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Getting focused: Using 80 GHz radar sensors for liquid level measurement

The introduction of 80-GHz radar sensors represents a paradigm shift in liquid-level measurement. High-frequency devices have ushered in an era of new possibilities for the process control industry, and it’s because of signal focusing. This article looks at things an 80 GHz radar sensor can do that its predecessors cannot.

Gregory Tischler, Tom Brewer
03/07/2017

Figure 1: The focused beam (yellow) of 80 GHz radar sensors avoids internal agitators that contact the 26 GHz radar beam (gray). Courtesy: VEGA AmericasUsing 80 GHz transmission frequency liquid-level measurement can improve signal focusing to allow accurate, reliable measurement in tanks with agitators, heating coils, and other internal obstructions. That's because a narrower emitted beam makes radar a realistic option for use on ball valves, and increased transmission frequency correlates to a smaller antenna making it ideal for retrofitting and use on smaller tanks. 

Focused beam avoids obstructions

Increased focus of the beam angle is the principal benefit of 80 GHz radar instruments-the one improvement that makes the rest possible. In every process, focus is crucial to accurate level measurement, and these new instruments emit the most focused signals on the market.

Plant operators have struggled with unfocused radar for decades-particularly in liquid-level applications. The wide beam angle of 26 GHz sensors (and 6 GHz sensors before them) made it difficult for radar signals to miss agitators, heating coils, and other vessel internals. For example, a radar sensor with a transmission frequency of 26 GHz and an 80 mm-diameter antenna was limited to a beam angle of approximately 10 deg. These instruments picked up reflections from vessel installations that distorted the returning signal, forcing users to make adjustments to monitor the true liquid level.

With an antenna of the same size, 80 GHz gauges emit a beam angle of only 3 deg. This allows them to be used in vessels with internal installations or heavy buildup on the walls. The focused microwave beam simply avoids these obstacles as if they aren't even there. This is welcome news in chemical and food production, where obtrusive internals are the norm and space is at a premium (see Figure 1).

An exciting benefit of increased signal focusing is the performance of 80 GHz radar sensors when mounted on ball valves. Historically, attaching a 26 GHz radar gauge to a ball valve and receiving an accurate level measurement has been a significant challenge. Ball valves contain many interior surfaces that reflect radar signals. To make matters worse, they often are used in combination with a bleed ring that can create even more signal noise. These reflections make it difficult to discern which signals are generated by the valve and which are from the product. This confusion forces operators to turn to instrumentation manufacturers for help, but the best solutions often are difficult to implement and may require periods of trial and error despite the best efforts of plant technicians. Before long, the manufacturer's tech is onsite servicing the 26 GHz radar.

The increased focus of high-frequency gauges means fewer signals are reflected by the valve's interior. This minimized noise creates a clear picture of the level inside a tank. Users across the world have experienced consistent level measurement mounting an 80 GHz radar gauge on garden-variety 3-inch and 4-inch ball valves. The bigger the valve, the clearer the signal, but 80 GHz sensors also are performing well on 2-inch ball valves. This is a big break for users who need ball valves to separate their equipment from their product but have avoided installing a radar-level device because the degree of measurement difficulty was too great. Radar's accuracy and low-maintenance reliability is now a viable option.

Superior focus makes for accurate measurement without adjustment even when mounted on a ball valve, but the increased focus of 80 GHz radar sensors creates other benefits for users. 

Small antennas take radar where radar has never gone before

Amping up a radar sensor's focus has an opposite effect on its antenna-its size decreases as transmission frequency increases. An 80 GHz sensor, therefore, does not require a large horn to focus its beam at the measured material. The signals take a narrow beam all on their own. The saved space makes a huge impact, particularly as it applies to retrofitting. Plants now can integrate the most advanced radar devices into an existing process without shelling out thousands of dollars for vessel modifications. For users who have longed for radar but could not afford a retrofit, this news is tantamount to a budget increase. Smaller instruments, however, aren't just good for old vessels; they also can help manufacturers stay nimble and market-responsive.

Figure 2: Small process fittings make 80 GHz radar sensors ideal for retrofitting onto existing tanks. Courtesy: VEGA AmericasThere's a trend in the pharmaceutical and chemical industries toward batch production. Batching allows operators to produce seasonal and low-volume products with less financial investment. Small batches are produced in small vessels, where conventional wisdom says using radar is impossible due to small process connections. Thanks to the compact antenna of 80 GHz radar sensors, that is no longer true, and operators no longer have to sacrifice accurate measurement in the name of space (see Figure 2). 

Enhanced resolution measures to the last drop

Any fan of cliché action films is familiar with this scene: An investigator stands behind a computer whiz in a laboratory full of expensive-looking surveillance equipment. They are watching a grainy piece of closed-circuit video, knowing that one of the pixelated figures on the screen is a criminal mastermind. The computer whiz punches a few keys and the word "enhance" appears in green letters on the computer screen. The blurry face gets a little clearer. The computer whiz clacks away at the keyboard and enhances the footage once more, revealing a crystal-clear image of the villain's face. This might be an example of lazy screenwriting, but it's a good analogy of the difference in resolution one would experience when switching from low-frequency to high-frequency radar sensors.

When the level of liquid in a vessel gets low enough, 26 GHz radar is unable to distinguish the signal returned by the remaining product from that of the tank bottom, and the user rightly thinks the vessel is empty when it isn't. This is the same as the action-movie computer whiz being unable to enhance surveillance footage. Limited resolution presents a natural handicap to process efficiency. Ultra-focused 80 GHz devices measure liquid down to the last millimeter in the tank, giving users accurate data that can help optimize their processes. It won't thwart a terrorist attack or prevent an elaborate heist, but the enhanced resolution of 80 GHz radar sensors helps users avoid waste. 

Limitations of high transmission frequency

There are certain scenarios where 80 GHz sensors are limited. In liquid applications where foam is rampant, a guided wave radar (GWR) sensor is usually a more appropriate solution than an 80 GHz sensor because a GWR transmitter emits a concentrated radar pulse conducted via a probe that will not be absorbed into the foam. That's not to say 80 GHz radar won't work in applications with foam, just that guided wave sensors typically work better.

Additionally, high-frequency instruments are not recommended for use on stilling wells or on standpipes. This is an instance where high frequency works against the application. Note that 80 GHz beams have more modes (think, pathways) to the product, and in the confined space of a stilling well, signals from those modes create a confusing level reading. Again, 80 GHz radar sensors will work, but 6 GHz sensors are going to output a measurement that is easier to read.

Looking ahead

This cannot be overstated: 80 GHz transmission frequency changes everything as it concerns liquid-level measurement. Superior signal focusing allows for accurate, reliable measurement in tanks with agitators, heating coils, and other internals. Plus, the narrower emitted beam makes radar a realistic option for use on ball valves. The increased transmission frequency correlates to a smaller antenna that is ideal for retrofitting and use on smaller tanks, and enhanced resolution lets users make the most of every vessel. The future of radar level sensors is here. That future is 80 GHz. 

Gregory Tischler is a product manager at VEGA Americas and is responsible for radar and guided wave radar sensors. He has almost 20 years of experience in the industrial automation industry, all with VEGA Americas. He is a voting member of the American Society of Mechanical Engineers Bioprocessing Equipment Process Instrumentation subcommittee, which is responsible for writing instrumentation standards for bioprocessing equipment, and he was also an active member of the Measurement, Control & Automation Association committee responsible for shaping new Federal Communications Commission rules for tank level probing radars (Section 15.256), which were released in 2014. Tom Brewer is marketing content specialist at VEGA Americas. Edited by Jack Smith, content manager, CFE Media, Control Engineering, jsmith(at)cfemedia.com.

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