(www.ktekcorp.com), a Prairieville, La.-based supplier of level instrumentation.
Through-air doesn’t contact the storage vessels’ contents or the process, notes Lauro Cantu Jr., radar specialist with Emerson Process Management’s Rosemount Division (www.emersonprocess.com/rosemount), Austin, Texas, which supplies process sensors. When using these non-contact sensors in processes, Cantu says one advantage is the user’s ability to isolate the sensor and have it still function. “You can isolate the unit through a valve or a Teflon window—the window acts as a shield—which gives higher safety and less downtime.” Other advantages are less maintenance, as well as use with more viscous fluids.
With antennae or horns, through-air sensors broadcast the radar wave onto the liquid or solid surface—and then receive the reflected signal with those antennae or horns, Hotard explains. These sensors typically find more use in large storage tanks, where high accuracy is required for inventory control and custody. “A lot of places where it has been applied are those places where other technologies won’t work,” he notes.
Higher efficiency
Two types of through-air radar sensors—frequency modulated continuous wave, or FMCW, and pulse—exist, notes Bogdan Cherek, president of ABM Sensor Technology Inc. (www.abmsensor.com), an industrial sensor supplier in Peterborough, Ontario, Canada. He believes the pulse system delivers higher efficiency. Existing pulse-radar systems—which have fixed power, transmit fixed pulse lengths and have fixed gain in their receivers—may encounter problems with different tank dimensions, Cherek believes. “Sometimes their (the unit’s) power is too high; sometimes, too low.” His company’s pulse-radar technology self-adjusts power and, overall, requires less power, transmitting in the low-microwatt range.
Guided-wave radar level sensors operate similarly to through-air ones, Hotard explains—except guided-wave’s antenna drops into and penetrates the surface of the product. Microwave energy is sent through the probe, which is mounted to the bottom of the tank, into the liquid, he says. “That creates a reflection when the dielectric constant of the liquid changes,” states Cantu. The dielectric constant, known also as permittivity, measures the ability of a material to withstand formation of an electric field within itself.
Typically, guided-wave radar level sensors find use in water and petroleum liquids, Cantu remarks, especially in oil-water separators and chemical industry vessels in which two liquids have a distinguishable interface. “One advantage of guided-wave is that it’s not influenced by tank obstacles or geometry,” he adds.
This technology has become more popular in the past two years, Hotard says. Noting that guided-wave has no moving parts, he asserts, “Regardless of a temperature, pressure or specific gravity change (of a vessel’s contents), it can give a true measurement.” Product buildup on the antenna is possible, though it will still operate, he notes, but users apply this technology to replace float technologies.
Guided-wave radar level sensors, which are also called time-domain reflectometers, use one or two wires, Cherek notes. But now, two-wire 4-20 milliamp (mA)-loop powered non-contact radar sensors are available, says Cantu. “[However], I’m starting to see that instead of using 4-20 mA, the units are being powered digitally by field buses such as the Foundation Fieldbus or Profibus,” he comments.
Historically, guided-wave has cost less than through-air, Cantu observes. That’s changed in the past few years, though, because installation and configuration of through-air has been simplified, he says. “Through-the-air is the future,” Cherek adds.
