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Barrel/Chamber length?

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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Barrel/Chamber length?

Unread postAuthor: TheTrooper » Tue Nov 20, 2007 3:33 pm

If I have a pressure chamber 15 Inches long with 1.5" diameter PVC pipe how long will a 3/4" barrel have to be to have maximum performance? Couldn't find anything on search specific to this.
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Unread postAuthor: LikimysCrotchus5 » Tue Nov 20, 2007 3:42 pm

if i remember correctly for pneumatics, it is .2:1 ratio is the minimun for the best performance in pnuematics. I always forget for combustins but i think it is .8:1 or 1:1 but dont make my word for the combustion. Could someone clarify for me if i am (which i probably am) wrong.

But barrel length should be nothing to worry about. You could have double the barrel length to the chamber since your are using exactly 1/2 of the chamber volume for the barrel volume. I have a gun similar to what you are building and i get pretty good performance out of mine but i would say that i could make mine longer and get a little better performance. You can see the gun here.
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Last edited by LikimysCrotchus5 on Tue Nov 20, 2007 3:52 pm, edited 1 time in total.
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Unread postAuthor: TheTrooper » Tue Nov 20, 2007 3:46 pm

Oh i get it so make the barrel 2x the length of chamber. I think thats what you meant.
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Unread postAuthor: LikimysCrotchus5 » Tue Nov 20, 2007 3:50 pm

Yea pretty much because you could have 1:1 ratio and still be fine in performance.
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Unread postAuthor: psycix » Tue Nov 20, 2007 4:17 pm

You are wrong:
A double diameter pipe has MORE then double volume inside
pi*radius*radius*length=volume

so
pi*0.75*0.75*15= volume of chamber =26.5 cubic inches
pi*0.375*0.375*30= volume of your barrel = 13.25 cubic inches
this would give a 26.5/13.25= 2:1 ratio and NOT a 1:1

Edit: you should get the longest barrel you still think its mobile enough.
If you want to run around much with ur gun, you want a short barrel, if u want raw power but not being mobile, just get one helluva long barrel. 120" if you want.
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Unread postAuthor: ALIHISGREAT » Tue Nov 20, 2007 4:41 pm

ahh another factor is the valve, what valve are you using?

the valve will affect the length of barrel needed becasue a valve with a slow opening time (a ball valve) need a long barrel to get good performance but a fast opening valve (a sprinkler, qev or piston valve) will get better performance than a slow opening valve from a short barrel.
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Unread postAuthor: DYI » Tue Nov 20, 2007 5:29 pm

We can only answer this question properly if you give us the pressure you intend on using.

Assuming you are using 120 psi, you will need a 3/4" barrel approximately 110" long for maximum performance. At 240 psi, you could use a barrel about twice as long.

If you would simply download GGDT, you wouldn't need to ask these questions.
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Unread postAuthor: LikimysCrotchus5 » Tue Nov 20, 2007 5:35 pm

Psyicx wrote: A double diameter pipe has MORE then double volume
so
pi*0.75*0.75*15= volume of chamber =26.5 cubic inches
pi*0.375*0.375*30= volume of your barrel = 13.25 cubic inches
this would give a 26.5/13.25= 2:1 ratio


you are contradicting yourself. I am pretty sure that 2:1 is double. I SAID that 1:1 is something your aiming for in a C:B ratio. But, it isnt 2:1 because your calculations are incorrect. Example:

Chamber = 3.14 * (.75 * .75) * 10 = 17.6625
Barrel = 3.14 * (.375 * .375) * 10 = 4.415625

17.6625:4.415625 = 4:1 C:B ratio

That means that you can multiply the barrel length * 4 so that you get a 1:1 C:B ratio which is what i told him to aim for. So therefore, i am wrong in my first post, but you psicyx didnt correct me, you just added more false info. So you can make the barrel 4 times the length of your chamber to get a good C:B ratio so that it would equal 1:1 which is what i told him to aim for.

The formula for the volume of a cylinder (which pvc sections are cylinders) is ¶r2h. So based off this formula, you get a 4:1 CB ratio.

your error up in your post is that you doubled the length of the 3/4 inch barrel. And you want to use the same length of pvc so that your calculations are accurate.
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Last edited by LikimysCrotchus5 on Tue Nov 20, 2007 5:47 pm, edited 3 times in total.
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Unread postAuthor: DYI » Tue Nov 20, 2007 5:44 pm

In most cases, 1:1 or higher is a horrible waste of gas. The only reasons to make a cannon with a C:B that high are
1. Using pressures below 50 psi
2. You can't make the barrel any longer (my problem)
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Unread postAuthor: LikimysCrotchus5 » Tue Nov 20, 2007 5:45 pm

so then what would be a reasonalbe ratio that would be gas effieicent?
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Unread postAuthor: DYI » Tue Nov 20, 2007 5:54 pm

At 120 psi, for a given chamber volume, 0.2:1 will be pretty close.

You can find the answers to all these questions by using GGDT. Don't tell me you're a 1st Lieut. and still don't have it!
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Unread postAuthor: LikimysCrotchus5 » Tue Nov 20, 2007 6:02 pm

I am trying to download it now but its not working. I got both of the files in a directory but it still has the run time error. Can you help me?
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Unread postAuthor: DYI » Tue Nov 20, 2007 6:58 pm

Ask someone like Pete for help, he really knows what he's doing when it comes to comps.
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Unread postAuthor: TheTrooper » Wed Nov 21, 2007 2:29 am

I will be using, hopefully, 100 PSI with a standard sprinkler valve found on the tutorial. Please make this clearer because several people are saying each other are wrong. just tell me how long i should make this 3/4" barrel to 15 inches of 1.5" chamber. It needs to be mobile.
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Unread postAuthor: psycix » Wed Nov 21, 2007 9:45 am

LikimysCrotchus5 wrote:
Psyicx wrote: A double diameter pipe has MORE then double volume
so
pi*0.75*0.75*15= volume of chamber =26.5 cubic inches
pi*0.375*0.375*30= volume of your barrel = 13.25 cubic inches
this would give a 26.5/13.25= 2:1 ratio


you are contradicting yourself. I am pretty sure that 2:1 is double. I SAID that 1:1 is something your aiming for in a C:B ratio. But, it isnt 2:1 because your calculations are incorrect. Example:

Chamber = 3.14 * (.75 * .75) * 10 = 17.6625
Barrel = 3.14 * (.375 * .375) * 10 = 4.415625

17.6625:4.415625 = 4:1 C:B ratio

That means that you can multiply the barrel length * 4 so that you get a 1:1 C:B ratio which is what i told him to aim for. So therefore, i am wrong in my first post, but you psicyx didnt correct me, you just added more false info. So you can make the barrel 4 times the length of your chamber to get a good C:B ratio so that it would equal 1:1 which is what i told him to aim for.

The formula for the volume of a cylinder (which pvc sections are cylinders) is ¶r2h. So based off this formula, you get a 4:1 CB ratio.

your error up in your post is that you doubled the length of the 3/4 inch barrel. And you want to use the same length of pvc so that your calculations are accurate.


Look over my calculations: i didnt use the same length for the chamber as for the barrel but I used 15" chamber (the one he has) and the 30" barrel (wich you supposed to do the chamber length x2 as barrrel lenght and then said something about a 1:1 ratio.)

My calculations arent incorrect, i just used other numbers. :)


Edit:
Oh i get it so make the barrel 2x the length of chamber. I think thats what you meant.

Yea pretty much because you could have 1:1 ratio and still be fine in performance.

And i proved it was NOT a 1:1 ratio.

Edit 2:
Ive read over some of your posts again and i conclude we had a miscommunication.
You are thinking I wanted to calculate a 1:1 ratio and made a mistake, but I just wanted to prove that 2x chamber lentgh barrel is NOT a 1:1 ratio.
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