Industry uses, or at least used back in 1975, according to a book on industrial processing electronics "Elektronik ohne Ballast" by Otto Limann, ch. 17, 17.55 (I have a 1980. Croatian translation of original book) radioactivity - the sensing principle is similar to smoke detectors' operation, only with measurement instead of threshold detection. 

Well, a scale might work if you measure the flow rate as change of overall weight and extrapolate the value into the future. That way you should be able to stop pouring early enough. Measuring the top container should give you all the sand that has already been poured even if it is still in the air.

But without more details about what you are doing your question cannot really be answered. How much sand are you pouring per second? How deep? What time resolution do you need? How constant is the flow of sand? Is there dynamic movement in the sand e.g. parts breaking free from a slope? ...

Paddlewheel?

is the tube transparent ? is the sand flow always compact, or with variable concentration with air gaps ?

mouse optic sensor could sense the movement given the fact that the sand does not change its structure much when flowing, if the sand is not compact, then laser profiling from all directions could estimate the profile concentration, plus the optical sensor that would sense the flow speed.

Any hints about the literature what is done already or is this a novelty ?

A caveat is that a load sensor wouldn't work, I need the reading while the sand is being poured.