Tutorial: Guided-wave radar level sensors

There are two main types of radar level sensors: through-air and guided-wave. Both technologies are very versatile, but guided-wave designs have some useful advantages.

Guided-wave technology sends the radar pulse down a probe that extends into the tank contents, either liquid or solid. Typically, they’re mounted from the tank top hanging down. The pulse hits the surface and is reflected back up the probe to the sensor, where the transit time is translated into a distance. This is very similar to through-air radar, which accomplishes the same action without the probe.

This technology gives a very precise continuous reading, is energy efficient (a consideration for potential battery-powered wireless connectivity), and can operate in difficult situations where other approaches fail.

The probe concentrates the energy pulse much as speaking through a tube concentrates the sound. This is particularly useful under certain conditions where a through-air pulse is less reliable:

Liquids that have low dielectric values (e.g., propane, butane);

Low specific gravity liquids;

Turbulent liquids;

Internal physical obstructions (e.g., baffles);

Heavy foam layers; and,

Problematic solids.

A guided-wave probe is often much smaller than a through-air antenna, which allows it to be mounted through a smaller opening or where equipment is more congested. While the probe helps with the signal, it can introduce its own problems:

In some extreme cases no product contact is permitted, which demands another technology entirely.

A unit can only measure as far as the probe extends. There is no theoretical limit, but there are practical issues when they get very long.

Probes can be either rigid or flexible, determined by practicality. An application with solids may not be suitable for a flexible probe. Similarly, liquid with high turbulence may cause a flexible probe to move around.

Sticky material can build up on a probe. Various designs can see through this to a point, but eventually it will interfere with an accurate reading.

In some cases, probes have been bent to read around corners. This is tricky, though, and you should consult with your provider as to practicality.

The probe does not have to be the full length of the tank unless you need readings all the way to the bottom. If the process only needs readings in the upper half of the tank, that’s all the length you need.

These points are generalities, so discuss specifics with your provider. Guided-wave radar sensors are available from a variety of suppliers:

Endress+Hauser IFM Efector K-Tek MagnetrolMonitor Technology Siemens For more on level sensing technology, check out these tutorials:

Choosing Level Sensors (Control Engineering, June 2007) Through-air radar sensors Magnetostrictive sensors

—Peter Welander, process industries editor, PWelander@cfemedia.com , Process Instrumentation & Sensors Monthly Register here and scroll down to select your choice of free eNewsletters .

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