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Finding volume

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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Finding volume

Unread postAuthor: no-limit » Mon Jun 23, 2008 1:07 pm

I was told this is the formula for finding cylinder volume. pi*r*r*length but these calculations don't make sense.

1" by 60" = 47.12
2" by 30" = 94.24
4" by 15" = 188.49

Sems to me they would all be the same. What am I doing wrong? I'm am trying to figure the C:B ratio and by this my 1" by 60" barrel would only require a chamber of 2" by 30". I am using 2 sprinkler valves using 2 chambers at a 1:1 ratio for each chamber making it 2:1 for the total chamber volume.
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Unread postAuthor: Gippeto » Mon Jun 23, 2008 1:20 pm

Your not doing anything wrong.

When you double the diameter of a circle (pi*r*r) the area increase by a factor of 4.

Example:

1" circle area =3.14*.5*.5 =.785in2

2" circle area =3.14*1*1 = 3.14in2

3.14/.785 = 4

For a pneumatic, I wouldn't recommend any more than 1:1. (unless you just want the work out. :wink: )

And just to satisfy my curiosity, why 2 sprinkler valves on a 1" barrel? Are you planning on firing the chambers one at a time?
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Unread postAuthor: no-limit » Mon Jun 23, 2008 1:30 pm

Someone once said that having 2 sprinkler valves would give twice the speed since they only have to open up halfway to achive full volume.

Someone also once said "c:b ratio's are mostly important with combustions. with a pneumatic, more air and/or higher pressure equates to more power. The larger the chamber (if pressure is held constant) will give you more even force on the ammo as the compressed air expands down the barrel."
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Unread postAuthor: MrCrowley » Mon Jun 23, 2008 2:04 pm

no-limit wrote:Someone once said that having 2 sprinkler valves would give twice the speed since they only have to open up halfway to achive full volume.

Not true. I remember reading that and I also remember posting a lengthy reply saying why it's not so.

It'll improve performance but wont even come close to doubling.
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Unread postAuthor: Pilgrimman » Mon Jun 23, 2008 2:06 pm

You mean 2 valves in parallel right? It will not necessarily give double the speed, but it will help with a longer barrel as the flow is improved by having 2 valves. With a pneumatic, a C:B of 1:1 is more than enough to get as much power as possible without diminishing returns. .2:1 is (roughly) a perfect C:B ratio in terms of efficiency, but 1:1 should be good for power.
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Unread postAuthor: no-limit » Mon Jun 23, 2008 2:14 pm

Yes they are running parallel. I'm launching rockets and a 60" would qualify as a longer barrel I think. Most of my launchers have a bigger chamber volume (more than 2:1) because it needs to stand up. This one will be less than a 2:1 ratio because of the design using 2 valves.

Improved performace is good.
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Unread postAuthor: psycix » Mon Jun 23, 2008 2:34 pm

That is true on one hand, and also you've got double flow. The downsides are that you've also got yourself a double as large pilot volume, and that it might become rather costly, you might be better of building a simple piston valve.
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Unread postAuthor: no-limit » Mon Jun 23, 2008 4:33 pm

For me there is no such thing as a "simple piston valve". The inside of a pvc pipe is way to filled with bumps to get a good seal. After my 4th attempt I have stopped that nonsense.
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Unread postAuthor: iisthemuffin » Mon Jun 23, 2008 6:22 pm

Im sorry to steer away from the conversation but will someone please tell me the formula for finding volume of the pipe? I currectly have a simple sprinkler valve cannon with a 2 inch by 1 foot chamber and a 1/2 inch by 3 foot barrel. I want to know the formula to find the volume of my pipes to see if this is efficient.
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Unread postAuthor: DYI » Mon Jun 23, 2008 6:30 pm

Im sorry to steer away from the conversation but will someone please tell me the formula for finding volume of the pipe? I currectly have a simple sprinkler valve cannon with a 2 inch by 1 foot chamber and a 1/2 inch by 3 foot barrel. I want to know the formula to find the volume of my pipes to see if this is efficient.


The formula for volume of a cylinder is given in both the first and second posts of this thread. Would it really kill you to read the thread first?
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Unread postAuthor: iisthemuffin » Mon Jun 23, 2008 6:33 pm

I read though the entire thread. But i thought that both of those were given for just a circle. I wasnt aware it was for a cylinder.
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Unread postAuthor: DYI » Mon Jun 23, 2008 6:37 pm

Circle area is pi*r<sup>2</sup>
Cylinder volume is pi*r<sup>2</sup>*length
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