What measurement precision do you want? I have a silo as tall as yours but the level measurement is very simple. Here's how it works:
Motor-drive paddles, mounted at the silo wall, run at a very slow speed at different elevations in the silo (a meter apart, from each other). If the level where a paddle is at becomes filled with powder, the material impedes the paddle and the small, geared-motor driving the paddle stops turning, activates a switch (a torque switch). You'll know the level of stock because the indications from bottom to the actual level is up!
We used to use a load cell. We'd use a cable hung in the silo to the bottom. A pneumatic cylinder would pull up with enough force to lift the fully buried cable. Then it would relax. The part of the cable still buried would be completely supported by the material in the silo. That meant the rest of the cable hangs as dead weight. The load cell just weighs this "free" cable and does the weight per foot calculation and you have the depth of the material.
