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Solving Level Measurement Problems With Guided Wave Radar Technology

Guided wave radar technology (GWR) has emerged as a level measurement staple in many industries around the world. Since its introduction in the late 1990s, users of GWR have discovered that it has unique characteristics that make it well suited for difficult level control challenges. In a recent article for Hydrocarbon Engineering, Magnetrol® Global Product Manager Bob Botwinski discussed the benefits of guided wave radar technology and how it can solve some of the most nagging application problems. Here’s what the article has to say about two common level measurement problems, radar echo and overfill capacity:

Radar echo
Much has been said in the radar world about the need for a strong signal—for example, a high amplitude transmitted signal to the medium being measured. It might seem like heresy to suggest it is not the real issue, but is it? In some ways, the radar signal is like the sound from a radio: if it needs to be louder, the signal is amplified. However, if there is a high noise level behind the desired signal, then the feedback is distorted. The same situation occurs in the radar world. This relationship between these “desired” and “undesired” signals is called signal to noise ratio (SNR). Strong amplitude is a “brute force” approach and is much easier to achieve than overall SNR. In practical use, the design with a greater SNR is more robust and far less likely to have issues with unwanted reflections than one that has an inferior value.

Modern radar designs strive to increase their SNR, and users would be wise to keep this lesser known trait in mind when choosing between the various devices offered in the market. Low dielectric, turbulence and other challenging conditions are made easier with a superior SNR.

Overfill capability
It is commonly understood that no level measurement technology is perfect in all applications – many have issues measuring accurately to the very top of the tank. The ability to read to the very top of the vessel is often called overfill capability. The most advanced of GWR designs remove this weakness that plagues so many devices in the radar category. This can be critical with media that are highly corrosive, toxic or otherwise dangerous in a spill.

European agencies such as WHG or VLAREM certify overfill proof protection, defined as the tested, reliable operation when the transmitter is used as an overfill alarm. It is assumed in this analysis that the installation is designed in such a way that the vessel or side-mounted cage cannot physically overfill. However, there are practical applications where a GWR probe can be completely flooded with level all the way up to the process connection (face of the flange). Although the affected areas are application dependent, typical GWR probes have a transition zone, or possibly dead zone, at the top of the probe where interacting signals can either affect the linearity of the measurement or, more dramatically, result in a complete loss of signal.

While some manufacturers of GWR transmitters may use special algorithms to “infer” level measurement when this undesirable signal interaction occurs and the actual level signal is lost, advanced designs, such as what is featured in the MAGNETROL Eclipse models, offer unique solutions by utilizing a concept called overfill safe operation. An overfill safe probe is defined by the fact that it has predictable and uniform characteristic impedance all the way down the entire length of the waveguide (probe). This allows the probe to measure the true level at all times.

This probe design has the ability to measure accurate levels up to the process flange without any non-measurable zone at the top of the GWR probe. Overfill safe GWR probes are a unique advancement because coaxial probes can be installed at any location on the vessel. Overfill safe probes are offered in a variety of coaxial and caged designs.

More Information
More about the benefits of guided wave radar technology can be found in the Hydrocarbon Engineering article, including information about GWR usage in tanks and bridles, combining GWR with magnetic level indicators, and nonstandard measurement techniques. In addition, visit the MAGNETROL radar solutions site to learn more about the radar technology we offer.