One bad meal can ruin a restaurant for a customer. The place can change ownership, bring on a new chef and get sterling reviews from critics, but that negative experience lingers. Not even assurances from a foodie friend are enough to erase an unsatisfactory night out. Bad meals do not die easily; they live on well after the bill is paid. Similarly, a process instrument can leave a sour taste in a user’s mouth if it does not produce accurate, reliable measurements. Radar measurement improved
For years, radar sensors have had a bad reputation for powder level measurement. Frankly, radar earned that bad rap. While the technology has always worked brilliantly with coal, grains and plastic pellets, it was never compatible with powders for a variety of reasons including size and chemical composition. Users who tried to make radar sensors work because they are low-maintenance, long-lasting and durable in difficult conditions ended up disappointed. If radar technology were a restaurant, powder users would drive by its parking lot without a second look. But now is the time for users to give radar another try.
Modern sensors with 80-gigahertz (GHz)-transmission frequency put radar technology back on the menu for powder level measurement. This article examines how increasing the frequency makes radar viable in powder applications and discusses radar’s advantages when compared to other level technologies.
No problem with particle size
Radar sensors were always a good fit for measuring the level of plastic pellets, but they were not accurate when used for fine powders. The lower densities of the smaller particles were difficult for low-frequency (6 to 26 GHz) sensors to measure because of their wavelengths. The early 6.3-GHz radar instruments operated with a 50-millimeter (mm) wavelength, and their modern 26-GHz cousins sported a then-industry-leading 12-mm wavelength. The beams from these sensors could not detect small particles. Think of trying to contact an individual fertilizer pellet with a standard rake; the difficulty that low-frequency radar sensors had detecting small powders is greater.
The 80-GHz radar sensors emit higher-energy (3.5- to 4-mm) microwaves that can detect particles smaller than 1 mm in diameter. The rake’s teeth are closer together, so contacting a lone fertilizer pellet is easier than ever. High-frequency radar sensors with large dynamic ranges are now a proven level solution in plastic powder applications.
Any dielectric will do
The inability to detect small particles was not the only reason end users may have given up on radar technologies. Product composition played a role in disqualifying radar sensors as well because older instruments were unable to measure powders with a low dielectric constant (dK). Low-dK products, whether solid or liquid, do not hold a large quantity of energy from a charge, and what little energy they do hold, they do not hold long. For decades, this reality has been problematic for end users, designers and manufacturers of radar sensors. Low-dielectric material did not reflect a usable quantity of energy back to the radar sensors. This concern has been alleviated by the availability of large dynamic sensor ranges.
Measured in decibels, dynamic range is an indicator of sensitivity. The popular video game console Xbox One S advertises its dynamic range as proof of lifelike, detailed graphics. In the process instrumentation industry, the term refers to the range of usable signals a device can detect. The larger the dynamic range of a radar instrument, the smaller the signals it can measure. Whether discussing video games or radar sensors, dynamic range measures the ability to capture small details. For radar sensors, these details are weak, difficult-to-detect signals.
Though the technology has been available for years, large dynamic range previously came at a prohibitive cost. It made no financial sense for instrumentation manufacturers to double the cost of sensors to narrowly expand the application scope for users. That expanded scope is now easy to achieve at a reasonable price. The most sensitive radar sensors on the market have a dynamic range of 120 dB, five times greater than that of their 26-GHz predecessors. These sensors can measure virtually any product, no matter how low the dK. This paradigm shift arose because, while the dK value used to be the most important variable when determining if a radar sensor would work, the dielectric value of a powder now does not determine if radar will work, but rather, which radar will work.
As radar sensor transmission frequency has increased, wavelength has decreased, allowing for more accurate measurement of small particles.
Noncontact instruments have longer service life
Mechanical level instruments that contact the product have always been used to measure powders of any particle size and dielectric value. Still, powder end users have been interested in noncontact sensors, including radar and ultrasonic devices, because bobs, yo-yos, rotating paddles and other contact devices are often maintenance intensive and damage prone. The main culprit of this damage is product buildup.
Buildup is a serious concern and a constant headache for end users who measure powder level with mechanical level instruments. Paralysis by buildup is common.
Moving gears and motors are prone to seizing up because of accumulated dust. Powders can also coat the weight of the yo-yo, gum up the connection between the bob and cable, or prevent a paddle from rotating at the proper speed. Any of these unfortunate scenarios may hinder a user’s level measurement accuracy.
To be fair, if a mechanical device is installed in a vessel, an up-to-the-second level reading may not be necessary for the process. However, end users should still be concerned about buildup because of the instrumentation’s return on investment.
The purchase of a level measurement device is an investment in the safety, reliability and success of an automated process. However, not all users look at it that way. Many think of a level instrument not as an investment, but as a purchase and choose the least expensive option. However, if buildup is present, it will not be long after installation before the least expensive level instrument needs new parts or must be sent for a repair.
These costs accumulate quickly. After a while, buying a new level measurement device will make more financial sense than continuing to repair the old one. End users can easily find themselves back where they started: making a level measurement purchase when they should be making an investment.
Noncontact instruments maximize an investment. They may be more expensive on the front end, but the initial cost is negated by reduced (and in most cases, eliminated) maintenance costs and a longer service life. With the ability to set and forget the level measurement, plant operators can allocate more resources to improving overall process efficiency. If buildup encroaches upon a radar antenna, its presence is neutralized because software filters out close-range signals. For longer sensor life and accuracy through buildup, noncontact level sensors are reliable investments.
Radar uraarnaffected by dust
Dust in the air is a common challenge for the accurate level measurement of all bulk solids, powders included. It tends to be worst during filling cycles in which stored and filler products collide. As discussed in the previous section, dust buildup can influence measurement accuracy and the ability of some equipment to function correctly. However, simply installing a noncontact level device is not enough to defend against the negative effects of dust.
Ultrasonic devices, for example, are a cost-effective, noncontact way to measure the level of bulk solids. They are compact, easy to install, and effective and accurate in vessels up to 50 feet. Ultrasonic sensors are successful in many applications, but they work best in mild environments, measuring stable products. A dust-heavy application is no place for ultrasonic instruments because sound waves are mechanical and require a medium (in this case, air) to transmit energy from one place to another. Dust in the air dampens sound waves, interrupting their transmission, mechanically preventing their propagation and making level detection nearly impossible. If measuring level during filling is important to an end user, an ultrasonic device is not a wise choice, and users should turn to radar.
Radar sensors emit electromagnetic radio waves that are unaffected by dust. Unlike sound waves, dust does not inhibit a radar signal’s transmission or its propagation. In fact, it has no affect on any radar sensor, regardless of transmission frequency. This is due to the wavelength. The diameter of dust particles is in the 0.5- to 1-micrometer range. The 80-GHz radar sensors have the shortest wavelength on the market, and dust is about 1,000 times too small to affect these 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 during a filling cycle. This is a major advantage in automated processes.
Conclusion
A bad experience, whether at a restaurant or with a process instrument, is often difficult to forget. Anyone who invested in a radar sensor to measure powder, only to have the sensor fail, has every right to hesitate trying it again. However, an 80-GHz transmission frequency changes everything. These sensors have the wavelength to detect smaller particles and the dynamic range to measure weak signals from low-dielectric products, and all of radar’s established strengths still apply. Noncontact measurement extends a sensor’s life, and the presence of dust has no bearing on measurement accuracy. Ultimately, radar level measurement for powder storage and handling is a viable option with this improved transmission frequency.
Gregory Tischler is a product manager at VEGA Americas, who 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.
VEGA Americas
