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Air chamber volume Ratio, PSI,,,

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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Air chamber volume Ratio, PSI,,,

Unread postAuthor: benchrest500 » Mon Jun 06, 2011 2:48 am

OK I'm new at this and just built my first air canon with 4" x 5' PVC air tank, 1" Rain Bird valve and 2" x 5' PVC barrel

I see a lot of high PSI guns on here but can't some of the high PSI benefits be overcome with larger lower PSI tanks?

In my mind if the projectile is well sealed you have (in my case) 120psi in the tank and a 5' x 2" barrel your actual PSI when it leaves would be calculated with the whole capacity of all plumbing.

So doubling diameter roughly quads capacity so when potato is leaving barrel my capacity increased roughly 25% and then I'm not sure if it's proportional but I'm guessing 25% reduction in PSI so 90PSI on exit?

So larger air tank more speed at same PSI?

4" x 2.5' air tank at 240PSI = 4" x 5' air tank at 120PSI ???
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Re: Air chamber volume Ratio, PSI,,,

Unread postAuthor: Crna Legija » Mon Jun 06, 2011 2:59 am

benchrest500 wrote:OK I'm new at this and just built my first air canon with 4" x 5' PVC air tank, 1" Rain Bird valve and 2" x 5' PVC barrel

I see a lot of high PSI guns on here but can't some of the high PSI benefits be overcome with larger lower PSI tanks?


yes, but you need to move that big tank around its much easyers to move 1l tank at 300psi then a 10l 100psi. you will also need a longer barrel to take advantage of the extra air.

also a small high pressure tank will preform better with everything els the same.
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Unread postAuthor: benchrest500 » Mon Jun 06, 2011 3:30 am

So try to avoid this????

Image

HA HA!


No but seriously, to get some better performance on the easy and cheap I like the larger tank, not like I'm going for concealed carry here and it looks impressive.

But YES I can see advantages to the high pressure moving through the valve faster and getting momentum up faster but the PSI will still be the same at the end of the barrel (at proportions mentioned in last post) and higher PSI at the end of the barrel should translate into faster projectile.

In my case I only bought the valve, had everything else laying around and had it built in under an hour,,,, sure is fun!! I just hook it up to shop air with a ball valve and air chuck (and regulator). I also have air on my work truck so that makes it portable.
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Unread postAuthor: Crna Legija » Mon Jun 06, 2011 4:16 am

they use low pressure so the pumpkin doesn't get demolished in the barrel, slow acceleration but the barrel length +chamber volume keep the velocity up
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Unread postAuthor: Technician1002 » Mon Jun 06, 2011 6:21 am

The big issue with large volume low pressure tanks is the need for a large valve orifice (less mass in the same volume of air) and you need a longer barrel so the lower acceleration can do it's job. A sprinkler valve is somewhat OK on a 100 PSI cannon, but severely restricts flow on a low pressure cannon with a large bore.

Pumpkin cannons use butterfly valves for high flow. I used a home built full orifice valve.

When done right the results are impressive. Below is a shot from my ABS cannon at about 70 PSI with an apple.
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