May-2016

Guided wave radar — today’s vanguard in level measurement

Guided Wave Radar (GWR), the increasingly popular, industrial, loop-powered transmitter we know today, burst on the scene in the late 1990s.

Bob Botwinski
Magnetrol International

Article Summary

The Magnetrol® Eclipse® Model 706, like most devices, is based on the revolutionary Lawrence Livermore National Lab patent that in 1995 Popular Science magazine called “Radar on a Chip” for $10.

Hard-fought Early Battles
At first the Eclipse GWR was almost shunned. Why would a customer use an “RF capacitance-looking device” with a probe? Non-contact devices had clear advantages over contact with ultrasonic and radar transmitters already carving out their own niche in the marketplace. Installing a probe seemed almost archaic. But, the probe WAS the secret.

What almost 15 years of experience has taught us is that the probe, the initial perceived weakness, is the real strength of the system. First, the probe offers a conductive path for the extremely low-energy signal to travel. This allows a maximum amount of energy to reach the surface where it is reflected and sent back to the transmitter for interpretation. Extremely low dielectric/low SG liquids like propane and butane can be measured with no problem. Non-contact radars can measure these liquids using a stillwell/standpipe that essentially provides a guided wave device, but at a far greater cost. DPs can measure these materials but are subject to SG variations that will greatly affect accuracy. Secondly, since the probe is a conductive path that maintains control of the signal, energy is not scattered within the tank (like non-contact radar) where it can encounter numerous objects that create false targets.

What has been revealed in the past 15 years is the special qualities of Eclipse Model 706 GWR that has allowed it to seep into the bag of instrumentation tricks we have come to count on every day. GWR, as a technology, has slowly become the standard in process and storage tanks around the globe. First it was used as a problem-solver. Then, as users gained confidence, it became a daily staple on their level measurement menu.

This article will not explore the simple, generic applications we know can be solved with almost any level measurement technology including GWR. Rather, we intend to highlight a few of the special areas where users have found particular success in solving nagging measurement problems and the Eclipse GWR has become their go-to technology as application knowledge and product performance have evolved.

Radar Echo — Is Bigger Always Better?
Much has been said in the radar world about the need for a strong signal, (i.e., a high-amplitude transmitted signal to the medium you are measuring). It might seem like heresy to say it is not the real issue, but is it? In some ways, the radar signal is like the sound from a radio to which you are listening. If you want it louder, you amplify the signal—an easy task. However, if there is a high noise level behind the desired signal, what you get is garbled. The same situation occurs in the radar world. This relationship between the wanted and unwanted signals is called Signal to Noise Ratio, abbreviated as 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 as they choose between the various designs offered in the market. Low dielectric, turbulence and other challenging conditions are made easier with a superior SNR—and the new Eclipse Model 706 leads the industry in this area.

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, the advanced design of the Eclipse Models 705 and 706 offer unique solutions by utilising 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 true level at all times.

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 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. The ability to read to the very top of the vessel is often called Overfill Capability.

European agencies like WHG or VLAREM certify Overfill proof protection, defined as the tested, reliable operation when the transmitter is used as an overfill alarm. Further, it is assumed in their 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.

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.

Guided Wave Radar in Chambers/Bridles and Magnetic Level Indicators
Bridles and chambers have become popular means of level measurement, first due to use with displacer transmitters and now as an efficient means of external mounting that allows isolation via shut-off valves. GWR has often been used in this configuration utilising coaxial probes. However, the recent popularity of single rod probes (primarily due to their cost and higher immunity to buildup) has raised a set of important performance issues.

Coaxial probes are the most efficient propagators of microwave energy, which is why everyone’s television signal is transmitted over coaxial cable. Single rod probes are inefficient in two key aspects:
- Launching the signal causes a large impedance mismatch at the top of the probe which creates noise that interferes with good target acquisition.
- Propagation of energy along the single rod probe is the least efficient of all GWR waveguides, which is not the best approach for optimal performance.

Both of these issues are resolved in the Eclipse Model 706 when its single rod probe is carefully impedance-matched to the typical chambers/bridles seen in the process industries. In this way, there is no top-of-probe mismatch and, when done very carefully, the single rod probe/cage combination effectively becomes a coaxial arrangement creating excellent propagation efficiency.

The most recent Eclipse Model 706 developments have included this probe/chamber matching design which yields excellent performance at the lower cost of a single rod probe.