Slightly dangerous? Yes!

..but no one is going to make a spud gun with anything else, so stop preaching this. 99% of people don’t have the capabilities or equipment to work with other materials. The danger is negligible when these things are only used a handful of times.

Hell, it’s even remote controlled, which means that they won’t need to be standing next to it if it goes off.

you need a life^

@pinky- no, he’s right. When liquid cracks the pipe it just leaks, the liquid doesn’t expand. When air does it it explodes and you see catastrophic failure.

I have seen both failures occur first hand, and I fortunately still have both eyes. I now take extra precautions- like if I DO store compressed air in a PVC pipe, I keep that pressure vessel in something that will contain the shrapnel, like a wooden box with enough air holes to vent the gas but contain the plastic.

What would help more in a forum like this though is a link to instructions on how to build a metal pressure vessel with available materials. Not just the vessel, but a large output orifice (by large I mean 3/4″) as well. Most of your metal pressure vessels are limited by 1/4″ output somewhere.

As someone who has seen PVC fail under pressure and is very thankful that no one was around it when it blew I can assure you its not a good material for storing compressed air. I think if you go on some manufactures websites they will even say not to use it for compressed air. It can and will go boom and little pieces of plastic end up everywhere. (150 psi air compresser on it by the way)

That said, I replaced the PVC line that blew with pex which isn’t rated for 150 psi either. Heck, at least when that fails it will just split.

BRB while I build more PVC air cannons to piss off Hackerspacer.

My life, if I’m willing to take the risk then you have no right to tell me not to. It won’t hurt you.

Quit your whining.

That’s a pretty neat little Maude killer (Simpsons referencde) and as for PVC storing compressed air and shrapnel, what about wraping in in carbon fiber or its cousin, duct tape.

According to the data on this site, its pressure limit is dependent upon the pipe’s length, as well: http://www.harvel.com/tech-specs-pvc-pipe-40.asp

It’s probably safe to assume that the danger level increases in some proportion to pipe length, wall thickness, damage (if present), and pressure level.

That said, the Schedule 30 stuff works freaking GREAT as air inlet tubing! And as a vacuum reservoir when you use a pipe with caps on both ends.

Also, if you say that inexperience isn’t a reason, and that people should read up before attempting a build, that’s all the more reason to have a warning with every build-log or instruction.

this might make a lot of people here cringe , but a good pressure vessel for compressed gas is 2l coke bottles

@Jerry_J: Safety may not have been a factor in the contest, although I agree, it should not be overlooked. I don’t see a presence sensor on the business end either. At 160psi, I’m betting the muzzle velocity of a tshirt could easily put out an eye. I think that if you’re going to enforce safety, you’ve really got to lay it out in the requirements.

@password: It’s better, but still not perfect. When a coke bottle explodes (at about 150psi) it doesn’t toss much shrapnel, but if you wrap a tshirt around it it’ll shred it pretty nicely. And anybody nearby will lose their hearing temporarily.

@Otacon2k Exactly what I mean.

Regarding the mention of soda bottles as air reserviors:

Most bottles that hold carbonated drinks are usually good to about 120 PSI. The people at aircommandrockets.com have used fiberglass and epoxy resin to reinforce 2L coke bottles and they’ve used them in rockets up to 250 PSI. If you were planning to use plastic bottles as an air tank, maybe fiberglassing them would make them much safer for use at 100 PSI. A sort-of hybrid air tank. It would also be much lighter than any other tank of the same volume. Just be careful with it.

@BacklitVillian

You would need sufficient vent area to allow the expanding gas to escape while maintaining the structural integrity of covering pipe, which might be hard to do.

Maybe there are other protective coverings that can be used? Chainmail? Kevlar?

I’d think a blow-off valve would be better than a containment vessel (think of water heaters and turbochargers), but those can sometimes fail as well. Another type of container would be the best choice (obviously), but this is merely prototype work in its bare naked form, IMO. I’m sure that many HaD’ers have made dangerous prototypes over the years, myself included.

Grabbing an idea off of the Internet and running with it, without a fundamental understanding of what’s really going on, is plain stupid, let alone dangerous, and I think that’s what most of us are saying… ???

Who cares about safety on something like this? The only reason they used an arduino was so that they could get out of the blast radius…if it was shoulder fired like that fireworks RPG thing a few days back it would be a different story…but no one can get injured if no one is nearby!

Aside from all of the comments, nobody else noticed they’re running 160psi through a 150psi valve? Bueller?

@Andrew: You can be stupid all you want, but if the chamber-turned-potential-bomb erupts with anyone else nearby.. then you’re involving others. Shrapnel from a small vessel like this will fly a good 100′ in all directions. The soundwave will temporarily deafen anyone within half that range.

@Hacksaw: Pretty sure he was referencing steel vessels which can be had for cheap $20-$40 locally or online, which usually only come with 1/8″ or 1/4″NPT fittings.

Pressure-rating aside you still have to calculate the derating figure. Most PVC has a specific maximum operating pressure for a set temperature (usually 72*F). Anything above or below this will change the properties of the pipe, and you have to use a derating calculation.

If the ambient air temperature, or the PVC temperature, hops to 90*F, a general derating figure of .75 for SCH40 PVC should be used. This immediately drops 160PSI rated 2″ pipe down to just 120PSI. With a fast-inflation (recharge) of the PVC chamber, the air temperature easily shoots over 100psi instantly and this can be a dangerous situation. Moreso with warm air.

If it’s cooler the PVC becomes brittle, so the slightest jar or sharp strike at just the right time can lead to failure.

Something else to remember are the fittings. Threaded fittings are more likely to introduce failure, than properly-glued fittings.

They’re pushing the PVC to the limit, and they don’t have to. They could spend $3 and get pressure gauges also, which would be wise, and another $5 for a safety-pop valve.

Also, they could’ve easily gone to 4″ PVC, double the chamber volume, drop the operating pressure to about 60psi (or even less), run a 1″ Rainbird valve and ended up with an even more efficient launcher, using less air, far below the de-rated PSI limit, which is far safer.

One of my frequent toys uses a ~200ci steel chamber with a 3/4″NPT outlet, 1″ valve, 28×2″ barrel, operates at no more than 40psi and can launch 30g projectiles more than a football field at ~215fps. And that’s a pretty inefficient setup (the 3/4″NPT is the chokehold).

Do the research folks. Hackerspace is right on target with this.