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C:B

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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Unread postAuthor: NoXi » Tue Jun 19, 2007 4:06 pm

Thanks turbo, but the thing is, im afraid the pvc will explode so i'll probably stick with copper the whole way.

Well, since this is going to be my first cannon, i'm going to make it simple. I'm going to use a schrader valve and probably a ball valve. Is there also an alternative to a ball valve that can hold a lot of pressure? If i do use a sprinkler valve, i don't know which one i can use or not. Also, do i need a blowgun for it? I will be going to Home Depot later today. I would like to know what brand and type of sprinkler valve i should buy.

Thanks
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Unread postAuthor: Jumpin Jehosaphat » Tue Jun 19, 2007 4:23 pm

You only need a blowgun if you want to pneumatically modify your sprinkler vlave, this makes it work better and without electricity.

I used an inline WaterMaster sprinkler valve on my gun. The main thing you should be concerned with is not to get one with a guide rod down the center of it because they are harder to mod.

If you are going to mod the valve, also pick up a 1/4" NPT thread tap so you can drill the correct threading into the valve, and some Teflon tape to secure everything.

Also, what do you mean by a lot of pressure? Most sprinkler valves are rated somewhere around 120 psi.
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Unread postAuthor: NoXi » Tue Jun 19, 2007 5:05 pm

I was talking about the ball valve. If i was going to use a ball valve, i was wondering how much pressure it can take
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Unread postAuthor: Jumpin Jehosaphat » Tue Jun 19, 2007 5:39 pm

If you use a copper ball valve, it probably has a high pressure rating. Sorry, but I'm not sure about pressure ratings.
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Unread postAuthor: NoXi » Tue Jun 19, 2007 6:56 pm

o ok, thanks. Since this is going to be my first pneu, i think ill jus use a ball valve. Then later, i use sprinkler. Thanks. Also, for the gauge, will any pressure gauge work? Thanks

edit:: Here is what i want to make my cannon look like.
Image
Also, was wondering how long the chamber should be when i have a 26" 1/2" barrel. Itd be nice if you can tell me what the lengths should be. Thanks
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Unread postAuthor: pvcmaster » Tue Jun 19, 2007 7:10 pm

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


I dont know if this is still helpful, but i found a great site for pipe sizes:
http://www.gizmology.net/pipe.htm
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Unread postAuthor: NoXi » Tue Jun 19, 2007 7:31 pm

cool, Thanks pvcmaster
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Unread postAuthor: Jumpin Jehosaphat » Tue Jun 19, 2007 8:22 pm

Any gauge should work as long as all the threading fits. Make sure you get one that can work at the pressures you plan on firing at.

I can help with the chamber length when I have more time.
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Unread postAuthor: lukemc » Tue Jun 19, 2007 9:17 pm

well sence its related i was wondering if someone has figured out how much air expands at what pressure. like say one cubic inch of air expands to 2 cubic inches at 50 psi so for a 50 psi gun optimal c:b would be .5:1. if someone knows this please tell because pneumatic C:B ratios are realy worthless without knowing whta pressures are being used
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Unread postAuthor: TurboSuper » Tue Jun 19, 2007 9:50 pm

Argh, don't use a ball valve. Sprinkler valves all the way.
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Unread postAuthor: NoXi » Tue Jun 19, 2007 10:23 pm

lol i would use a sprinkler valve but it seems a little to confusing. If someone can make a more descriptive tutorial on how to use it, i would use one.
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Unread postAuthor: noname » Tue Jun 19, 2007 10:33 pm

Just screw it in and hook up a couple batteries to the solenoid for a basic valve. If there's any problems, just consult the troubleshooting guide.
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Unread postAuthor: NoXi » Tue Jun 19, 2007 10:46 pm

Ok, thanks. Ill think about using one. Can someone help me show how long the pipes should be when the barrel is 26 inches? Thanks
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Unread postAuthor: Jumpin Jehosaphat » Wed Jun 20, 2007 2:27 pm

NoXi wrote:Ok, thanks. Ill think about using one. Can someone help me show how long the pipes should be when the barrel is 26 inches? Thanks

Remember this for future use.
It's easy to figure out what lengths of pipe you should use to get the C:B ratio you want (Unless you are trying to be super exact, then it is hard)
Pipe measurements give you the diameter of your pipe, you need the radius, so divide it by 2. (radius of 1/2" pipe is 1/4") Once you have that plug your numbers into the volume formula.
V= pi X r² X L
The approximate volume of 26" or 1/2" pipe can now be found.
V= 3.14 X (.25)² X 26
V= 5.1 cubic inches
You know the highest you want your C:B to be is 4:1 which means your chamber is 4 times the size of your barrel.
So, the largest chamber without being overkill is going to be 4 times the volume of your barrel
5.1 X 4
20.4 cubic inches.
Now work the volume formula backwards for pipe with diameter 3/4" (radius is .375")
V= pi X r² X L
20.4 = 3.14 X (.375)² X L
20.4 = .441786.... X L
L= 46.22
This means the maximum amount of chamber pipe you want you use is about 46" and a C:B ratio of 4:1, if you want to figure out how much to use for a different C:B ratio, just do the same thing I did.

lukemc wrote:well sence its related i was wondering if someone has figured out how much air expands at what pressure. like say one cubic inch of air expands to 2 cubic inches at 50 psi so for a 50 psi gun optimal c:b would be .5:1. if someone knows this please tell because pneumatic C:B ratios are realy worthless without knowing whta pressures are being used

C:B ratio is a good estimator for a guns performance as far as I understand.
If you are some sort of efficiency nut you could use of of the gas laws.
P1 X V1= P2 X V2
To use this formula you must use absolute pressures, the pressure of our atmosphere in psi is 14.7psi, you use this value for P2. The value 50psi, is gauge pressure, to make it absolute pressure you must convert it to absolute pressure by adding the pressure of the atmosphere, which makes it 64.7psi.
64.7 psi X 1cubic inch = 14.7psi X V2
V2= 4.4cubic inch
1 cubic inch of air at 50 psi expands to 4.4 cubic inches of air.
I'm not entirely sure how that is helpful, but there it is if you have any use for it.
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Unread postAuthor: joannaardway » Wed Jun 20, 2007 3:30 pm

If you've still got pressure questions, common UK semi-rigid copper pipe at 26.3mm bore (1.03") has a working pressure of 582 psi.

I'd guess that US pipe would be vaguely similar.
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