pic: T Shirt Shooting Robot

[EricH]

Originally Posted by gallo26

A leak warning would be hearing the noise. I'm almost positive that you will hear it, and we have on past tanks. You hear it on robots with pneumatics or anything with leaks. Its the exact same thing if there's a leak in an inner tube or a track ball.

Wait... Let me get this straight. You have never heard a leak in PVC. You have heard it on pneumatics or "anything [air-filled] with leaks". (The edit is because you don't exactly hear a water leak in the same way--or a steam leak.)

Am I correct?

Because if I am, you probably don't know what an air leak in PVC sounds like. It'll likely sound similar, yes... But it won't be the same. You see, an FRC robot's pneumatic system has two types of leaks. One is from a punctured tube, the other is from a poorly sealed valve. A bike tire or trackball sounds about the same as a puncture tube; a poorly-sealed valve will be hard to hear due to the slow nature of the leak. And in fact, I'd rather you had that than a leak in the PVC itself.

What a pneumatic tube, a bike tire, and a trackball have in common is that they are "soft" plastics--thin-walled, flexible, and somewhat "bang-resistant"--that is, they won't make a big mess if they do burst. There are still FIRSTers who can tell you of the doublers in 2004, which, if mistreated, could go all at once on a slight hole. Being flexible, they can bend a bit out of the way. PVC pipe is not nearly as flexible, especially at the size you're using. And if you do develop a leak, you've probably got about 1 shot to realize you do before something happens. You might not hear it, you might not notice a pressure drop, but something's going to give and you hope it isn't catastrophic. That's why we're pushing you guys to switch to a safer material, such as metal--PVC isn't exactly "bang-resistant". (Metal isn't either, but it can sure take more pressure than PVC.)

[gallo26]

You can hear the leak of the escaping air, and if you submerge it under water you see the bubbles. Our PVC has leaked before, and you can hear the air escaping. and I know that PVC with running water you don't hear it, but you do see the water that it leaks. We had a rather bad gluing job done on an older version of our cannons and you can clearly hear the leak as well as see the pressure drop. So sorry, but i don't think your correct.... because i do know what it sounds like, and I have seen them in water

[Daniel_LaFleur]

Originally Posted by gallo26

You can hear the leak of the escaping air, and if you submerge it under water you see the bubbles. Our PVC has leaked before, and you can hear the air escaping. and I know that PVC with running water you don't hear it, but you do see the water that it leaks. We had a rather bad gluing job done on an older version of our cannons and you can clearly hear the leak as well as see the pressure drop. So sorry, but i don't think your correct.... because i do know what it sounds like, and I have seen them in water

*sigh* ... so much misinformation...


PVC leaks cannot be heard as they will cause the PVC to fail violently. The leaks that you describe are leaks at the bonds or joints, not in the PVC itself.

Lets consider PVCs pressure rating for a moment. PVC is only pressure rated for liquids (and for a very good reason):
Water is incompressable, much like a steel block ... if you push down on a steel block with 50 PSI it will not compress and if you release that force instantly the steel block will not move.
Air, on the other, hand is compressable, like a 50 pound spring. Apply 50 psi to it and the sprin compresses. release that force instantly and the spring not only goes back to it's original shape but rockets off of the surface it was on.

Now on to PVCs material properties:
PVC is rigid polymer. This allows it to flex while maintaining it's strength, however once it is flexed too far it will crack and the PVC pieces will then (violently) attempt to return to their original shape. PVC is extruded which means that it's wall thickness is not consistant. Also each time it is flexed it gets reduced in strength (each pressurization cycle flexes the PVC).

If the PVC pipe had pressurized liquid inside then when the PVC pipe fails all of the pressure within the pipe is released but does not violently explode because it is like the steel block and was not compressed.

If the PVC pipe had pressurized air inside then when the PVC pipe fails the air will violently rush out the failed spot which will continue to apply pressure to the now failing spot causing it to fail further. Pieces of PVC at this point can be ripped off and carried (or shot) by the escaping air.

In short, a failure of the PVC (not a bond or joint leak) will not be 'heard' until it violently fails. There will be little to no warning. The PVC is not rated for pressurized gas, and the shock loads that a cannon put on PVC are far beyond what it is designed to hold.

Now I'm not saying that yours WILL fail, but considering that you will expose the public to any failure you have, I would suggest that you take every precaution possible to ensure the publics safety. FIRST (and the FIRST teams) do not need the political ramifications from a publicity stunt gone bad.

P.S. @ Krunch dude. The chart you referenced is out of ASTM D1785 which specifically states that PVC is for liquids only and that those numbers do not reflect gas pressurization.Read Abstract here.

[robself705]

Here is our t-shirt cannon

http://www.youtube.com/watch?v=xKCwbomY82c