Try searching on silo level indicators. I grew up on a farm - the silo was where we put chopped hay and cord. A "silo" is also used to describe a storage structure (usually round) used for all sorts of granuallar and powdered material. Not sure what your background is so I thought this might help.
We used a lot of level indicators in one area that I used to work in - for plastic pellets that were similar in size to grains of wheat. There was quite a difference - static electricity and ethylene gas were concerns that we had to address. There have to be a million kinds of them, each with their own cost, complexity, reliability, etc.