but the FMCW method provides a good way to rescale these short times to a frequency spectrum ideal for analysis with modern signal processing techniques. Due to internal tank echoes, waves on the surface of the liquid etc., a lot of information is required from the tank for selection of the correct echo, and this is provided by the tank spectrum. Conditions in the tank change over time, as the tank and ship are not quite rigid, and as layers of cargo will change the reflections from different parts of the tank; this means measured signals from the empty tanks are generally obsolete next time the tank is empty. Different manufacturers have used various methods for hardware solutions over time. Many units use a well-known piece of cable as a reference for achieving a good level of accuracy in the rescaling from time to frequency, but other methods are possible and are used on high precision refinery gauges. The cable solution in the early models, however, made it possible to use circuits with low power consumption. Today low power counter solutions offer a better solution for controlling the frequency.
At Saab Marine Electronics, the second generation TankRadar Marine (TRM) was launched in 1985, followed by the third generation G3 in 1995, both of them with a number of continuous improvements and cost reductions. The G3 system included liquid gas tanks, and the system was fully integrated with some cargo handling capabilities.
In 2002 G3 was followed by the STaR system - a system with "intelligent" transmitters enabling much higher accuracy and the possibility to include a cost-effective redundancy where two or three electrically independent radar level transmitters could be sharing the antenna and encapsulation. Typically one of the level transmitters is used as a high-level alarm instead of another system.
The TankRadar® STaR with its 3-in-1 functionality: independent radar based level gauging, high-level and overfill alarms.
- Radar waves never get stuck.
- Radar waves are not affected by te atmosphere above the product in the tank.
- The only part located within the tank is the antenna.
- No moving parts - High reliability.
- High accuracy.
- On the TankRadar®systems, the Electronic Box can easily be changed under closed tank conditions.
Learn more about radar measurement
Fast Fourier Transformation (FFT) is the simplest and one of the most widely used signal processing techniques.
FFT works with relatively low resolution and accuracy. The powerful Digital Signal Processor (DSP) in TankRadar ® Pro performs fast FFT calculations of the incoming tank signal, and provides a frequency spectrum that is analyzed to find the liquid surface. Since the frequency is related to the distance between the transmitter and the reflecting object, the spectrum can be mapped to the corresponding tank levels.
The illustration shows the main principle of the FHAST method for detecting the surface echo.
The TankRadar ® gauges use our patented method for detecting the surface echo, called FHAST for Fast High Accuracy Signal Technology. The signal is filtered in a digitally controlled analog filter.
First, a filter removes any echoes smaller than a threshold value. Then a narrow filter is applied around the frequency corresponding to the surface echo. The remaining frequency is compared with the frequency calculated in the previous sweep, resulting in a very accurate signal with a frequency of only a few Hertz.
The diagram shows the typical error caused by an interfering object inside the radar beam, with the FMCW method and with the MIP-mode. As you can see, there is very little influence on the MIP-mode measurement.
MIP-mode radar measurement is a new and unique method for signal processing. The MIP-mode improves the accuracy to levels never previously experienced in the level gauging industry. MIP-mode instrument accuracy is ± 0.1 mm.
MIP-mode calculates the number of phase shifts and fractions of phase shifts, created when the distance to the surface changes. MIP-mode also uses the FMCW mode in the background as a backup and as a reference for restarting MIP-mode if it has been interrupted.
In the figure above the result of an interfering echo from an object, incorrectly placed within the radar beam, is shown with both the traditional FMCW method and with MIP-mode. As you can see the error caused by the interfering echo is greatly reduced with MIP-mode measurement.
Multiple Echo Tracking (MET ®) is a feature available with the new TankRadar ® Pro series. MET ® improves measurements in echo-disturbed regions. MET ® increases the resolution of the radar "image" so that the sensor can identify and separate echoes more easily, and improve the accuracy in echo-disturbed areas.
MET ® provides even more advanced algorithms which efficiently handle interfering echoes. An agitator, for example, can cause interference when the sensor attempts to differentiate echoes from liquid and agitator surfaces. MET ® measures on the disturbance as accurate as on the surface level, making it much easier to distinguish between the echoes.
The Echofixer is a software module available with the new TankRadar ® Pro series, that improves the reliability of the level gauge when used in tanks with agitators or other false echo sources.
When there are echo-disturbing objects in the tank such as agitators, baffles, mixers, pipes, heating coils, etc, the Digital Signal Processor (DSP) must be able to distinguish between the actual liquid surface and the interfering echoes. The Echofixer improves the ability of TankRadar ® Pro to separate disturbing echoes from the surface echo.
The Echofixer registers echoes according to certain criteria, thus making it possible to use sophisticated methods for adapting the measurement to various situations. Dynamic and real-time updated information from previous measurements is used to make intelligent decisions based on the collected information by the gauge itself. The Echofixer can also use information provided by the operator as a complement to the automatically collected data.
The illustration shows the FMCW (Frequency Modulated Continuous Wave) radar principle.
The TankRadar ® level gauges work with a method called Frequency Modulated Continuous Wave, FMCW. Instead of measuring time, these gauges transmit signal sweeps with a constantly changing frequency. The frequency of the reflected signal is compared with the frequency of the signal transmitted at that moment. The difference between these frequencies is proportional to the distance from the gauge to the surface.
The TankRadar ® level gauges use frequencies between 9.5 and 10.5 GHz during the signal sweep. The frequency of the received signal is subtracted from the frequency of the transmitted, to obtain a signal with a frequency of a few kHz. This signal can be further processed to get an accurate distance to the surface. measurement
There are no health hazards in handling the TankRadar ® gauges. As the emitted power from each transmitter is so low, there is no health hazard even when you are very close to the antenna.
Most international standards state that a power density of up to 1 mW/cm 2 is considered safe for continuous exposure.
The power density close to the antenna is 0.001 mW/cm 2 and further down in the tank it is much lower.
The transmitted microwave power is less than 1 mW.
As a comparison it might be interesting to know that in sunshine you are exposed to a power density of 100-150 mW/cm 2, and that a cellular phone emits 1000 mW/cm 2.
When lightning strikes a tank farm, large currents can be induced in the long cables. The TankRadar ® system can stand substantial impact such as field cabling being struck by lightning.
Both power supply and signal processing are galvanically isolated from the field cables. Filters, varistors and fuses protect the electronics from electrical overload.
Learn more about marine radar measurement
Sensitivity is the single most important factor for a reliable and accurate measurement. Many systems operate on the limit of their capability and may fail if any condition turns bad.
High sensitivity is needed with difficult conditions like:
- Deep tanks
- Dirty antenna
- Waves on surface
- Products with low reflectivity (like oil)
High sensitivity is achieved with:
- Large antenna
- Good electronics
- Good signal processing
Attenuation in Antenna and Electronic Unit
The radar gauge must be able to work with around 80 dB 2-way attenuation in antenna and microwave unit. Many manufacturers can handle only 60-70 dB attenuation (between 100 and 10 times less sensitive). The antenna and the microwave unit are the most important parts affecting the sensitivity.
There are mainly two types of antennas for radar level gauges: cone or horn antennas and parabolic antennas. The size of the antenna opening affects the sensitivity of the antenna.
A large antenna has:
- A narrower beam
- Higher sensitivity
A small antenna has:
- A wider beam
- Lower sensitivity
An antenna with a narrow beam is easy to locate in tanks with internal structures.
High sensitivity is required when there are any difficult conditions in the tank, such as in very deep tanks or with foam or waves on the surface.
See also a description of how the relation between received and transmitted power is affected by the size of the antenna.
Regardless of the type of antenna, it is only a small area of the product surface that reflects the signal. This area is located where the surface is approximately perpendicular to the antenna and corresponds in size to a 4° cone from the antenna.
With a narrow radar beam a larger portion of the power is reflected from the surface. This results in a high sensitivity.
The illustration shows the radar principle FMCW (Frequency Modulated Continuous Wave).
The Rosemount TankRadar ® level gauges work with a method called frequency modulated continuous wave, FMCW. Instead of measuring time, these gauges transmit signal sweeps with a constantly changing frequency. The frequency of the reflected signal is compared with the frequency of the signal transmitted at that moment. The difference between these frequencies is proportional to the distance from the gauge to the surface.
The Rosemount TankRadar ® level gauges use frequencies between 9.5 and 10.5 GHz during the signal sweep. The frequency of the received signal is subtracted from the frequency of the transmitted, to obtain a signal with a frequency of a few kHz. This signal can be further processed to get an accurate distance to the surface.
The illustration shows the inside of a tank with the Parabolic Antenna.
We sometimes get this question from our customers. There are no health hazards in handling the Rosemount TankRadar ® gauges when they are powered. As the emitted power from each gauge is so low, there is no health hazard even when you are very close to the antenna. Some data below illustrates this.
Most international standards state that a power density of up to 1 mW/cm 2 is considered safe for continuous exposure.
The power density close to the antenna is 0.001 mW/cm 2 and further down in the tank it is much lower. The transmitted microwave power is less than 1 mW.
As a comparison it might be interesting to know that in sunshine you are exposed to a power density of 100-150 mW/cm 2 .
The illustration shows the main principle of the FHAST method for detecting the surface echo.
The Rosemount TankRadar ® gauges use our patented method for detecting the surface echo, called FHAST for Fast High Accurace Signal Technology. The signal is filtered in a digitally controlled analog filter.
First, a filter removes any echoes smaller than a threshold value. Then a narrow filter is applied around the frequency corresponding to the surface echo. The remaining frequency is compared with the frequency calculated in the previous sweep, resulting in a very accurate signal with a frequency of only a few hertz.
With this method it is possible to achieve a very high accuracy. It uses the calculating power very efficiently, focusing on reliable and fast results.
Q: Are the microwaves used by the Rosemount TankRadar® hazardous to your health?
A: Not at all. Only 0.5 mW is emitted from the gauge. As a comparison direct sunlight accounts for 100 mW/cm2 , a cellular phone for1000 mW.
Q: Is radar technology for level gauging expensive?
A: Due to the technology the system is virtually maintenance free and will be a very good investment over it's lifecycle (more than 20 years). Purchase price varies of course depending on application and volumes but is normally slightly higher than servo gauges. After 3-5 years a Rosemount Radar is always cheaper than a servo gauge since the customer doesn't have to spend any money on spare parts to keep his system accurate and functional.
Q: Can Rosemount guarantee no drift of accuracy after let's say 10 years?
A: Yes, since the instrument has an internal digital reference without any measurable drift the instrument will stay accurate over it's entire lifetime.
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