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help a noob out :)

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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help a noob out :)

Unread postAuthor: JDAX23 » Tue Jan 08, 2008 3:40 am

k... i have read a lot of the forums about pneumatics as well as a lot on wiki, and checked out dozens of user's cannons... i have made a couple of combustions in the past, but i feel it is time to step up my game, so i would like to make an over under pneumatic. I understand how the sprinkler valve works, as well as how to mod it, and the blow hose also... my question, which im sure ill be yelled at, is, going for power, does it matter what diameter the chamber/barrel is, or just as long as the chamber has about twice the volume of the barrel, it will have more power?...(any examples would be appreciated)

thanks in advance for any help you guys are willing to give,
and im sorry if i offend anyone for me being new to this, but i did reserch a lot

o and one other thing... does it matter where you place the sprinkler valve, by this i mean on the over part or the under part, or between the elbows, for better performance?

once again thanks in advance to those who help out
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Unread postAuthor: MrCrowley » Tue Jan 08, 2008 3:53 am

Well for a fixed barrel length, a bigger chamber will always increase performance until it's not really noticeable.

More or less just try and have as long of a barrel as possible that suits your needs, like portability, practicality etc.

If you have any desings in mind, just post the specs or a diagram here and we will help you out with a good C:B ratio, which isn't as important in pneumatics, as combustions.

Yes it can matter, if you put it on the bottom you will have some 'dead space' which is empty space between projectile and valve, this space will have to be filled by air, which will give the air more room to expand, lowering the pressure and overall performance.

Same if you put the valve on the elbows.

Best place is as close to the projectile as possible, e.g on the 'over part'.

Though some guns have shown to have a performance increase in dead space, but nothing you need to worry about at this stage.

Thanks for searching first btw. :)
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 08, 2008 4:28 am

One helpful point is that for a given quantity of air, less volume and higher pressure is a plus.

For example, a 10 inch long chamber at 100 psi will give significantly better power than a 20 inch chamber at 50 psi. The bigger the chamber, the more power, that's a general rule, but bigger chamber also means harder to pressurise and more noise.

As MrCrowley suggested, find the longest barrel that's practical enough for you, and what pressures you can work with, and we can give you some reasonable advice. Remember, shorter blowgun hose = quicker opening valve, the best thing is to tap it directly to the the valve.
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Unread postAuthor: JDAX23 » Tue Jan 08, 2008 10:47 am

thanks for the help... that makes sense what you have said about the chamber and barrel sizes, so say i want my barrel to be about five feet, and say either 1 1/2 inches or 2 inches in diameter, about what size chamber should i use

and ya jsr i have see you make that comment about the blow gun hose before lol so ya i will prolly have a really short tube or just mount it directly...

ummm so with the barrel at 1.5 or 2 inches, would a 1 inch sprinkler valve be an alright size?, and then when reducing or raising the sizes between the pvc parts, should i go for a more gradual decrease for maximum air flow? and ill work on a diagram i havent quite figure that out.

thanks so far guys!!
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Unread postAuthor: Hubb » Tue Jan 08, 2008 11:29 am

A 1" sprinkler will be fine. As far as gradually reducing the fittings to make for a better airflow, it is better to be safe than sorry. By that, I mean I've never seen a pressure rated fitting that gradually reduces. They are usually kept for DWV (and that's a negative). Honestly, I don't think you'll notice a difference either way.

On to chamber size. A C:B isn't really of that much concern in a pneumatic, but a good starting point would be around 1:1.

As far as your diagram, PVC Arsenal posted a helper for that a while back but his link is broken. I took the liberty of uploading the file into RapidShare (as it was too big to make an attachment here).

PVC_Designer.exe

(This link may be broken as well, as RapidShare only holds a file for 90 days. If it is, let me know and I will upload it again).
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Unread postAuthor: JDAX23 » Tue Jan 08, 2008 12:00 pm

hey thanks the link worked fine so ill get on that...so it doesnt matter if say the barrel is 1.5 inches as well as the chamber? and if thats the case would they have to be the same length?... however im thinking of having the barrel an inch and a half at around 4 to 5 feet in length, and i dunno about the chamber maybe 2.5 to 3 inches by maybe like 3 or 3.5 feet?

o and if i want to be able to interchange different siae barrels is it safe by somehow getting the ones that just screw in?
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Unread postAuthor: Hubb » Tue Jan 08, 2008 12:14 pm

That should work nicely.

As far as interchangeable barrels, it simply a matter of screwing them on and off. Most, would glue an adapter (or bushing and adapter) to the barrel to screw into the valve.

Oh, and don't worry about teflon on the barrel threads, unless you absolutely want to.

As far as the program, it will not do scaled drawings. It more of a sketching program to give a general idea of what your thoughts look like.
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Unread postAuthor: JDAX23 » Tue Jan 08, 2008 12:19 pm

alright well thank you very much hubb i think i may run to home depot and buy the sprinkler valve, pieces and try to find a blow gun... but just to check again, the diameter/lenght of the chamber is not as important correct, like will i get that much more power out of a 3 inch diameter vs a 2 or 2.5 inch?
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Unread postAuthor: Hubb » Tue Jan 08, 2008 1:03 pm

A post in this by JSR (third post down) may clear up that question for you.
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Unread postAuthor: JDAX23 » Tue Jan 08, 2008 1:13 pm

hmmm so based on those it seems to be that the larger barrel needs less psi to shoot it as fast if i read that correctly cause and 50 psi the 20 inch long shot it close to the next one that helps though thanks
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Unread postAuthor: grumpy » Tue Jan 08, 2008 1:25 pm

hubb017 wrote:A post in this by JSR (third post down) may clear up that question for you.


hubb, the only problem i have with those figures, is that pressures over 140 psi are not safe or practical for pvc. those figures are ok for metal chambers but not pvc.
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Unread postAuthor: JDAX23 » Tue Jan 08, 2008 1:28 pm

ya thats what i was wondering... cause for pvc i dont think i would go over 100 psi, just cause im a pansy :( lol
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 08, 2008 1:35 pm

pansy!

:D

hmmm so based on those it seems to be that the larger barrel needs less psi to shoot it as fast if i read that correctly cause and 50 psi the 20 inch long shot it close to the next one that helps though thanks


Those figures show chamber volume and pressure - so larger chamber at less pressure is worse than smaller chamber at more pressure, even if physically speaking it's the same quantity of air

The point of that exercise was to show the importance of high pressure, to drive the point home that you should use the maximum pressure you can. 100 psi will still give you decent power, just make sure your chamber volume is at least roughly equal to your barrel volume.
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Unread postAuthor: JDAX23 » Tue Jan 08, 2008 1:46 pm

ya im man enough to admit it lol... plus this is my first pneumatic so i dont know if i wanna try anything over 100 psi for a little bit... but i think what im gonna do is maybe have interchangable barrles so i can mess around with those but im thinking mainly 1.5 inches or 2 at 4 or 5 feet and then the chamber will be like 3 inches by like a foot and a half or two
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 08, 2008 1:50 pm

For a 2 inch barrel and 3 inch chamber, the barrel should be roughly twice as long as the chamber, so your dimenstions are about right.
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