There is no compressed gas used for this. It is strictly atmospheric pressure and a vacuum. With both ends of the pipe sealed with tape, a vacuum is pulled on the pipe. When the tape on one end is cut, atmospheric pressure pushes air into the pipe. Since the pipe was in a vacuum there is little to no air resistance to hinder the acceleration of the ping pong ball. As the air rushes in, some of it bypasses the ping pong ball and gets between the ball and the tape on the exit side of the barrel. With the ball moving up the barrel, this creates a piston in a cylinder effect and compresses the air in front of the ball, which blows the tape out and allows the ping pong ball to exit the barrel. I don't have any means of measuring the velocity of the ball, but I did read/watch a video posted on h-talk and the physics instructor (Purdue) estimated a muzzel velocity of nearly 600 feet per second. We have not fired for distance yet. That will be the next thing we try. I'm guessing that due to the non-aerodynamic design of the ball and it's lack of weight, it will not go very far.
Wisdom has been chasing you, but you have always been faster.
In your darkest hour, when the demons come, call on me brother and we shall fight them together.
A Veteran is a person, who at some point in their life, wrote a blank check payable to the United States of America for payment up to and including their life.
Gene Castagnetti-Director of the National Memorial Cemetery of the Pacific in Hawaii
Reply
Originally Posted by doc havoc
It`s better to be silent and thought the fool; than speak and remove all doubt. 
! -George Carlin
It was part lesson (vacuum and atmospheric pressure), part stress reliever and part fun. We didn't waste any class time on this. It was assembled and tested during breaks and outside of class. This cannon shoots subsonic. They (the students) want to build a supersonic cannon next. I don't think that will happen on my watch. I see way too many things going wrong.
