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Finding volume of a cylinder.

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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Finding volume of a cylinder.

Unread postAuthor: iisthemuffin » Wed Jul 16, 2008 12:33 pm

I know this is an over-asked question and i am sorry for that but i feel that im missing something. Ive got a sprinkler valve cannon and im not sure of how to find the volume of the pipes. Ive read a few different things and i tried a formula that DYI gave me. I trust that the formula is right seeing as how he is a very smart man. But i get weird numbers. Maybe my C:B is really all over the place or maybe im just doing it wrong.

So if someone could figure this out for me i would appreciate it.

Barrel:
Length - roughly 3 feet
Diameter - 1/2 inch

Chamber:
Length - roughly 1 foot
Diameter - 2 inches

If you are so kind as to figure this out for me, if my C:B is really bad please, if you dont mind, offer suggestions on how to improve this.

Once again, Im sorry for the repeated question.
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 16, 2008 12:40 pm

Just use the formula - but since I'm in the mood for spooning ( ;) MrCrowley)

Barrel volume approx 7 inches<sup>3</sup>

Chamber volume approx 37 inches<sup>3</sup>

More than enough volume for a powerful shot, but you can afford to shorten your chamber to 2 inches if you want. Remember, for a given amount of air, halving the volume but doubling the pressure will give more power.
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Unread postAuthor: biggsauce » Wed Jul 16, 2008 12:43 pm

Are you getting a large cb ratio?
Do you want a .8:1 cb ratio? (For a pneumatic, this isnt neccessary)

If you're looking for .8, try a barrel about 20ft long (rough number)

Try some of the calculators available on spudwiki if you want some verification of your numbers
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Unread postAuthor: iisthemuffin » Wed Jul 16, 2008 12:45 pm

Oh sorry about that. For some reason it never dawned on me that i could use google. Im sorry about that guys. But thank you.

JSR, Are you serious about 2 inches? Ive never heard of that before. And are you saying that what im at right now is powerful enough? Im not loosing performance am i?
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 16, 2008 1:18 pm

I don't have GGDT here, but if you're using 100 psi or so you can have afford to have a barrel volume around 4 times bigger than your chamber.

Of course, bigger chamber = more power, but also more effort to pump it up and more noise.
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Unread postAuthor: iisthemuffin » Wed Jul 16, 2008 1:58 pm

I usually take it to 120 psi. I could probably go a bit higher but would rather not risk ripping the diaprhagm on the sprinkler. I dont have a real compressor so i use a 12v comprssor. It does take about two minutes thought. I took it to a friends house and filled it with a real compressor. Took maybe 2 seconds.

But your saying if i cut the chamber down i will lose power because i have less volume? Im not too concerned about the pumping time so i wouldnt cut it down soley for that purpose.
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 16, 2008 2:08 pm

iisthemuffin wrote:But your saying if i cut the chamber down i will lose power because i have less volume? Im not too concerned about the pumping time so i wouldnt cut it down soley for that purpose.


While the pressure behind the projectile is greater than the friction on the projectile from barrel drag and air resistance, it's going to continue accelerating. This means that you can still have performance from a chamber much smaller than the barrel provided your pressure is reasonable.

However, more volume = more power. There is a point where no matter how much more you increase the volume, the corresponding gain in power will be insignificant. I can't punch any numbers here but offhand I'd say that if you doubled your chamber size, there wouldn't be a really noticeable difference in performance, unless you also increase barrel length. Keeping everything constant, increasing barrel length will give you more power while making it more efficient and less noisy.
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Unread postAuthor: iisthemuffin » Wed Jul 16, 2008 2:16 pm

Ok i follow what your saying.

Now im looking into buying a 3/4 inch barrel for it. Would this offer more or less power? I would think more. Is that right? I cant really tell you why i say more. Just kind of leaning in that direction. If you dont mind, please explain why it would give more or less power.
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Unread postAuthor: ramses » Wed Jul 16, 2008 2:27 pm

You would most likely get more power yet, because there will be 2.25x more force on the projectile. If you are buying a new barrel, i would suggest getting a longer one, like 5 feet.
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 16, 2008 2:29 pm

Applying the same amount of pressure to a larger projectile gives more power because there's more area for the air to push on. For example, with 10 psi on a projectile with a base area of 1 square inch, that's 10 pounds of force on it. If your projectile has a base area of 2 square inches, automatically you get 20 pounds of force on it, that's double the power.

However, it's not as simple as that. It depends on your valve flow as well. It could be that if your valve is small, then increasing the barrel diameter means the air can fill it at a lower rate, meaning you do get more area for the air pressure to push on, but that pressure is going to be decreased.
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Unread postAuthor: iisthemuffin » Wed Jul 16, 2008 2:36 pm

I think i get what your saying. Your saying if i have enough flow, it will increase the power because more pressure will be on the projectile correct?

Im using a 1 inch modded sprinkler valve. I think this would be enough flow wouldnt it? The flow of the valve is already bigger than the flow of the barrel. Also, wouldnt using the 3/4 barrel increase performance just because the flow from the valve to the barrel is even less restricted than with a 1/2 inch barrel?

ramses wrote:You would most likely get more power yet, because there will be 2.25x more force on the projectile. If you are buying a new barrel, i would suggest getting a longer one, like 5 feet.


Worth while suggestion. Ive already decided on that, but i do appreciate your opinion.
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 16, 2008 2:40 pm

Note that a 1 inch valve doesn't neccesarily have the same flow as a 1 inch barrel, but in this case upgrading to a 3/4" barrel would definitely be a good thing, if you keep the same barrel length and chamber volume you'll have a launcher that's significantly more powerful, efficient and quiet.
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Unread postAuthor: iisthemuffin » Wed Jul 16, 2008 2:46 pm

Yea i was just thinking that it doesnt nessacarily have a one inch flow. It depends on how much air the diaphragm will let through.

So, your saying 3 feet is good with a 3/4" barrel. Would lengthening the barrel further increase power more?

Also could you give me a rough estiamte of how much it would increase power? Would it double or what?
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 16, 2008 3:20 pm

I'd need GGDT to give you an idea of figures, but a 3/4" barrel of the same length with everything else being equal gives you the potential for double the power. As to barrel length, make it as long as you can afford, even and 8 foot 3/4" barrel for a 1 foot 2" chamber would give you a good ratio, and a very noticable increase in power.
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Unread postAuthor: iisthemuffin » Wed Jul 16, 2008 3:24 pm

I can get 10 feet for like 6 dollars. But there is a problem, Since 3/4 inch PVC pipe isnt big around it tends to bend when you have long pieces of it. (Im sure you knew that) Also my cannon is inline, so i cant really put a barrel support on.

Do you have suggestions to fix the bending?
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