Noncontact radar level technology is widely used, offering accuracy to 2 mm, range approaching 200 meters, and tolerance of extreme pressures and temperatures with high stability, no moving parts and little maintenance. Steady improvements in signal processing that allow it to accommodate a broad range of vessels and process materials, along with reduced power consumption (including two-wire versions that operate on 4-20 mA) and lower costs have made it the first choice of many instrument specifiers for liquid level.
But conventional noncontact radar is restricted to 26 GHz, which leads to compromises in antenna size and beam angle that limit its ability to accommodate foam, buildup and spurious reflections from mounting fittings, tank walls, baffles, agitators and heaters, especially in tall, small or narrow vessels. This 26 GHz frequency also reflects poorly from some liquids, as well as many solids, which reduces sensitivity.
These limitations are most apparent in solids level measurement, prompting engineers to wish for higher frequency. In the past few years, the FCC has granted this wish by allowing the 80 GHz band for certain low-power applications, including automotive distance measuring systems (for collision-avoidance and interval control) as well as radar solids level sensors.
Along with resulting economies of scale, the pleasing performance of 80 GHz solids sensors has invited instrument makers to adapt them for difficult liquid applications. One of the first to do so is VEGA, which reports its new VegaPuls 64 is “the first radar level gauge on the market for liquids that measures at a frequency of 80 GHz.”
Higher frequency allows tighter focusing of the radar beam, so it can avoid interference with obstacles such as heating coils, baffles or agitators. A radar sensor with a transmission frequency of 26 GHz and an 80 mm-diameter antenna is limited to a beam angle of approximately 10°. With an antenna of the same size, VegaPuls 64 has a beam angle of only 3°. At any given distance, a 3° beam is about one-fourth the width of a 10° beam, which allows the sensor to be used in vessels with internal installations or heavy buildup on the walls because its focused microwave beam can be positioned to avoid the obstacles.
The narrower beam also allows the sensor to fit in nozzles or standpipes, such as those often used to access buried tanks, or to make sensors accessible to tank rooftop walkways, where a conventional sensor beam gives spurious reflections by striking the nozzle walls and flanges. It also allows a ball valve to be installed between the sensor and the vessel, so the sensor can be safely isolated or removed for service.
Alternatively, the higher frequency can be used to allow a smaller antenna. The 80 GHz VegaPuls 64 antenna is approximately the size of a quarter (3/4 in.) instead of a coaster (2-3 in.) at 26 GHz, which allows it to be used in compact process fittings for confined spaces in small vessels. The small sensor size also allows applications in pilot plants and laboratories, where conventional radar sensors won’t fit. It also performs accurate measurements closer to the antenna without “ringing.”
The 80 GHz sensor also has a broader dynamic range, which improves measurement certainty, and makes it suitable for a wider range of applications. Where a 26 GHz sensor would be limited to 96 dB, the 80 GHz sensor can be set at 120 dB, which is 200 times as sensitive. Media with very poor reflective properties, such as those with a low dielectric constant like oil and liquefied natural gas (LNG), can now be measured with more certainty, and the sensor can be tuned to accommodate a broader range of foam, turbulent product surfaces, condensation or buildup on the antenna.
The availability of 80 GHz radar is not only raising the level of safety—and allowing the next step toward self-driving cars—it’s coming to keep an eye on your more difficult liquid level applications.