It would be helpful to know how large the bins are. I work at a place that uses a lot of powders and pellets of varying density in our processes. I have used vibrating rod, fork, paddle wheel, capacitive and ultrasonic sensors. If the bins you are using are shaped properly and the material flows out consistently (without rat-holing or bridging), you could use any one of the above sensors if mounted properly. I try to avoid capacitive due to changes in ambient humidity in Michigan which can affect proper readings. The rod, fork and paddle sensors detect material at one place in the vessel, but are very sensitive to the angle they are mounted. If mounted horizontal, then bridging of the material on the sensor can occur. I have used and like the Bindicator and Binmaster products for these type of sensors. For ultrasonic sensors, I have had good luck with the Ranger and Ranger One sensors from Flowline. I have been intrigued by the 3D radar sensors that are offered by Endress + Hauser, but have not used them yet. These are ideal for large bins or silos that do not empty or fill evenly.