[Mike Nawrot]

When it comes to air cannons, there's one general rule of thumb I keep in mind: PVC+Pressure= Explosion and blood stains on the metal shop floor. Just don't do it. If you want a pressure tank without spending too much money, sears offers a $20, 5 gallon craftsman tank, rated for 135 psi. That's the equivalent of 13.6 feet of 3" PVC. That's a lot of air. Our cannon, which is based on this tank, shoots t shirts a good 200 feet, and we have a relatively bad barrel to tank ratio (there's a lot of wasted air, but having a massive barrel is more trouble than it's worth. We're working on it). The operating pressure is limited to 100 psi by the kit's auto-shut-off switch, and just in case, the relief valve is at 120 psi. The only PVC component that holds any pressure is the sprinkler valve, which is rated for 150 psi, so there's a decent safety factor. It's all mounted on a robot, so it's remotely operated. I'd consider it safe, but there's no such thing as too safe when it comes to pressurized air. So general rules:

1. A safety factor of 1 generally isn't good enough. It's like having exactly 0" clearance between moving parts. It works on paper, but that's it.

2. Nobody stands in front of the barrel. Everybody should be behind the cannon, except for those in the crowd, but by then, air resistance should have slowed the projectile enough to prevent it from hurting anyone.

3. Test everything countless times before using the cannon in any crowded places.

4. Having a safety relief valve is a good idea

5. PVC DOES NOT SAFELY HOLD PRESSURE. Even if it says it's rated for pressure, that's at standard operating conditions. PVC will shatter easier in the cold, and even if it's at the rated pressure, any sort of unusual stress or strain will cause it to explode, leaving nasty cuts on anyone in the blast radius. From what I hear, stitches on your face aren't fun.

Overall, accidents can be prevented with the use of common sense, and by reading the labels. Have fun, be safe, avoid hospital trips, medical bills, and law suits.