Ultra Sonic coverage angle problem in Cylinder
If the beam hits the wall at an angle of 15 degrees or less, it will reflect, also at 15 degrees, i.e. continue in approximately the same direction.

If the beam hits imperfections in the wall, a fraction may be reflected upwards and result in a low intensity return. If it hits the contents, you hope for a larger intensity return.

So:


Discriminate between small returns and larger ones. Note however that a large return from the bottom of the tube will be attenuated, so the discrimination threshold must reduce as depth increases, i.e. reduce as the travel time increases. 
Polish the walls of the pipe!