Ultra Sonic coverage angle problem in Cylinder

Question

I have 2.5 meter long height, 5cm radius cylinder. This cylinder can be used as a trashcan and I want to install an Ultra Sonic Ranger like SRF05 to see how much space left in it.

Cylinder has long height compared to radius and SRF sensor will be installed on the top of cylinder. And I read these sensors have a coverage angle of 15degrees.

Would it be possible if sensor's beam angle produce a false distance by hitting Cylinder walls before actually reaching the bottom of the object? I mean Cylinder is really slender and I'm worried about the surface around since SRF sends beam in a 15degree circular angle instead of linear straight line.

More about Ultrasonics:

http://www.sensorsmag.com/sensors/acoustic-ultrasound/choosing-ultrasonic-sensor-proximity-or-distance-measurement-838

It would be great if it could detect in a straight line.

Answer 1

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:

1. 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.
2. Polish the walls of the pipe!