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how to determine the best chamber?

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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how to determine the best chamber?

Unread postAuthor: jsefcik » Fri Aug 07, 2015 1:58 pm

Im onto my next build using a 2.5 inch pneumatic relief valve( qev on steriods) now my recent one was a 1.5 inch valve i used 2 inch pvc as a chamber,,for this bigger valve should i use 3 inch? Or whats the best route? Thanks
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Re: how to determine the best chamber?

Unread postAuthor: matti » Fri Aug 07, 2015 3:27 pm

If your valve is 2.5" then you should use at least same size pipe for chamber :roll: I don't think using bigger pipe for chamber does much if anything to power.. it's more about how big chamber you want and how long it will be.
And again.. GGDT helps you with selecting chamber volume.
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Re: how to determine the best chamber?

Unread postAuthor: jsefcik » Fri Aug 07, 2015 7:24 pm

2.5 inch pvc is what im probably going to use 2.5 inch is 60 bucks for all the fittings,, 3 inch is like 30-35 bucks alot cheaper, ill prob use 5 feet of barrel and chamber,,,would you mind plugging those into ggdt for me?
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Re: how to determine the best chamber?

Unread postAuthor: mark.f » Sat Aug 08, 2015 8:05 am

Screenshot-GGDT v4.6.png
Screenshot-GGDT v4.6.png (21.71 KiB) Viewed 472 times

It doesn't work. :(

Seriously though, can you be a little more specific?
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Re: how to determine the best chamber?

Unread postAuthor: jsefcik » Sat Aug 08, 2015 8:44 am

mark.f wrote:
Screenshot-GGDT v4.6.png

It doesn't work. :(

Seriously though, can you be a little more specific?


Does the shape of the chamber make any difference? Im wondering ? Explain what you need more of,im using 3 inch pvc for the chamber defintly 5 feet , barrel i was told double my chamber which woulf be 10 im not using that long of a barrel thats insane in my eyes
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Re: how to determine the best chamber?

Unread postAuthor: matti » Sat Aug 08, 2015 9:05 am

I think best way for you is just to make it, put a little bit longer barrel then your chamber is and thats fine. If you want, you can simulate your cannon power using the mentioned GGDT but you have to learn to do it yourself, because we cant know what pressure your using or what you are shooting etc.

We will help you with the GGDT if you have questions and the program is free and very simple to use.
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Re: how to determine the best chamber?

Unread postAuthor: Ragnarok » Sat Aug 08, 2015 9:23 am

jsefcik wrote:Does the shape of the chamber make any difference?

Yes, but it's trivial in most cases, and not simple to calculate (GGDT doesn't even try to model it, and HGDT only concerns itself with the difference it makes to igniting the mix).

Explain what you need more of,im using 3 inch pvc for the chamber defintly 5 feet , barrel i was told double my chamber which woulf be 10 im not using that long of a barrel thats insane in my eyes

That's a matter of C:B ratio, where the point at which the diminishing returns become less than worthwhile is a matter of personal preference.

You can model said diminishing returns, but it depends on valve flow, valve speed, barrel diameter, intended projectile, chamber pressure and a couple of other things. (Like whether you're fixing your chamber volume or barrel volume, although you have already said that)

I dropped this old diagram of mine into another topic recently:
Image
This assumes a fixed barrel volume, and simulates the results of relative chamber sizes (x-axis) against relative muzzle energy (y-axis) with three different combinations of valve and projectile (blue, red and green lines).

As you can see, the difference means that these systems diminish at very different rates when the C:B ratio is changed. The blue line is still significantly improving at C:B ratios of above 3:1, but the green line is pretty much maxed out by 1:1.

My best advice on the matter is to decide how long you're prepared for the barrel to be, then decide how big the chamber should be. For my sort of launcher, that would generally be in the same region as the barrel volume, give or take a factor of about two.

However, the only way to figure out for certain what C:B ratio is worthwhile to you is, as matti says, learning GGDT and experimenting yourself.
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Re: how to determine the best chamber?

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