After sending out a few cannons, I decided to send out a “Tips” sheet in the cannon packaging to help people get started. I’m reproducing it here for those who didn’t get it:
Vacuum Cannon Tips
- The pipe suppliers are used to providing for large scale filtration systems. They care not about little nicks and scratches on the pipes. I sand off the more obvious burrs (so the pipe might look a bit battered) but they will have no effect on the performance of your cannon.
- Please don’t underestimate how loud it is close up; it makes your ears ring for some time afterwards if you fire it with no protection. The shooter DEFINITELY needs ear defence of some sort because they won’t have their hands free to cover their ears.
- Flanges (pictured) do make it easier to prepare and fire. There are many ways to obtain some. You can use cardboard, old mini-whiteboards, 3D printing… Thomas found that his tech department were very happy to be involved, knocked out laser cut flanges within a day and enjoyed the test firing process!
- The tapered holes for the valve make a very good seal, but if in doubt PTFE plumbing tape will make the seal even better. Thomas has not needed it apart from the early prototype.
Please warn your colleagues! You don’t want an evacuation because of a suspected shooter on the premises. This has happened. “Duds” are possible, but can be mostly avoided with the following approach:
- Don’t forget to roll the ping-pong ball all the way down the tube. This is easier before you add the tape because of the ball’s tight fit.
- It is worth taking care with the taping of each end. The rear just needs to be secure. The front needs to be secure but not too secure! Take care on the front, cut a square piece of tape and carefully fit it to the front of the tube. Try to fold back and stick as little as possible. You want the least folding and sticking that will support the vacuum. Trial and error will help you work out the optimum. We believe the tape blows off, so you want that to happen before the ball arrives.
(If you have flanges, this step is much easier, just cut a square and make sure it is securely pressed all round the circle of the tube and gently stuck to the flange.)
- Let the pump run for a while. You know very quickly if you have a bad seal because you will hear cracks from the tape and then the frustrating hiss of air being sucked in. If it is working nicely it gets to a pretty good vacuum very quickly but if you wait 10 seconds or so it continues to increase.
- Pierce the rear tape. Scissors are fine. It fails catastrophically and very very loudly. It is worth checking there is no tape hiding in the tube before loading the next ball.
- Please share your experiences on twitter @physicstp or Facebook.