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Getting the frequency right

30 May 2017

Low and mid frequency non-contacting radar devices can now be complemented by high frequency technology. Per Skogberg explains which of these might be the right solution for a particular level measurement application. 

Radar is currently the preferred technology for performing accurate and reliable level measurement. Low (6-11 GHz) and mid frequency (24-29 GHz) non-contacting radar devices have been installed in a broad range of challenging applications. The availability of high frequency (75-85 GHz) technology now increases the options available to end users. However, the different frequency bands available all have certain strengths and weaknesses.

Radar instruments emit microwaves to measure the distance to a liquid (or solid) surface. The wavelength is inversely proportional to the microwave frequency – the shorter the wavelength, the higher the frequency. Wavelength and frequency are fundamental attributes of a radar, and the physical properties of low, mid, and high frequency radar signals have a big impact on the suitability of each frequency when exposed to typical level measurement applications. High frequency microwave signals, for example, suffer more attenuation when propagating through a medium, resulting in a weaker signal return. 

Turbulence
Process liquid turbulence can adversely affect the accuracy of radar level measurement. Waves and ripples present on the liquid surface are detrimental to high frequency devices. The short wavelength of high frequency devices means that the signal reflection will be scattered and dispersed by such surface movements, rather than reflected to the antenna, which can cause up to 90% of the signal strength to be lost. Low and mid frequencies emit longer wavelengths, therefore enabling them to perform better.

Dirt and contamination on the antenna affects the radar signal’s strength and direction. Low and mid frequency signals can pass through such build-up virtually unaffected, but with high frequency signals, more of the energy is absorbed by the dirt, and the beam’s direction could also be diverted. For a radar with a narrow beam angle, this can result in the return echo not being directed straight back at the antenna, causing loss of signal strength. Low and mid frequency technology is, therefore, more suitable.

For dense and thick foam, such as from beer, molasses or latex, low frequency works best. For lighter foam, mid frequency performs very well. For condensation, the design of the antenna is also important. Antennas with flat, horizontal surfaces should be avoided in these applications.

Vessel size
Frequency also affects the beam width and angle from a radar device’s antenna. Here higher frequency signals enable smaller beam angles using smaller antennas, which is beneficial as it helps the beam to avoid obstructions such as agitators found inside many tanks and vessels. However, if there is an obstruction directly below the nozzle a narrow beam is likely to be completely blocked while a wider beam can still measure accurately.

In very small tanks – typically about 0.5-1.5m high – the size and placement of nozzles can be a limitation. The short measuring range and the requirement for small antennas means that high and mid-frequency technology are attractive options here. However, the challenges posed by process conditions such as condensation, contamination, turbulence and foam do need to be considered where applicable. Small beam angles can also be achieved by low frequency radars, but this requires larger antennas.

Bulk storage vessels often have floating roof tanks which require level measurement to be performed via still-pipes. Low frequency radars offer better choice here as they are less sensitive to build-up on pipe walls, slots, and pipes that are not completely straight. High frequency radars have difficulties in such situations. Bulk storage tanks can also have significant roof movements. The narrow beam width of high frequency radars makes them sensitive to tilting, and can result in the reflected signal missing the antenna opening.

When measuring solids, the best frequency choice will depend on the application in question. Low and mid frequencies can handle dust, condensation and coarse solids, while high frequency works well with very fine powders. Typically, condensation is challenging for high frequency radars, but with solids a further problem arises, as condensation, combined with some solids, will produce material build-up which will quickly clog small nozzle openings and cover the small antennas of high frequency radars.

Conclusion
For industrial level measurement applications, radar has become the technology of choice. The introduction of high frequency devices has expanded the options available to end users, especially for installations in small vessels and those with very small process connections. However, it is important to take all the process conditions into consideration and evaluate the strengths and weakness of each frequency before selecting the most appropriate solution.

Per Skogberg is product marketing engineer for Rosemount Process Level Radar Instrumentation at Emerson in Gothenburg, Sweden. 


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