E-Zine September 2011
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Radar sensor geometries generally include parabolic, cone/horn, stick/rod, and stilling well designs. Parabolic sensors are typically large (over 8-inch) and produce a strong radar beam with a relatively small beam angle that can measure large distances and/or exhibit custody transfer performance. Cone/horn sensors can be installed in nozzles as small as 2-inch and exhibit good performance. Some stick/rod sensors can be inserted into yet smaller nozzles but generally produce radar beams with relatively large beam angles. Stilling well systems can also be installed in relatively small nozzles and should be considered when the liquid in the vessel is turbulent. Stilling well systems provide a calming effect while containing radar energy to produce a more focused beam in the stilling well.
The material itself can cause the intensity of the reflected radar signal to degrade when the material exhibits poor reflective qualities. Notwithstanding other problems, radar level measurement is typically limited to materials with a dielectric constant greater than approximately 1.5. Liquids with lower dielectric constants do not reflect well and may cause the level measurement to be erratic or fail to operate. FMCW radar level measurement systems can typically operate at lower dielectric constants as compared to pulsed radar level measurement systems. Notwithstanding the above, some non-contact radar level measurement systems can measure the level of materials having dielectric constants as low as 1.05.
Radar level instruments often can measure through foam, so their measurements generally represent the level of the material. However, applications have been reported where the foam either attenuated the radar return signal too much to permit a measurement or was able to cause a large enough “spurious” return to an erroneous level in the vessel.
Excerpted from The Consumer Guide to Non-Contact Level Gauges