Using through-air radar on low dielectric materials in petroleum industry applications

Recently, a refinery customer came to use with a level application. Our team determined that it would be a perfect fit for radar level gauges, IF they turned on a Siemens radar algorithm called CLEF, that would let the radar measure accurately all the way to the bottom of the tank.

What is CLEF? How does it work? And why does it matter?

We’ve talked a little in the past about Siemens echo processing, and using echo position detection to accurately determine true level, regardless of obstructions. But in low dielectric materials, even Siemens echo processing algorithm needs a boost.

In an almost empty flat-bottomed vessel, a low dielectric material can reflect an echo weaker than the echo from the vessel bottom. On the echo profile, the two echoes become merged. This can cause an incorrect reading in an empty or almost empty vessel, because the radar device may report a material level equal to or lower than empty.

Siemens CLEF tank drawingThe Constrained Leading Edge Fit (CLEF) algorithm extracts the position of the first of two echoes that are close together, and uses the leading edge of the merged echoes to determine the true material level.

The value for CLEF Range must include the difference between Far Range and Low Calibration Point, plus any level above the Low Calibration Point to be managed by the CLEF algorithm. CLEF is used from Far Range up to the level defined by CLEF Range (see diagram). Above that point the Center algorithm is used. In applications with low dK materials, Siemens recommend settings CLEF Range to 0.5 m (1.64 ft) and Position Detect to Hybrid.

Before CLEF and after
The charts above show an echo profile on a tank with low dielectric material, to the left is before CLEF is applied, to the right, after. Before CLEF, the system would detect the echo at the bottom of the tank, ignoring the true echo. After CLEF is applied, the echo accurately detects the level of the material instead of the “louder” echo from the bottom of the vessel.

The user manuals for Siemens radar level detection systems include the instructions on how to enable CLEF, including the adjustments necessary if you’re using a stillpipe for measurement.

Lesman Instrument Company is the authorized Siemens representative in Illinois, Indiana, Missouri, Paducah KY, Wisconsin, and Michigan’s Upper Peninsula. If you are located outside that area, you can find your local sales office or get technical assistance by calling 800-365-8766.

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