C:B

NoXi · Mon Jun 18, 2007 11:48 am

Hi, i am going to be building my first pneumatic cannon out of copper. I was wondering it there is a certain C:B ratio. I have searched but found nothing on this topic.

Thanks

noname · Mon Jun 18, 2007 11:54 am

1.5:1 is a good ratio for a strong cannon, anything larger than 4:1 won't increase performance too much.

NoXi · Mon Jun 18, 2007 12:26 pm

Ok thanks for the fast reply. BTW, should i jus use the same calculator on burntlatke or is there one for pneumatics? Also, i was wondering if they sell 1/4" copper pipe

Jumpin Jehosaphat · Mon Jun 18, 2007 12:33 pm

There are links to a ton of great calculators at everybody's favorite wiki.
Here is a link
http://www.spudfiles.com/spud_wiki/inde ... plications

One that works great for me is GGDT
http://www.thehalls-in-bfe.com/GGDT/

They sell 1/4" copper pipe at many hardware stores.
Do you already know how to solder copper pipes?

NoXi · Mon Jun 18, 2007 12:52 pm

Jumpin Jehosaphat wrote:There are links to a ton of great calculators at everybody's favorite wiki.
Here is a link
http://www.spudfiles.com/spud_wiki/inde ... plications

One that works great for me is GGDT
http://www.thehalls-in-bfe.com/GGDT/

Fixed^^

They sell 1/4" copper pipe at many hardware stores.
Do you already know how to solder copper pipes?[/url]

Ok thanks alot, and yes i do know how to solder copper pipes.

Jumpin Jehosaphat · Mon Jun 18, 2007 1:03 pm

Whoops, I wasn't paying attention to how I used the URL tags. I fixed my previous post.

Good luck on your cannon, I look forward to seeing it.

NoXi · Mon Jun 18, 2007 1:24 pm

thanks, but that ggdt is really confusing! I was planning on having a 3/4" copper pipe for my chamber and 1/2" barrel but i dont know how long it should be cuz the calc is to confusing.

Jumpin Jehosaphat · Mon Jun 18, 2007 3:47 pm

It is a little confusing, especially the part about valves, but it's thorough.

For now, decide how long you want the barrel to be, then figuring out a good chamber size will be easy

NoXi · Mon Jun 18, 2007 5:07 pm

well, i was thinking of making the barrel around 18 inches.

A-98 · Mon Jun 18, 2007 5:15 pm

well, as noname said, anything above 4:1 isnt giving you much, but it will make it really loud (relative to a lower C:B) . ide also say that 1.5-2:1 is a good C:B

NoXi · Mon Jun 18, 2007 7:08 pm

o, ok thanks, the thing is, the ggdt is confusing, how am i supposed to know the OD of a copper pipe.

Jumpin Jehosaphat · Mon Jun 18, 2007 7:14 pm

Well an 18 inch barrel would have a volume of about 3.53 cu. in.
Since a 4:1 ratio is the highest you want to go for efficiency, the largest chamber you will need is going to be about 14.14 cu. in.
This means the longest you would want your chamber to be is 32 inches, which would just look dorky because your chamber would be longer than your barrel.

If your chamber were the same length as your barrel, its volume would be about 7.95 cu. in.
This volume would give you a 2.25:1 ratio which is plenty good.

I recommend using an 18 inch (or slightly less if you think it looks better) chamber. This will give you a powerful gun that is not super loud, unless you like super loud like me. Look at the gun in my avatar, it's C:B is 18:1, even with that long barrel, its so loud I use ear protection when I fire it.

NoXi · Tue Jun 19, 2007 1:08 pm

Lol nice cannon. I was wondering if you could connect pvc to copper. I have a 1" sch 40 pvc and it'll be cheaper for me to make one since copper is pretty expensive. IF not, how much pressure can the copper chamber take before it explodes? Thanks

TurboSuper · Tue Jun 19, 2007 2:05 pm

My cannon is a PVC chamber, followed by a sprinkler valve, followed by a copper barrel, with steel close nipples connecting them. Yes, as long as everything is NPT threaded (or whatever standard you're using), it will connect just fine.

Make sure to torque everything well with a pipe wrench.

Jumpin Jehosaphat · Tue Jun 19, 2007 2:19 pm

A copper chamber won't explode so much as tear apart which is one of its advantages. PVC will explode and shrapnel. I'm not sure how much your copper pipe can handle. I just checked a 1" sch 40 piece of PVC, and its rated to 450 PSI. The smaller the pipe you use, the higher the pressure rating will be. Copper pipe of the same diameter as PVC pipe can hold more pressure also. You said you wanted to use 3/4" copper pipe... I would look up a chart to double check, but you're probably safe even in the 500-600 psi range. Finding a valve that can operate in that range can be difficult, so never forget your valve is just as (if not more) explosion prone as your chamber. You should also keep in mind that standard air compressors rarely go over 150 psi.
Something you should decide on now is what kind of compressed gas you plan on using in your cannon.