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Pneumatic CB Ration

A place to ask general spud cannon related questions.
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Pneumatic CB Ration

Unread postAuthor: keep_it_real » Sun Dec 31, 2006 8:47 pm

Dunno if this has been said (well I know it's been said but maybe not this way) but here's how i calculated it. I'm not positive if it works like I think but the pressurized air in the chamber will expand until it reaches atmospheric pressure. After that, it seems the projectile will create a vacuum inside the barrel (if there is an ok fit/seal). This could slow it down. So there should be a point where the projectile (tater) gets pushed by the air but doesn't get sucked back in. This works best for a semi auto valve because the chambers are a lot smaller. If this was a one shot pneumatic, the barrel would be freakin' gigantic. Anyway, does that make sense...

p1 = average chamber pressure (psi)
v1 = chamber volume
p2 = atmospheric pressure (approx 14.7 psi at sea level)
v2 = barrel volume (including volume between valve and projectile) This is the variable we are finding

pv = pv (Charles law i think)
(p1 * v1) / p2 = v2

v2 - (space between valve & tater) - v1 = barrel volume

piR^2 * height = cylinder volume
R = ID of PVC / 2
barrel volume / (piR^2) = BARREL LENGTH!!!

k now I'll find the actually CB Ratio for a gun with an average psi of 100 and a barrel with an ID of 3/4

p1 = 100
v1 = 3
p2 = 14.7
v2 = ???

(100 * 10) / 14.7 = 20.4

volume between valve and tater = 2

20.4 - 2 - 3 = 15.2 (barrel volume)

(pi * .375^2) = .4416

15.2 / .4416 = 34 inches.

You probably don't need all 34 inches because the last little bit of pressure inside the chamber isn't going to push the tater much faster. So call it 30

It seems kinda long but it makes sense.
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