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Question For the Smart Guys...

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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Question For the Smart Guys...

Unread postAuthor: jeepkahn » Fri Mar 27, 2009 1:00 pm

I noticed this, this past weekend but forgot to ask about it....

On my modular gun(the toolie valve), when I launch stuff with either the 2" or 2.5" short barrels(24"), a second or 2 after firing the sealing disc "burps", but it's sucking in not blowing out...

Theories anyone??? do you think it's creating a vacuum inside the chamber due to gaseous inertia(similar to exhaust scavanging in internal combustion engines)? or is it because there's adiabetic heating during firing and as the air cools it's creating a vacuum?
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Unread postAuthor: rp181 » Fri Mar 27, 2009 2:14 pm

How tight fitting is the projectile? It could be acting like a piston, pulling a vaccuum. If this is true, your barrel is too long.
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Unread postAuthor: jeepkahn » Fri Mar 27, 2009 2:27 pm

rp181 wrote:How tight fitting is the projectile? It could be acting like a piston, pulling a vaccuum. If this is true, your barrel is too long.


Reread my post , slowly, the problem occurs when I use my short barrels, but not my long barrels, the projectiles are very tight, and it's a 300in3 chamber and 75in3 barrel, 4:1 C:B ratio...Overkill, yes, but I only use the short barrels for launching tennis balls and rubber balls for my dogs to fetch....from 200yds away
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Unread postAuthor: daxspudder » Fri Mar 27, 2009 2:49 pm

your experiencing an effect similar to the eductor effect as the air leaves the cannon after the projectile, friction is pulling air from behing the muzzle, causing a vacuum for a short stint which pulls on your diapraghm making a burping sound
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Unread postAuthor: psycix » Fri Mar 27, 2009 3:00 pm

? or is it because there's adiabetic heating during firin

No, the opposite. Adiabatic chilling.
As the air expands it gets cold.
When later on the air heats up, a bit pressure is created.

Are you sure that it is sucking air in? How do you know that?
How loud is the burp? Video?

I think dax's theory is also possible. The air has a certain velocity going outward and this causes it to pull a vacuum.
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Unread postAuthor: jeepkahn » Fri Mar 27, 2009 3:51 pm

that's what I was describing as "internal combustion exhaust scavenging"...

the air exits so quickly that it literally outruns itself and creates a vacuum where there used to be pressure...Eductor effect...

normally it happens several seconds after firing(not split seconds), when I first heard it I thought it was post launch farting, that's when I decided to find out what was happening, I'd fire a tennis ball and then I would put my hand over the barrel and it would suction itself to my hand once it started burping..... It also does it, if not worse, when I dry fire it... Which is what made me think "eductor", because it's a 4" chamber firing through 4"x2" bushing(which is tapered)through a 2" camloc( actual I.D. about 1.8") and back to a 2 0r 2.5" barrel, which in theory could produce supersonic air flows(once the projectile leaves the barrel, or if dry fired) that could in effect create a vacuum in the chamber... and that would also explain why it doesn't do it with my ong barrels because there isn't enough pressure left after the projectile exits the barrel to create the requisite pressure required for accelerating the air....

Or am I way off base???
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Unread postAuthor: SpudFarm » Fri Mar 27, 2009 4:11 pm

easily described as this:
everything has mass. when a molecule from the chamber want's to get out due to speed it will du that.
but when it is atmospheric pressure in the chamber after discharge the air is still moving out the barrel pulling air out from the chamber.
we all know what you get is you pull/suck the air out? vacuum/burping!
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Unread postAuthor: jeepkahn » Fri Mar 27, 2009 4:50 pm

SpudFarm wrote:easily described as this:
everything has mass. when a molecule from the chamber want's to get out due to speed it will du that.
but when it is atmospheric pressure in the chamber after discharge the air is still moving out the barrel pulling air out from the chamber.
we all know what you get is you pull/suck the air out? vacuum/burping!


eggzactly!!
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Unread postAuthor: daxspudder » Fri Mar 27, 2009 6:25 pm

but if its SECONDS later, that sounds more like an issue with the pvc contracting and expanding due to thermal changes.... when you discharge pressure your raising the temperature of your chamber a few degrees for every so many psi, but it gets diffused quickly and usually doesn't affect PVC otherwise you'd hear more about semi autos shattering... but its possible that yours is due to the 2"x4" reduction getting "excessively" hot... the burp could easily be the pvc expanding when it "cools" back down...
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Unread postAuthor: ramses » Fri Mar 27, 2009 6:35 pm

no: the chamber cools as it de-compresses!

the only reason I can think of is that the cooled gas in the chamber warms up, creating a tiny bit of pressure that then overcomes the static friction of the valve, and "pop" a tiny bit of air comes out.
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Unread postAuthor: jeepkahn » Sat Mar 28, 2009 7:53 am

daxspudder wrote:but if its SECONDS later, that sounds more like an issue with the pvc contracting and expanding due to thermal changes.... when you discharge pressure your raising the temperature of your chamber a few degrees for every so many psi, but it gets diffused quickly and usually doesn't affect PVC otherwise you'd hear more about semi autos shattering... but its possible that yours is due to the 2"x4" reduction getting "excessively" hot... the burp could easily be the pvc expanding when it "cools" back down...


I messed with it last nite and here's what I found...

The burping doesn't start until I set the launcher down, or bump it with my leg, or whatnot(probabably the jarring causing the seal to become imperfect), because i could dry fire it with no barrel and if I placed it gently on the table and it wouldn't burp until I would reach into the barrel opening and push on the sealing face and would get a huge "suck" of air into the chamber... In case I didn't specify already, the valve is a spring reseating valve(which is why it closes and traps the vacuum)...


I'm not really concerned about it, but the phenomena is quite interesting to try and figure out...
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Unread postAuthor: john bunsenburner » Sat Mar 28, 2009 8:56 am

Ok simple; When you fire the moving particals of air go out and creat a vacume, before they can all renter the valve shuts letting little or no air in. When you bump the valve it vibrates and air starts filling back in, this air holds the valve open, maybe more and more air goes in until the valve is pulles open further and further, it then reaches a point where in a matter of a spilt second al the air is sucked into the chamber to equilize, balance, the pressure in the chamber and outside of it, and you hear a burp.
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Unread postAuthor: jeepkahn » Sat Mar 28, 2009 9:03 am

john bunsenburner wrote:Ok simple; When you fire the moving particals of air go out and creat a vacume, before they can all renter the valve shuts letting little or no air in. When you bump the valve it vibrates and air starts filling back in, this air holds the valve open, maybe more and more air goes in until the valve is pulles open further and further, it then reaches a point where in a matter of a spilt second al the air is sucked into the chamber to equilize, balance, the pressure in the chamber and outside of it, and you hear a burp.


That was my theory from the begining, i just wanted some extra opinions...

the interesting part is that when it burps, it sounds like a drawn out tuba note lasting about 2 seconds or more...
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Unread postAuthor: john bunsenburner » Sat Mar 28, 2009 9:51 am

That only means that the valve doesn't open alot...
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Unread postAuthor: psycix » Sat Mar 28, 2009 3:40 pm

Or it simply closes fast again, and your valve just happens to close right near the optimal vacuum.
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