"Measuring through foam and heavy vapors—GWR is preferred for foam and vapor applications, because the signal stays in contact with the cable or probe. So, it generally “sees” through the media to find the liquid level. FSR may work with some types of foam and vapors, but it is often a trial-and-error application. Its signal may lock onto the top of the foam as the level, or get absorbed by the vapors. Low-frequency FSR (10-24 GHz) generally has a better success rate than higher-frequency (80 GHz) radar due to larger wavelength that doesn’t get absorbed as easily."