E-Zine July 2011
Non-contact radar level measurement sensors emit a radar signal towards the material and measure the remnants of the signal that are reflected from the material.
Some radar level measurement systems determine the location of the material level using the time-of-flight that the radar signal takes to travel to and return from the material. The distance between the sensor (antenna) and the surface of the material can be calculated as one-half of the measured time-of-flight of the pulse times the speed of the radar signal. Mechanical dimensions can then be used to determine the level in the vessel.
Other radar level measurement systems modulate the frequency of the radar signal. In doing so, the received signal can be compared to the transmitted signal to determine the frequency difference between them. The frequency difference and mechanical dimensions can then be used to determine the level in the vessel. These frequency modulated continuous wave (FMCW) techniques are often better able to discriminate level than pulsed radar level measurement systems. This is because the frequency spectrum that is produced is more amenable to signal processing analysis than is the echo signal returned when using pulse radar techniques. In some applications, the peaks in the frequency spectrum of an FMCW radar unit can be correlated with various levels, such as the actual level, some liquid-liquid interfaces, and stratification due to vapor in the vessel.
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Excerpted from The Consumer Guide to Non-Contact Level Gauges