Measurement technology comparison: 80 GHz radar sensors vs. ultrasonic transmitters for level measurement

Not long ago, we published a paper that digs into whether 80 GHz non-contact radar sensors present an alternative to guided wave radar devices in liquid level applications. If you missed that comparison, you can download it here. Non-contact and guided wave radar sensors are hardly the only level measurement solutions available in process automation, and further comparisons are useful to the users who work with these instruments every day. This paper compares 80 GHz radar sensors with ultrasonic devices for liquids and solids, and discusses how each technology performs relative to several (but certainly not all) real considerations users have to make when choosing a level measurement.

User Considerations:

Setup

Radar and ultrasonic instruments operate generally the same. Both are downward facing technologies that emit a signal that reflects from the product surface. In the case of radar, said signal is a radio microwave, and in the case of ultrasonic, the signal is a high-frequency sound wave. Both technologies use the signal’s time of flight to determine product level, and each is easy to set up and calibrate. A user can have either technology up and running in a matter of minutes.

Users will find ultrasonic level instruments have an installation limitation that 80 GHz radar devices do not. These high-frequency sensors emit focused signals and thus require a minimum of undisturbed space. This is especially useful in tanks that contain agitators, heating coils, and other internal obstructions. Ultrasonic signals can travel a narrow path in their own right, down to 4 or 5° in some instances. However, this requires some effort upon installation; such effort is not required by 80 GHz radar sensors.

Advantage: 80 GHz radar

Changing temperatures

Steady, predictable temperatures are something of a luxury in the process industry, as so many reactions depend upon heat. One of the major advantages of radar level sensors is a near invulnerability to shifts in temperature, and 80 GHz devices uphold the old standard. History is not quite as rosy for ultrasonic devices in applications with changing temperatures. Temperature fluctuations alter the speed of sound waves, changing the ultrasonic signal’s time of flight and leaving the level measurement inaccurate. Users will find it is better to play it safe with tried-and-true radar technology.

Advantage: 80 GHz radar

Foaming

Foam is the natural enemy of both microwave and ultrasonic signals. Foaming absorbs signals, literally making them disappear. Complete absorption only happens in extreme cases, but it’s a serious consideration, particularly in the chemical and food production industries. Sensors have come a long way in measuring through foam in recent years, and liquid level measurement through foam is now better than it has ever been. However, even 80 GHz radar sensors with high dynamic range and the most sensitive of ultrasonic devices are no match for thick foam. Go with a guided wave radar instrument in the case of extreme foaming.

Advantage: Draw

Product reflectivity

The high sensitivity of VEGA’s 80 GHz radar sensors make it compatible with even the most unreflective low-DK liquid product. No matter how difficult a liquid might be, one of these new sensors can measure it accurately. Ultrasonic devices also measure accurately despite difficult product characteristics, but a word of warning: Turbulent surfaces tend to disrupt the accuracy of ultrasonics, but not with radar sensors.

Advantage: Draw

Dust and condensation

Although one would never find condensation in applications where one finds dust, we’ll conflate these two conditions because of their similar effect on ultrasonic level sensors. Sound waves are mechanical and require a medium to transmit energy from one place to another. In an ultrasonic instrument, the transducer diaphragm acts as that medium—the diaphragm must move to produce a signal. Condensation on a transducer can significantly dampen an ultrasonic signal, preventing their propagation, and making level detection nearly impossible. Dust in the air also dampens ultrasonic signals, which is why an ultrasonic device will not do in bulk solids applications where it’s important for a user to measure during filling.

Radar sensors are unaffected by condensation and dust. As the transmission frequency of radar level instruments has increased, their wavelength has decreased, making them more susceptible to condensation. To compensate, manufacturers have developed ever-more-sophisticated sensitivity software that ignores a signal if bounces back to the sensor too quickly. So even 80 GHz radar sensors, with a wavelength between 3.5 and 4.0 mm deliver accurate level readings in condensation-heavy applications. 

The subject of dust and radar brings us right back to wavelength. The diameter of dust particles is in the 0.5-to-1.0-micrometer range; about 1,000 times too small to affect 3.5-4 mm waves as they travel through the airspace. This means that inside an industrial plant, radar can accurately measure the level of bulk solids and powders when dust is rampant and/or during a filling cycle.

Advantage: 80 GHz radar

Tank size

Both ultrasonic and 80 GHz radar devices perform well in small vessels. This has always been the case for ultrasonics, but radar sensors haven’t performed well in smaller tanks until 80 GHz sensors hit the market. These sensors have small antennas with correspondingly small process fittings that make them suitable for small tanks often found in pharmaceutical processes. 

In terms of larger tanks, both radar and ultrasonic sensors perform well. But when using an ultrasonic sensor, users should be ready for a tradeoff. In order to measure accurately over long ranges, ultrasonic sensors require a much larger opening at the top of the tank. If one has a tank with a large opening or if one can afford a retrofit, an ultrasonic sensor is a fine solution. If not, radar is the way to go.

Advantage: 80 GHz radar

Results: Non-contact 80 GHz sensors offer several advantages over ultrasonic transmitters

In most applications, users would be wise to choose an 80-GHz radar sensor over an ultrasonic device. Ultrasonic devices are a cost-effective, non-contact, accurate means of level measurement, but they aren’t compatible with harsh environments. They are like the quarterback whose team can count on to hit the opening receiver, but fails to deliver in tight coverage or under pressure from pass rushers.

If radar instruments were a quarterback, they’d be able to throw a tight spiral into a small window, even when on the run. Users don’t have to sacrifice performance when temperatures change, when dust fills the air, and when condensation covers the antenna. 80 GHz sensors provide the added benefit of emitting a narrow beam that avoids vessel obstructions.

Users should reserve ultrasonic transmitters for applications where the product and the environment are stable. The water and wastewater industries are full of reservoirs and basins that are perfect for ultrasonic level measurement. For more unpredictable, more challenging level applications, the 80 GHz radar sensor is the right instrument for the job.