No single frequency is suitable for all applications in the CPI. Understanding the principles behind the technology will help in selecting the right device
Radar has proven to be a reliable technology for level measurement in silos and tanks. However, there have been discussions recently about the capabilities of high radar frequencies: 80 GHz is often called a universal talent for all applications. But does this apply without restriction? Will all previously used frequencies be replaced? To answer these questions and to understand the possibilities in general, some background knowledge about radar technology is required.
There are two types of radar used for level measurement: pulse radar and frequency-modulated continuous-wave (FMCW) radar. Today, FMCW is the favored technology that all major industrial process instrumentation manufacturers rely on.
In FMCW devices, radio waves whose frequency is modulated over a bandwidth are continuously emitted, and the reflections are collected by a receiver. The difference between a transmitted and received wave of a given frequency is measured, and that is proportional to the distance to the surface at which it was reflected (Figure 1).
Thus, the level measurement by radar is primarily a non-contact distance measurement from the measuring device (mounted on top of the vessel) to the surface of a medium to be measured. By entering the vessel geometry and medium properties, such as density, the device can calculate level, volume or mass. In contrast to ultrasound, radar is independent of pressure and temperature. Viscosity and density do not affect the measurement. Despite this insensitivity, there are some factors that do have an influence on FMCW measurement, which are briefly described in this article to help find the best device for a certain application.
Signal dynamics and bandwidth
As each emitted frequency is reflected, a large received spectrum results is created.However, waves are reflected not only by the medium, but also by all surfaces in a vessel, including tank internals. The exact differentiation of all targets detected by the radar is only possible via a high signal dynamic, also known as high measuring sensitivity: the more signals reflected by a target that can be received by the device, the clearer or higher this point rises in the spectrum over the noise, and can be identified. This high dynamic is given by most FMCW devices available on the market.
As the bandwidth of the radar widens, the resolution of the fast Fourier transform (FFT) spectrum increases and the individual targets are indicated by narrower, more accurate peaks. The bandwidth over which the frequency is modulated determines the number of different signals reflected from a target. A 24-GHz radar device typically modulates between 24 and 26 GHz and thus has a bandwidth of 2 GHz, while an 80 GHz typically modulates in the range between 78 and 82 GHz and thus has a bandwidth of 4 GHz. With 4 GHz, for example, it is possible to differentiate between targets that are only 10 cm (4 in.) apart. With 2 GHz, these targets cannot be distinguished under the same conditions.
As a side note, the ability to distinguish close objects does not (yet) include the possibility to do interface level measurement. For the moment, only time-domain reflectometry (TDR) guided radar (with radar waves guided along a cable or rod) is able to differentiate two different levels of product inside one tank.
Focusing and antenna size
For a long time, the bandwidth was limited by the performance of the microchips that were available. Today, the bandwidth is limited by the antennas and their designs that have to transmit the frequency spectrum. Radar waves do not propagate point-focused like a laser signal, but rather in the form of a lobe or angular beam (Figure 2). In order to influence the opening angle or the focusing of the angular beam, there are two possibilities. First is the frequency used: the higher the frequency, the smaller the aperture angle due to the shorter wavelength. The angular beam width of an 80-GHz system with 4 GHz bandwidth at 10 m (33 ft) distance is only 30% as wide as that of the 24 GHz with 2 GHz bandwidth (0.5 to 1.75 m or 1.6 to 5.7 ft). The second possibility is the antenna diameter: the larger the diameter, the more focused is the angular beam.
For the chemical process industries (CPI), this can be easily transferred to several possible areas of application: in high narrow silos, the radar beam should not come into contact with the silo wall or tank internals, since both should not be measured. Therefore, the radar angular beam must both be focused and kept as narrow as possible, as provided by an 80 GHz radar gage with a large antenna. In other applications, the strong focus does not make sense; for instance, in tanks with moving surfaces, or if the signal does not meet at right angles with a flat liquid surface due to the flange not being mounted horizontally. A very small opening angle would be counterproductive because the radar waves reflected at the product surface would not return to the antenna, hence no signal evaluation is possible. Here, a 24 GHz device with a small antenna is the better choice.
Reflectivity and frequency
In addition to the angle, the properties of the product surface also determine how many radar signals are reflected and how they are received: the higher the reflectivity or dielectric constant ( Er), the higher the amplitude of the reflected signals. Liquids reflect very well: water with an Er value of 80 is one of the most reflective products, with about 65% of the emitted energy and signal received back. Acids and alkalis with an Er value of 20–30 reflect approximately 40% of the signals. Liquid hydrocarbons with an Er value between 1.6 and 3 will reflect 5% of the emitted energy even at a low radar frequency of only 10 GHz, which is sufficient for a measurement. An 80-GHz radar would also work well in this application.
In contrast to liquids, bulk solids generally reflect very poorly: radar transmitter manufacturers state an Er value of approximately 1.4 as the lowest value that can still be measured reliably and safely. While the reflection coefficient of a flat liquid surface does not change with the frequency, the backscattering on fine-grained bulk goods, such as granules or powders, increases significantly with a higher frequency. The 80-GHz radar is therefore the first choice here, because of the high dynamics it is able to clearly display the level line, even in the case of a heavy dust development (for example, during the filling process of a silo or stock pile). The better resolution of its 4 GHz bandwidth also helps to distinguish the signals from interference and medium, even if they are close together.
This completes the description of the main parameters of FMCW radar level measurement. The antenna design, as well as its position and orientation, were only briefly described, since both are strongly application-dependent. It should now be clear that there is no universal frequency for all applications. In the following, typical cases and application recommendations are briefly shown. They will guide you in the right direction, but will not replace a detailed evaluation of your application by an expert who can guide you to find the right combination of frequency and antenna type suited for your application.
Further information can be found in Ref. 1.
1. FMCW Radar Level Measurement Systems, Krohne white paper, https://bit.ly/2UrSYFu, explains the influencing factors of the measurement in more detail, as well as the differences between different antenna types.
Jonathan Pradel has been working for the Krohne Group (Duisburg, Germany) since October 2016 when he joined Krohne France (Krohne S.A.S., Romans Cedex, France; Phone: +33-47-50-54-415; Mobile: +33-69-99-41-548; Email: email@example.com), the “center of excellence” and main production site for level instruments within the Krohne Group. He holds a master’s degree in international management from Southampton University, in England, and a bachelor degree in International Business from the Grenoble Graduate School of Business in France.
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