Radar is one of the most common level measurement technologies in the instrumentation market. Whether measuring liquids or solids, from chemical plants to wastewater facilities and for ready-mix cement manufacturers, radar provides accurate, reliable level measurement without the need for ongoing maintenance and frequent recalibration. Radar sensors are not only a trusted means of pinpointing level control; they are easy to use as well.
When the process automation community discusses radar for level measurement, the conversation veers in one of two directions: through-air radar and guided wave radar (GWR.) This article explains how through-air radar operates and it’s advantages and limitations in industrial applications.
How through-air radar works
Radio microwaves are transmitted by the antenna system of the sensor to the measured product, reflected by the product surface, and received again by the antenna system. The microwaves travel through the air, hence the label, through-air radar. The time of flight from emission to reception of the signals is precisely proportional to the distance to the product surface: The longer the time of flight, the greater the distance. This distance is inversely proportional to the level in the tank. The greater the distance, the lower the level.
Many parameters determine the strength of the signal returned to a radar sensor. An agitated, turbulent surface will affect the reflected signal strength, and distance to the product surface will, too. Even normal process conditions such as build-up on the antenna system may influence a return signal. Signal strength also depends partially on chemical composition, as not every product reflects microwaves equally.
Conductive products reflect almost all microwave energy and non-conductive products reflect only a portion of the energy. Non-conductive products with low dielectric constant, oil for instance, produce weaker signals than those with high dielectric constant, such as water. The range of signals a radar sensor can detect is called its dynamic range. Sensors with large dynamic range are sensitive enough to register weak signals as well as strong ones. Radar sensitivity varies from manufacturer to manufacturer and they even differ from sensor to sensor in a manufacturer’s instrument line.
The focus of the microwave beam depends on a radar transmitter’s antenna size and its transmission frequency. The smaller the antenna, the wider — and less focused — the beam. The larger the antenna, the more focused the beam. That’s why the development of liquid level sensors operating at a transmission frequency of 80 GHz was such enormous news in the process automation industry.
Radar level measurement with 80 GHz
At VEGA, we have seen the practical benefits of our 80 GHz level sensors, the VEGAPULS 64 and VEGAPULS 69, in over 70 000 installations worldwide, every year. In containers and silos with many internal installations, enhanced focusing helps reduce the influence of noise created by microwave energy reflecting back to the antenna from something other than the product surface. Noise is commonplace in vessels with agitators, baffles, or heating coils, and the walls of the vessel itself sometimes create it. Noise is a problem because it distorts level measurement, but it can be overcome.
Advantages of through-air radar sensors
The most obvious advantage to using a through-air radar sensor is that it is a non-contact level measurement that requires less maintenance due to a lack moving parts and a lack of contact with corrosive product. Users also do not have to worry about the level sensor being damaged or blocked by adhesive solids.
