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Multiple shots question? regulator vs. ball valve

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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Multiple shots question? regulator vs. ball valve

Unread postAuthor: jmez1996 » Tue Sep 27, 2011 4:14 am

I was experimenting with the pressure of my gun as shown here:
http://www.spudfiles.com/forums/files/177_168.jpg
in regards to the pressure drop after each shot. To use the gun i open the ball valve after the fill, then fill it to 260psi. I then close this valve and this isolates the firing tank from the main air reservoir.
This ball valve is opened and closed between shots to refill the firing chamber. Doing it this way i get what seems like 5 decent shots.

Today i left the pump plugged in when i cycled the gun. And noticed the pressure drops being:

FIRST SHOT: 260psi
2nd: 145psi
3rd: 80psi
4th 50psi
5th and final (with ballvalve open): 30 psi

Is means a pressure reduction of (roughly) 40% each shot.

These shots all seem relatively similar though, although the first does seem stronger.

It seems that i would get much more shots, and it would be much more efficient if i let it drop pressure by say 30psi each shot? instead of a percentage of the tank. Would this be possible if i replaced the ball valve with a regulator? any other ways to get more even shots? any suggestions of a good small regulator that would fit into the stock instead of one of the huge ones off a compressor?

cheers.
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Unread postAuthor: Crna Legija » Tue Sep 27, 2011 4:40 am

Yes but if you set your reg at say 150psi you would only get one shot because after the first shot your tanks is already under the set pressure. They are only good if your have 500psi in a tank and set it at 100psi.

That is why lots of air people like hammer valves they self regulate and will give the most consistent shots for one charge.
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Re: Multiple shots question? regulator vs. ball valve

Unread postAuthor: jackssmirkingrevenge » Tue Sep 27, 2011 5:00 am

jmez1996 wrote:These shots all seem relatively similar though, although the first does seem stronger.


Seem.

You cannot tell without a chronograph, or at least some sort of medium that allows you to measure depth of penetration, using consistent projectiles.

It seems that i would get much more shots, and it would be much more efficient if i let it drop pressure by say 30psi each shot? instead of a percentage of the tank. Would this be possible if i replaced the ball valve with a regulator? any other ways to get more even shots? any suggestions of a good small regulator that would fit into the stock instead of one of the huge ones off a compressor?


What you want is a momentary valve that is only open for the few milliseconds that the projectile is in the barrel, anything else is wasted. This is why most commercial PCPs use hammer valves as Crna Legija mentioned.

Your firing chamber is probably far too big anyway, what is the chamber:barrel volume ratio?
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Re: Multiple shots question? regulator vs. ball valve

Unread postAuthor: jmez1996 » Tue Sep 27, 2011 5:58 am

jackssmirkingrevenge wrote:Your firing chamber is probably far too big anyway, what is the chamber:barrel volume ratio?


Not 100% sure but around 1.5:1 (chamber to barrel) would it have much better efficiency at 1:1 ratio?
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Unread postAuthor: jackssmirkingrevenge » Tue Sep 27, 2011 6:28 am

One thing you can try is this:

Leave the pump connected so you can monitor psi, and the ball valve open. Fire by lightly pulling the blowgun trigger and releasing it immediately, and see what sort of pressure drop vs apparent power you get.

I would strongly recommend you set up some way of evaluating power, for example multiple sheets of thin plywood or aluminium sheet, or say some opened up soup tins, equally placedbox'o'truth style.
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Unread postAuthor: jmez1996 » Tue Sep 27, 2011 7:06 am

ohhh i see what you are sayingg now.. so what if i tried a similar set up but without the ball valve and instead a attachment to the output to the blowgun similar to that used by wyz2285 in this: http://www.spudfiles.com/forums/semi-au ... 21372.html
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Unread postAuthor: jackssmirkingrevenge » Tue Sep 27, 2011 7:26 am

While wyz2285's system works, ideally the blowgun trigger should be actuated by a hammer valve for better consistency - though once you do that, for 0.177" you're better off using the hammer valve directly as in this good tutorial also by wyz2285.

Try your current setup as suggested, maybe with the ball valve just cracked open to limit flow from the main chamber, and see what results you get.
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Unread postAuthor: wyz2285 » Tue Sep 27, 2011 10:24 am

jackssmirkingrevenge wrote:good tutorial also by wyz2285.


That valve is used here, use 5.5 mm lead pellets, 8 shots for 400 psi, +-30 cm cubic chamber.
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Re: Multiple shots question? regulator vs. ball valve

Unread postAuthor: jimmy101 » Tue Sep 27, 2011 4:51 pm

jackssmirkingrevenge wrote:
jmez1996 wrote:These shots all seem relatively similar though, although the first does seem stronger.


Seem.

You cannot tell without a chronograph, or at least some sort of medium that allows you to measure depth of penetration, using consistent projectiles.

What jack said. You have to remember that the energy in the shot scales as the square of the velocity. It might seem to you that the second shot was similar to the first but if your first muzzle velocity was 300 fps and the second 250 fps then the second shot actually has 250<sup>2</sup>/300<sup>2</sup>=0.7 the energy (assuming the same ammo weight). So a 15% drop in muzzle velocity is actually a 30% drop in energy. If the next shot is at 200 fps then the energy has dropped by 56% even though the velocity only dropped by 33% compared to the first shot.

Chrono's are pretty easy to setup, a laptop and a microphone is all you need if you have access to a fairly large open area (to reduce echoes that make analysis of the recording tricky).
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