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Easy QEV semi auto question

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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Easy QEV semi auto question

Unread postAuthor: Biopyro » Sun Apr 05, 2009 3:43 pm

Image
When I finish pumping, I shut the ball valve and disconnect the pump, which empties the area between the schrader and the BV.
If I leave the cap with the schrader in when I fire, (of course) get a slightly underpowered shot, and can open the ball valve shut it, then vent the schrader to fire again. It works great for plinking.

However; when I go to fire the second shot, I open the BV and nothing happens. No leakage, no nothing. I close the BV, vent the schrader, then open the BV and it fires as I'd expect it to. It does this for another 2 shots, then the last one, it just has a patheticly underpowered shot.

Why do I have to vent that little bit of dead space twice per shot? I don't mind doing it, I'm just curious about the theory behind it.
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Unread postAuthor: Brian the brain » Sun Apr 05, 2009 3:56 pm

Does this happen consistently?

I mean, it might have been a freak incident...
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Unread postAuthor: sgort87 » Sun Apr 05, 2009 4:40 pm

It's because the space between the shrader and ball valve is not big enough to bring the overall pilot pressure down enough. By venting it out again, the pressure comes down enough to allow the QEV to actuate.

Just because you relieve some pressure from the QEV's pilot, it doesn't mean it's enough to move the piston/diaphragm. When you vent it off a second time, it reaches the point where the force on the pilot side is less than the force on the chamber side, meaning the piston will pop back and fire until the force behind the piston is greater than the chamber side again, sealing the barrel port off for another shot.

If you want to be able to fire like this without venting twice, double the space between the ball valve and the schrader. But this will make your first shot slightly more powerful than it is now, and the second shot a little less powerful.
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Unread postAuthor: dudeman508 » Sun Apr 05, 2009 5:03 pm

sgort87 wrote:It's because the space between the shrader and ball valve is not big enough to bring the overall pilot pressure down enough. By venting it out again, the pressure comes down enough to allow the QEV to actuate.

Just because you relieve some pressure from the QEV's pilot, it doesn't mean it's enough to move the piston/diaphragm. When you vent it off a second time, it reaches the point where the force on the pilot side is less than the force on the chamber side, meaning the piston will pop back and fire until the force behind the piston is greater than the chamber side again, sealing the barrel port off for another shot.

If you want to be able to fire like this without venting twice, double the space between the ball valve and the schrader. But this will make your first shot slightly more powerful than it is now, and the second shot a little less powerful.


Why would the size of the dead space between the shrader and the QEV have to be bigger for the second shot. I think its because after your first shot their equal preshure on both sides of the ball valve, so no air moves to pilot the QEV. You could fix this by releasing the pressure between the bv and shrader by pressing in on the shrader after every shot.
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Unread postAuthor: sgort87 » Sun Apr 05, 2009 5:09 pm

He does.
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Unread postAuthor: Hotwired » Sun Apr 05, 2009 5:18 pm

I knew you weren't paying attention.

It can still hide a few tricks even years later ^^
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Unread postAuthor: dudeman508 » Sun Apr 05, 2009 6:21 pm

vent the schrader


sorry my bad
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Unread postAuthor: psycix » Mon Apr 06, 2009 3:26 am

I thnk sgort's theory is right.
Try making the volume behind the BV larger.

The point is that when the gun is at a higher pressure, this does not happen because the absolute pressure difference is larger. (The pressure difference percentage is the same! The pilot volume expands to a certain volume.)
It just doesnt have enough pressure difference in the second shot.
This all has to do with the difference between the frontal and back surface area's.


How about filling from the chamber side using a compressor, prepressurized tank or CO2 tank?
This will make you keep up to "first shot pressure" thus one time venting.
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