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C:B ratio question

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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C:B ratio question

Unread postAuthor: freddy1919 » Thu Jun 19, 2008 9:53 pm

ok so im fairly new at this, just built my first combustion cannon last week, now i want to build a pneumatic one. All im looking for is best lengths for a 3" chamber and a 2" barrel, using a 1" inline sprinkler valve. I've searched through the forums and I've tried the different HHGT's (or whatever it is) but i keep getting errors. I just want a simple answer from people with experience. I plan on shooting potatoes, if that helps. thanks
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Unread postAuthor: starman » Thu Jun 19, 2008 10:09 pm

Take a good look through the pneumatic cannon showcase. Sounds like you've got an idea of what you want...take some time out and do some research on it...there are literally dozens of them in there that are similar to what you described.

The best approach is to get a design going and ask questions when you need help.
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Unread postAuthor: freddy1919 » Thu Jun 19, 2008 10:19 pm

i have looked through. I've seen things like .8:1 or something like 1.25:1 but some people agree and others don't. I've tried interpreting those numbers as lengths, and as cubic inches, but when I try and plug it into the HHGT (again I forget the abbrev.) and change the numbers that I do understand, it still comes out with errors. i have been looking and maybe I just don't know enough overall, so i just want to see what others have done
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Unread postAuthor: TurboSuper » Thu Jun 19, 2008 10:19 pm

Mmm...1:1 is a good ratio for a pneumatic.

Increasing the chamber volume for a fixed barrel will always yield a performance increase. However, this gain is not linear, after a certain point, a large chamber will be little more than a waste of space. Usually, going past 2:1 is silly.

Some people say .8:1 or .2:1 because it's more efficient in terms of power for size, but compressed air isn't really a rare resource...

Also, a 1" sprinker valve may choke the flow to a 2" barrel a bit. Not much you can do about that without going the piston route, unfortunately.
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Unread postAuthor: starman » Thu Jun 19, 2008 10:55 pm

freddy1919 wrote:i have looked through. I've seen things like .8:1 or something like 1.25:1 but some people agree and others don't. I've tried interpreting those numbers as lengths, and as cubic inches, but when I try and plug it into the HHGT (again I forget the abbrev.) and change the numbers that I do understand, it still comes out with errors. i have been looking and maybe I just don't know enough overall, so i just want to see what others have done


c:b ratios are based on volume. For a basic potato gun 1:1 is fine, some like 1.5:1. It really doesn't matter that much shooting spuds anyway. Also, you're going to want to keep the psi well under 100 as the power will tend to just vaporize the potato.

Don't worry too much about those modeling programs just yet. GGDT is probably what you are talking about. Just get this sprinkler valve gun working first.
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Last edited by starman on Thu Jun 19, 2008 10:59 pm, edited 2 times in total.
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Unread postAuthor: niglch » Thu Jun 19, 2008 10:57 pm

I would do more research on the wiki and the forums before you jump into pneumatics. Using a 1" sprinkler valve on a 2" barrel would restrict flow and probably wouldn't work too well unless you built a piston valve. Also, C:B ratios don't matter as much in pneumatics as they do on combustions. For a gun made out of PVC, 2:1 works fine in my opinion but pressure matters much more than volume. You should probably try building something simpler with a 1" barrel and maybe a 2" chamber. In my opinion it will be easier to build and it's power will impress the crap out of you even though it's not too big. This is especially true if you've only shot a simple aerosol-powered combustion before. Anyway, good luck.
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Unread postAuthor: Pilgrimman » Thu Jun 19, 2008 11:08 pm

I would buy a 2" ball valve if you are uncomfortable making a piston cannon. A sprinkler valve is simply too small to give good performance, unless you use multiple valves. A C:B ratio of 1:1 or 2:1 is probably sufficient for this application, as efficiency isn't a primary concern. I'm definitely looking forward to the result, though! Welcome to Spudfiles, the best forum ever!
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