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Constant Pressure with Innertubes

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Constant Pressure with Innertubes

Unread postAuthor: Atlantis » Mon Jul 28, 2008 9:58 pm

The basic idea I have is to have an innertube as my chamber housed inside a PVC tank, I'm wondering how much the elasticity of the innertube would keep the pressure near. I basically need constant pressure for multiple shots. It's either that or adding some elastic to a bike pump and letting that stay at constant pressure, I know that method works. I'm hoping for something around 30 to 50 PSI, prolly for Nerf darts or airsoft purposes.
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Unread postAuthor: FishBoy » Mon Jul 28, 2008 10:03 pm

what kind of valve would you be using?
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Unread postAuthor: Atlantis » Mon Jul 28, 2008 10:07 pm

I'm using a sprinkler valve, I plan on having a main chamber before the valve that gets refilled by the innertube chamber with a very small inlet between the two chambers to keep the valve from dumping the innertube chamber. I left some stuff out sorry.
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Unread postAuthor: FishBoy » Mon Jul 28, 2008 10:20 pm

ok, i think i get it, the innertube acts as a compact on-board air tank which fills a small chamber which is the discharged into the barrel?

i was actually thinking about making a nerf gun for multiple shots that would go like this-
semi-large pvc air tank-> ball valve-> very small chamber-> modded sprinkler valve-> barrel

my question is, what is the advantage to putting the bike innertube inside the air tank instead of having a plain pvc one?
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Unread postAuthor: Atlantis » Mon Jul 28, 2008 10:23 pm

Well I figured the innertube would compress the air in the chamber as shrinking the volume therefor keeping the pressure the same, like a balloon just with a PVC cage to keep it from popping.
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Unread postAuthor: FishBoy » Mon Jul 28, 2008 10:29 pm

so once the air tank got low, you would have more pressure than if you had the same amount of air in a solid pvc tank, thus making it more efficient?
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Unread postAuthor: iknowmy3tables » Mon Jul 28, 2008 10:34 pm

inner tube, how about slingshot tubing
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Unread postAuthor: Atlantis » Mon Jul 28, 2008 10:44 pm

Basically. Any elastic tubing should work, I just might have access to some free bike innertubes soon from a scrapped bike. I'll prolly use a bunjee corded bike pump so I can basically cock the gun once for multiple shots.
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Unread postAuthor: Carlman » Tue Jul 29, 2008 2:21 am

this is how the nerf magstrike works ( i bet you allredy knew that though)

it has a 'bladder' which keeps pressure constant.
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Unread postAuthor: ALIHISGREAT » Tue Jul 29, 2008 3:18 am

have you tested the elasticity and burst pressure of an inner tube yet? your idea definitly has potential to work though.
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Unread postAuthor: Ragnarok » Tue Jul 29, 2008 9:25 am

I have severe doubts this idea will actually work well at all.

For one thing, the inner tubes will probably expand out to reach the walls, and then pressure will build up as normal. You might be able to layer the tubes to limit that - but I've had to do that in the past, and it's a fookin' nightmare. Even then, there are limits to how many layers you can have, and I don't think it will be enough to prevent either the aforementioned effect at any real pressure.

Secondly, even if it does work, the pressure will not remain constant - and that can be proven.

It's interesting in theory, but I'd sooner rig something with a regulator in it.
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Unread postAuthor: iknowmy3tables » Tue Jul 29, 2008 1:32 pm

the pressure is obviously not going to be constant it's all about an extended life low pressure source
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Unread postAuthor: Leonard » Tue Jul 29, 2008 2:24 pm

Just use a regulator between your large pvc/metal chamber and your small chamber. No need to open ball valves or anything, the smaller chamber just get's filled back up automaticly from the large chamber through the regulator.

And a regulator is not that expensive. Simplier, more reliable and faster.
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Unread postAuthor: btrettel » Tue Jul 29, 2008 4:14 pm

Having used regular bike tubes (expensive) and latex tubing (cheap and available on McMaster-Carr) before in water guns and small Nerf guns I can't recommend it much for spud guns.

The flow would be limited to diameter of the tube. With bike tubes that's less of a problem but you still can't have very high flow.

The pressure is relatively limited. The pressures reported on McMaster-Carr are not an "operating pressure," rather, a peak pressure. The actual operating pressure is about 65% of the reported pressure from my experience with one tube. For the 35 PSI tube on McMaster-Carr this means your pressure is about 20 to 23 PSI. And the pressure decreases from the operating pressure about 25% as the tubing contracts.

The energy density is rather poor for spud guns. They work okay in water guns and I recommend them there but if you want a lot of air, you'd have to have a lot of tubing.

The tubing isn't very durable and will require replacement eventually. Unless you limit the expansion (which would limit your capacity and pressure) tiny tears in the tubing that accumulate with use will reduce the pressure.

It's too much of a hassle. Regulators make more sense in spud guns.
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