To get an accurate level measurement, a non-contact sensor has to be able to ascertain where the surface of the measured substance is. And while liquids generally “find their own level,” every solid piles in its own special way. Some lie wide, shallow, and evenly; others collect in pillars with sharp slopes. Added to the variety of the collecting shapes is the shape of the individual pieces: smooth or jagged, rectangular, round, or flat. All of this makes for chaotic readings when using reflecting sound waves or electromagnetic waves.
Unless you have a True EchoTM PRS Dual Frequency Pulse Radar Level Transmitter for Solids.