I was wondering if anyone happens to know a decent level sensor which could measure the length (0.400mm - 0.700mm) of a powder tightly compressed within a small tube? The sensor must be outside the tube.
I am looking for a pro-level sensor, not a toy.... 
Digital micrometer/caliper?
Need more detail.
Does level change, or are you going to be presenting multiple tubes to sensor for evaluation? Change how fast? Tube opaque? Power?
One route might involve leaving the world of Arduino (oh no!) and using a simple camera to "snap" an image, and using simple graphics processing to "see" level. While I think the press are over-excited about the Pi, and mis-understand what the hobbyist/ student "needs", I have to admit that the Pi's camera, does make me think that if I REALLY didn't want to use an old laptop and webcam for something like this... which ** IS ** what ** I ** would do... (I have to have some reason to tell my wife for keeping the old laptops, after all)... I must admit that the Pi would cross my mind... for THIS!