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how much psi will 1 foot of 2 inch pvc hold?

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 much psi will 1 foot of 2 inch pvc hold?

Unread postAuthor: iisthemuffin » Thu May 29, 2008 3:30 pm

i bought a 5 foot long by 2 inch piece of pvc for my first cannon and i have some left...the entire 5 foot is rated at 280 psi...how much will a foot of this hold? if i divide it by 5 i get 56...i dont think that is how you do it tho since i have seen chambers about that size and people pump them way hire...so how much do you think a foot of that would hold?
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Unread postAuthor: SpudFarm » Thu May 29, 2008 3:33 pm

its rating is 280psi no matter how long it is.

it will burst much higher than that
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Unread postAuthor: iisthemuffin » Thu May 29, 2008 3:39 pm

really? i thot it did matter how long it was...i had about a foot of 3 inch on a cannon and my compressor would start to slow down at 60 psi on it...kind of like the pipe was getting full...so i switched to 2 feet and it never slows...can you explain that please?
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Unread postAuthor: joestue » Thu May 29, 2008 3:46 pm

The larger pipe depletes the tank.

If this wasn't a Really stupid simple question, you would have to provide a thousand more details.
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Unread postAuthor: SpudFarm » Thu May 29, 2008 3:46 pm

small compressor, bigger air volume, owner that don't understand pressure etc?
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Unread postAuthor: iisthemuffin » Thu May 29, 2008 3:49 pm

i dont understand what your saying? so it slowing down didnt mean anything?
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Unread postAuthor: SpudFarm » Thu May 29, 2008 3:52 pm

there is absolute NO reason for the compressor to slow down.
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Unread postAuthor: iisthemuffin » Thu May 29, 2008 3:55 pm

ok just to be clear...the length of my chamber does not matter? as long as its pressure rated the one foot piece will still be 280 psi?
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Unread postAuthor: MrCrowley » Thu May 29, 2008 3:57 pm

It's all about pressure differential.

Say you charged your compressor to 100psi, if your chamber is bigger then the volume of the compressor, then 100psi will drop down to a lower pressure in the chamber because it has more room to expand.

Remember the compressor is compressing air, if you give this air more room, like a chamber, then the air is going to decompress and the pressure will be lower because it has expanded. There will also be less energy stored in the air particles because it's not as compressed, less energy means that there will be less force acting on your projectile, so your cannon wont be as powerful with a lower pressure.

How old are you? 3 years ago when I was 13 and didn't even take physics, I understood this.

You need to go on to the SpudWiki and also go into google and learn about the basic principles behind pressure. You can't keep asking questions like this and expect to be taken seriously here.


Edit: Yes. Learn your damn physics. Length has nothing to do with it, the longer the pipe the longer it will take to fill up with a compressed gas, 280psi is 280 pounds per square inch. (Assuming that when you say longer, you mean the volume is increasing)

It doesn't matter the volume, the fact that you even think it matters is worrying considering you want to build a potentially lethal device.
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Last edited by MrCrowley on Thu May 29, 2008 4:03 pm, edited 1 time in total.
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Unread postAuthor: iisthemuffin » Thu May 29, 2008 4:01 pm

im 16 and i do understand what your saying about the air decompressing...ive never done this before...im new to the whole thing...sorry about the dumb questions...im just trying to learn
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Unread postAuthor: MrCrowley » Thu May 29, 2008 4:10 pm

Do you take physics at school?

It's probably too late by now, but this is really simple stuff you should've learnt as a compulsory subject. At junior year in highschool physics was part of science, we hardly learnt anything but it was more then enough to grasp anything I read about pressure.
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Unread postAuthor: iisthemuffin » Thu May 29, 2008 4:12 pm

my school sadly does not offer physics...i know you probably get alot of people asking stupid questions but im just learning man...i dont intend to make anyone mad by asking rediculously stupid questions
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Unread postAuthor: MrCrowley » Thu May 29, 2008 4:19 pm

It would help if you knew physics related principles before building a Spudgun. We aren't a physics website, we are a Spudgun website. You really need to know the basics of physics before you join because we can't always teach you them.

Go to google, and knock yourself out reading about pressure, motion, force, mechanics and fluids. Also may help if you learnt a bit about combustion, car engines work on the same principle.

You either have to pick stuff up from reading on the forum, or learn about physics from searching google for articles. Most members already know enough physics to get by with spudguns, that's why we don't discuss something so basic.

I'm not going to say you didn't ask a stupid question, because you did. But it can be easily understood if you just learnt about how pressure works. Just learn the principles somewhere else if you can't here, but we are more then happy to have you here if you want to learn.
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Unread postAuthor: iisthemuffin » Thu May 29, 2008 4:24 pm

thank you very much...i will start looking stuff up here shortly...but its good to know that the length doesnt matter because i would much rather have a foot of 2 inch pvc than the 2 foot by 3 inch chamber i had from the first cannon i made...once again i apologize for the dumb question and would like to thank you for guiding me in the right direction
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Unread postAuthor: BC Pneumatics » Thu May 29, 2008 4:30 pm

Length is the only dimension of pipe that you can change without altering the pressure rating. Changing the diameter will <a href="http://en.wikipedia.org/wiki/Inverse_proportion#Inverse_proportionality">inversely</a> effect the working pressure. Changing the thickness (Schedule) of a pipe will also alter how much pressure it can hold.
As MrCrowley pointed out, the rating is in pounds per square inch. As you may or may not know from math courses, 'per' is another word for 'divided by'. The pressure rating has already been divided by the pipe's dimensions (though not just length) to give you a number that will work for any length of pipe.
If we had to divide it out ourselves (like you tried to do with 280/6) we would not be given a pressure (which is a force/area) on the side of a pipe, but something like "2 inch PVC, 5 Feet, 105,504lbs max" We would then have to divide the 105,504lbs by not only the length of the pipe, but the length times the circumference. (Surface area of an uncapped cylinder = circumference times length) This would be 105,504lbs/(6.28" x 60"), or... 105,504lbs per 376.8 square inches. 105,504lbs/376.8square inches reduces to 280pounds/1square inch, or 280psi.
Since the rating is already given to us in terms of how much each individual square inch can hold by itself, it does not matter how many of them we have.
If we had a group of weightlifters that could hold 500lbs per person, then 10 weightlifters could lift 5,000lbs together, and 20 could lift 10,000lbs. It doesn't matter how many you have, it is still 500lbs each. If we have a longer pipe, we can hold more energy overall since we have more square inches, but it is still divided up at a rate of 280lbs on each square inch, 500lbs per lifter.
Hopefully this helps you understand why length doesn't matter, instead of just letting you know that it doesn't.
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Last edited by BC Pneumatics on Thu May 29, 2008 4:37 pm, edited 1 time in total.
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