Q&A: Radar Level Measurement Applications

Dec. 16, 2005

In the following Q&A, engineers at Magnetrol Intl., a global provider of flow and level technologies, respond to questions from Flow Control about the past, present, and future of

In the following Q&A, engineers at Magnetrol Intl., a global provider of flow and level technologies, respond to questions from Flow Control about the past, present, and future of radar level measurement for industrial applications.

Q: What is the difference between guided-wave and through-air radar? What are the pros and cons of each method?

A: The basic principal of operation is the same for both technologies. High-frequency electromagnetic energy is transmitted into a vessel. A reflection occurs at the point where there is an impedance change between the vapor space and liquid level. This impedance change is caused by the two media having different dielectric constants; the higher the dielectric constant of the level medium, the larger the amplitude of the reflected signal.

The difference between the two technologies is that through-air radar (TAR) sends its energy out into the open air. In this way it can be attenuated by the vapor space in which it is traveling, and reflections from objects other than the liquid level (false targets) can cause performance issues. Guided-wave radar (GWR) transmits energy down a probe (waveguide) where it is focused. Very little energy is lost down the probe, therefore very low dielectric media can be measured. Although using a probe virtually eliminates the variables that can influence TAR, the disadvantage is that GWR is a contact technology.

TAR will be considered first as most users prefer a noncontact solution. Larger vessels with few false targets, containing a high dielectric medium and little headroom, are more easily outfitted with TAR.

The more difficult high-temperature, low-dielectric applications, or those containing foam or turbulence, are better suited for GWR.

Q: What is the difference between frequency modulated continuous wave (FMCW) and pulse radar? What are the pros and cons of each method?

A: Today”s noncontact radar is offered in two basic types — pulse and FMCW. Pulse technology sends bursts of energy that travel to the surface and back. The bursts are centered around the design frequency of six GHz or 24/26 GHz (manufacturer dependent) and are typically one nanosecond in length. As electromagnetic energy travels at approximately one nanosecond per foot, the bursts of energy are approximately 500 nanoseconds apart to allow the energy of one to settle down before the next one is fired. Typical loop-powered transmitters have a range of 65 feet (130 feet round trip or 130 nanoseconds).

Pulse radar somewhat more complex than FMCW due to the use of equivalent time sampling (ETS). ETS converts the real-time radar measurement (in nanoseconds) into equivalent time (milliseconds), where it can be analyzed quite easily with simple circuitry.

FMCW, or frequency modulated continuous wave, operates very differently from pulse. FMCW does not operate at one fixed frequency, rather it operate with a base frequency plus a sweep. It is constantly transmitting energy while sweeping between the two frequencies. For example, some manufacturers use nine GHz as their base frequency and a bandwidth (sweep) of one GHz therefore sweeping between nine and 10 GHz. The energy travels to the surface and back, where it is mixed with the originally transmitted signal. The resulting IF (intermediate frequency) is the difference between the original transmission and the return signal. Since the return signal is delayed by an amount proportional to the time it took to travel to the surface and back, the IF is proportional to level change.

FMCW becomes complex due to the use of an FFT (Fast Fourier Transform). The FFT is a complex, mathematical conversion that must be used to transform the time-based radar information into a frequency-based spectrum. False targets can then be analyzed and rejected. This consumes both extra time and power.

What are the pros and cons of each method? There is some evidence that suggests FMCW radar is ideal for large, storage vessels with no obstructions while Pulse does a better job in the smaller, process oriented vessels. However, in general, both approaches do an admirable job and any discussion usually deteriorates into one manufacturer arguing in favor of their particular approach. Since the popular use of radar is new to the process industry only time and experience will yield an objective answer.

Q: In what sort of application environments is radar typically not a good fit?

A: The effectiveness of radar level measurement is based on its ability to receive and interpret a reflection from the surface of the level medium. Any issues that complicate this will reduce the reliability of the measurement. The key issues are as follows:

Extremely low dielectric — Propane (E=1.6) and butane (E=1.4) are so low that a standpipe or stillwell is necessary for effective measurement. If so, users can employ a guided-wave transmitter with a coaxial probe.

Moderate-to-heavy foam — Foam typically absorbs the energy, it is “stealthy." If there is enough foam, there is no useful reflected energy present for measurement.

Severe turbulence — Similar to foam, turbulence reduces the useful energy, in this case by scattering. Unlike foam, often turbulence can be addressed with proper gain and averaging adjustments.

Mixing blades — Stationary objects in a vessel can be easily rejected with a good false target rejection routine, but mixing blades pose a special problem. When running, they are usually not an issue. However, they often stop at different locations, which makes effective false target cancellation difficult. This needs to be handled on a case-by-case basis. Higher frequency transmitters (narrower beam angle), proper antenna choice, and mounting location will improve performance.

Dish-bottom vessels — These can sometimes cause problems when empty. The dish acts as an excellent reflector throwing energy around the vessel sometimes confusing the transmitter.

Q: I”ve spoken with some analysts that say radar level and wireless are a natural fit and are generating significant interest for a variety of industrial applications. How is wireless being used with radar-level systems to help users solve real application issues?

A: Radar has found acceptance in the inventory/tank gauging market. Line-powered radar transmitters are capable of highly accurate level measurements allowing for more precise control of inventory in large storage vessels. Introducing wireless data transfer allows users to manage inventory worldwide from a single data collection point, schedule allocations and production runs, and maintain accurate records of available product.

Radar transmitters are also used monitor remote sumps and lift/pump stations where wireless transmitters send notification alarms of high level or equipment malfunction

Q: How concerned should users be concerned about security when implementing a wireless-based radar level measurement system? Are there any specific types of applications where security concerns make wireless-based radar level measurement an infeasible solution?

A: There is nothing unique to applying wireless to radar level devices that make it more or less secure than any other wireless level device. However, including reliability, security is one of the major concerns with using today”s wireless networks.

With methods such as frequency hopping, proprietary protocols, and 128- or 194-bit encryption capabilities (256-bit coming soon), interception of periodic wireless transmissions of data are unlikely.

However, careful consideration should be given when applying wireless to process control. Interference with wireless transmissions can be caused by natural sources (weather), physical obstructions (Who parked the truck between the transmitter and receiver?), and the mischievous hacker or disgruntled employee.

Q: How do you see radar level technology evolving over the next five years? What sort of technology improvements can users expect to see?

A: Radar level measurement was first introduced over 25 years ago for measuring shipboard vessels in the marine industry. At that time a radar transmitter was large (lifted by crane), complex (often a technician was needed to calibrate), expensive ($15,000-$20,000), and power hungry (line-powered). Today radar transmitters are small, simple to use, competitively priced, and loop-powered. The direction of process control radar is heavily influenced by the direction of telecommunications and automobile manufacturing. These are much larger industries, which can drive innovation and change. For example, the auto industry is exploring the use of radar for collision-avoidance sensors and intelligent cruise control. 24GHz is presently used with new work being done at 77GHz. Just as consumers have watched satellite TV dishes shrinking, radar level transmitter antennas will shrink in proportion to the increase in frequency. Our process industry will benefit from the advancements in others.

Both TAR and GWR technology will continue to evolve as faster electronics become more readily available. Increasing the Frequency of operation of these devices decreases the wavelength and results in better resolution and a narrower beam width.

These improvements in performance will also be accompanied by new antenna and probe designs that will be necessary to utilize radar in more applications.

For More Information: www.magnetrol.com

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