We are often asked about measuring levels for bulk solids. This is easily done with point-level technologies, of which we have a few. However, if one is after a continuous-level, then we turn to ultrasonic sensors – if it makes good sense.
Unfortunately, solids often present more variables than do liquids:
No Self-Leveling - Solids often pile up and develop a steep angle of repose.
Heavy Dust - The air is often thick with dust during the fill process, and can block an ultrasonic wave.
Soft, Absorbent Materials - The surface of the solid must reflect the ultrasonic wave back to the sensor. Not a problem with hard surfaces.
Our level sensing Application Engineer, Jon Cox, wrote a nice article on this very topic for Process Magazine. He explains how the angle of repose, heavy dust, and surface reflection affect non-contact measurement technologies such as ultrasonic, radar, and load-cells. Read his article here.