Level Sensors Radar Replaces Displacer Level Sensors

Author / Editor: Dominik Stephan* / Dominik Stephan

How much is actually in there? Level sensors allow you to peek inside closed silos and containers– To learn more about the latest trends and developments, we paid a visit to Magnetrol, a Belgium-based supplier of level controls. For radar can detect airplanes and speeding drivers, but it can do a lot more than that.

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Through-air or guided wave: Radar helps to precisely determine the filling level in silos or vessels.
Through-air or guided wave: Radar helps to precisely determine the filling level in silos or vessels.
(Pictures: Magnetrol/Fotolia © Argus)

Level detection should not be a problem. A broad range of different technologies is available for measuring fluids and bulk solids. However, the details need to be carefully considered. The media as well as the silos and containers themselves can pose a significant challenge. The sensors must be compatible with any objects on the inside such as ladders, baffle plates and agitators. Vapor and foam can lead to incorrect results, and the properties of the medium such as density and conductivity can preclude the use technologies like capacitive sensing.

That is one reason why pulse emission is often regarded as the trump card in level detection: A radar, microwave or ultrasonic pulse is emitted into the container from above. When the pulse hits the medium’s boundary surface, part of the energy is reflected back and can be detected as an echo. The reading is derived from the travel time of the incoming signal without any for physical contact with the medium.

“Ideally, detection should always be non-contact,” explained Marc Baert, General Manager of Magnetrol in Zelen, Belgium. The company introduced its first radar level sensor in 1998, a step that revolutionized Magnetrol’s portfolio. Radar sensors are now the company’s flagship products, replacing displacer and float-based instruments. “Floats have their limitations. Contamination and density variations, for example, are problems which we have solved by using radar.”

Level Measurement at the Speed of Light

The principle of pulse measurement is simple, but the technology is complex. Travel times in the nanosecond range create the need for high-speed acquisition and calculation. Different waveforms can require elaborate adjustments or are directly dependent on the medium (as is the case with ultrasound).

Few non-contact techniques are able to produce dependable results regardless of the container contents. Even fewer operate without the need for calibration or regular cleaning. The one exception is radar. There was talk of using it for level detection back in the 1970’s, but the first compact radar level sensors were not introduced until 1989.

Radar level sensors use microwave pulses above 2 GHz which travel at the speed of light. The wave travels a distance of one meter in 3.4 nanoseconds. The speed remains essentially the same in a vacuum or any type of gas. The advantage is obvious, namely precision, non-contact detection for almost any medium. However, radar does have one crucial disadvantage. Objects inside the container or changes in the container cross-section create “false” echoes which interfere with detection.

There is one variant which sidesteps these problems: On guided wave radar systems, the pulses are sent down a metal probe. At the boundary surface, sufficient energy is reflected back to determine the level in the container. The signal travels through vapor clouds and past any objects. Agitators pose the only remaining problems. However, even for that problem the company has a solution. “We can supply a curved probe on our latest guided wave radar systems,” reported Baert. This feature is very welcome in the pharmaceutical and biotechnology industries.

Two Sides of the Same Coin

Magnetrol delivers around 20,000 sensors a year, particularly to customers in the oil, gas and petrochemical industry. However, demand also exists for the Belgian company’s sensors in other industries where there is a need for maximum precision. As detection becomes increasingly accurate, rugged and versatile, development efforts are now focused on data evaluation.

Other trends such as wireless data transmission are not a significant factor at the moment, reported Annick Halterman, Global Service and Technical Support Manager. “Many customers actually insist on the two-wire standard.” Non-contact and guided wave sensors are two sides of the same coin, said Halterman. Rather than deciding on one standard, the important thing is to know which technology is better for any given application. The sensors complement each other very well, sometimes even in the same system.

* *The author is a PROCESS editor.

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