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Pnumatic C:B ratio guidence needed

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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Pnumatic C:B ratio guidence needed

Unread postAuthor: Lucasberg » Tue Aug 26, 2008 9:54 am

My uncle wanted me to make a spud gun for out desert tip this coming up weekend.

I had found some plans for a 2.5inch barrel and 4 inch chamber pneumatic gun. It said it would be able to shoot tenis balls and what ever else would fit in the barrel.

My problem is my wife threw away the plans!!! I don't remember the length for the barrel or the chamber. To be extra safe after reading horror stories on here I spent $100 for pressure rated 2.5 inch and 4 inch pvc at a wholesale which only sold 20ft pipes.

I have searched for about 7-8 days and tried the program on here which I could not get to work. After reading so many different C:B ratios for 1:1 to 9:1 & it doesn't really matter with pneumatics, I don't know what to do. The math problem I have seen for it is greek to me , I've been out of school 15 years.


If some one feels some kindness in the bottom of their heart to help me out here are my specs

I'm going for an over under gun that would be hand holdable, have enough power to shoot tennis balls at high velocity as well and large spuds or what ever else I can cram down it.

I guess if some one could tell me something like for every --inches of 2.5 barrel I would need -- inches of 4 inch chamber I could scale it up to how big I wanted it?


Any help would be great. I promised I have searched for days but I want to be super safe.
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Unread postAuthor: starman » Tue Aug 26, 2008 11:11 am

FYI, volume of a cylinder is radius * radius * pi * length. You can use that to find volumes of barrels and chambers.

To directly answer your question, 4" pipe has roughly 2.5 times the volume as 2.5" pipe. So for example, for a 1:1 c:b ratio, if you have a 12" chamber you would use 30" barrel.

Keep up your study on your pneumatic project. We aren't going to spoon feed you, but we'll help point the way for you.

Welcome to Spudfiles!
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Unread postAuthor: jonnyboy » Tue Aug 26, 2008 11:14 am

The only thing c:b ratio is about is the noise a pneumatic gun makes when fired.

If you don't want alot of noise you want a 1:1 ratio so all the air will be expended by the time it reaches the end of the barrel. A five foot 4 inch chamber with a ten foot 2.5 inch barrel is about one it should work fine.
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mobile chernobyl wrote:I can shoot a Canuter Valve off my '82 Chevy Ram F150 AT LEAST 3/4 Mile with 'ma cannon made of soup cans duct taped together, then I just squirt some bacardi 151 in the chamber and hold up my cigarrete lighta and WHOOSH! That thing flies at least 3/4 mile
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Unread postAuthor: jimmy101 » Tue Aug 26, 2008 11:28 am

I guess if some one could tell me something like for every --inches of 2.5 barrel I would need -- inches of 4 inch chamber I could scale it up to how big I wanted it?

OK, we'll go for a 2:1 CB ratio. The CB ratio is the volume of the chamber divided by the volume of the barrel. The chamber and barrel are cylinders and the volume of a cylinder is Pi*r<sup>2</sup>L where Pi=3.1415, r is the radius of the pipe (which is 1/2 the diameter of the pipe) and L is the length of the pipe.

To calculate inches chamber to inches barrel for a particular CB;
CB = (vol chamber)/(vol barrel) = (Pi*r<sup>2</sup><sub>chamber</sub>L<sub>chamber</sub>) / (Pi*r<sup>2</sup><sub>barrel</sub>L<sub>barrel</sub>)

Canceling and simplifying we get
CB = (D<sup>2</sup><sub>chamber</sub>L<sub>chamber</sub>) / (D<sup>2</sup><sub>barrel</sub>L<sub>barrel</sub>)
where D is the diameter of the barrel or chamber.

Rearranging a bit gives;
L<sub>barrel</sub>/L<sub>chamber</sub> = (D<sup>2</sup><sub>chamber</sub>) / (D<sup>2</sup><sub>barrel</sub>) / CB

You have pipe that is D<sub>barrel</sub>=2.5" and D<sub>chamber</sub>=4" and we are going for a CB of 2.
L<sub>barrel</sub>/L<sub>chamber</sub> = (4<sup>2</sup>) / (2.5<sup>2</sup>) / 2
L<sub>barrel</sub>/L<sub>chamber</sub> = 16/6.25/2
L<sub>barrel</sub>/L<sub>chamber</sub> = 1.28

rearrange to
L<sub>barrel</sub> = (1.28 )L<sub>chamber</sub>

So, for every 1" of chamber we want 1.28" of barrel to get to a CB of 2.

If you go back and plug in a CB of 1 you get 2.56" barrel per inch of chamber.



What kind of valve will you be using? If you use a ball valve you might want to use a longer barrel since ball valves open slowly.

It will be pretty much impossible to get spuds that'll fit tightly in a 2.5" barrel.

Have fun :D
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Unread postAuthor: Lucasberg » Tue Aug 26, 2008 11:29 am

Thanks Guys, That helped alot.

After I build a gun for this weekend I'm sure I will build many more because I will have at least 15-17 feet of PVC left. I will post pics when Im done.
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Unread postAuthor: Lucasberg » Tue Aug 26, 2008 11:33 am

I tried to get a 1 inch sprinkler to work on a small scale test but it keeps leaking about 2 psi a second out of the little hole the solenoid covers so I got a 1 inch ball valve.
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Last edited by Lucasberg on Tue Aug 26, 2008 11:36 am, edited 1 time in total.

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Unread postAuthor: jimmy101 » Tue Aug 26, 2008 11:36 am

jonnyboy wrote:The only thing c:b ratio is about is the noise a pneumatic gun makes when fired.

The CB does affect the amount of noise a gun makes but it also affects the gun's performance (measured as muzzle velocity). A CB=4 gun will outperform a CB=1 gun.

... you want a 1:1 ratio so all the air will be expended by the time it reaches the end of the barrel.

That aint right either. With a CB of 1, when the projectile reaches the end of the barrel there is still half the original pressure pushing it. That is hardly "expended". To fully "expend" the air in the chamber the CB ratio needs to be down around 0.1~0.2 (for an intially pressure of 120 PSIG and assuming a reasonable amount of friction).
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Unread postAuthor: jimmy101 » Tue Aug 26, 2008 11:47 am

Lucasberg wrote:I tried to get a 1 inch sprinkler to work on a small scale test but it keeps leaking about 2 psi a second out of the little hole the solenoid covers so I got a 1 inch ball valve.

The sprinkler valve should not leak. Think what would happen if you installed it in a sprinkler system!

There is an easy fix, a more difficult fix and a "sideways" fix:

Easy fix: Attach the valve to a garden hose (pay attention to the correct flow direction for the valve) and flush water through it and cycle it a few times with a 9V battery. It is possible that there is just crud inside the valve that is keeping either the diaphram or the solenoid port from sealing properly. Usually this process will fix a new valve that leaks. If it doesn't, you've plenty of reason for returning the valve for a replacement or your $$$ back since the valve is unuseable for its intended purpose.

More Difficult Fix: Take the valve apart and give it a good cleaning. This'll probably void the waranty.

"Sideways" Fix: Who cares if it leaks out the solenoid! Remove the solenoid and fill the hole with epoxy. Mod the valve to be activated by a blowgun. (search the spudwiki for valve modifications).

(My personal preference is to never permanantly fill in the solenoid hole since it can't be undone. If you mod the valve you can usually leave the solenoid in place and can tirgger the valve either way.)
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Unread postAuthor: Lucasberg » Tue Aug 26, 2008 12:36 pm

I took it apart about 5 times and even took out the spring and stretched it a little before I realized where it was leaking from. It bugs me that it leaks and I cant fix it.

Would that be better performance wise over a 1 inch ball valve if I modded it to work by blogun ?

Thanks for all your help Jimmy
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Unread postAuthor: jimmy101 » Tue Aug 26, 2008 1:53 pm

Lucasberg wrote:Would that be better performance wise over a 1 inch ball valve if I modded it to work by blogun ?

Yes. The ball valve is about the worst kind of valve there is, it opens too slowly to be very effective. The un-mod'd sprinker valve would be the next best. The mod'd sprinker valve will usually be the best of these three valve types.
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Unread postAuthor: Pilgrimman » Tue Aug 26, 2008 3:02 pm

I had the same leak problem with a sprinkler valve once. I fixed it by taking the screws out, putting extreme pressure on the area where the solenoid screws in, and replacing the screws, starting with the screws closest to the solenoid. So instead of tightening them like you would lugnuts on a car, you want to go with the screws closest to the solenoid first, then backwards, and finally the screws near the "in" port last. I hope that helps... BTW, can't wait to see some pics!
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