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coaxial pilot valve 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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coaxial pilot valve question

Unread postAuthor: Arborman » Sat Apr 21, 2007 12:14 pm

I have another little q for my pneumatic panzerschreck (which I have finished the piston for).

To put the trigger in the position it would have been on the real one, I am going to need like a brass ball valve attached 3 feet of air hose.

After looking at all the pilot volume caused by the hose, I was thinking of mounting a sprinkler valve on the cleanout pug on the back and having the ball valve pilot that.

If anyone understands what the heck im saying that answer these questions.

1. Would the sprinkler valve setup work?
2. If so, can I attatch it with a .75" pvc male npt to male npt adapter thing? (my valve is .75")
3 Would just the ball valve work?
4. Would any significant advantage be gained by adding the sprinkler valve.

and one more thing

ON MCMASTER, I normally get this option:
non rated
sch 40
sch 80

I am assuming that non rated means dwv. Can't use that. Would sch 40 be ok to use in a pneumatic?
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Unread postAuthor: noname » Sat Apr 21, 2007 12:21 pm

It all depends on the sizes you have. Ball valve sizes, chamber size, barrel size....etc.
2) Yes.
5) Sch 40 is fine, Sch 80 can hold a bunch more pressure, and non rated is DWV.
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Unread postAuthor: iPaintball » Sat Apr 21, 2007 12:43 pm

1) Yes
2) As long as you use a 3/4" sprinkler valve, yep.
3) I wouldn't because it is not as realistic as a blowgun trigger, and, like you said, there is a lot of pilot volume.
4) A sprinkler valve would pilot the valve a LOT faster than a ball valve, thus giving you more power, and you can also use a more realistic trigger. (blowgun)
5) noname already answered
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Unread postAuthor: Arborman » Sat Apr 21, 2007 5:15 pm

one more thing.

I can never find pressure rated 4" screw cap thingys.

The hex ones on mcmaster ar only sch 80 and are like $20.

Any ideas?
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Unread postAuthor: scatdawg » Sat Apr 21, 2007 7:18 pm

just one response to your questions. the length of hose to your piloting trigger (whether it be a ball valve, sprinkler ,or blowgun) will make no difference in the volume that you have to dump. you could use 50 feet of airline. you only are dumping the volume of air that is behind the piston to the bumper. you have pressurized air all the way to the trigger so you only have to dump an equal volume of air out that matches the volume behind the piston.
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Unread postAuthor: rad14701 » Sat Apr 21, 2007 8:03 pm

Ummm... No, scatdawg, that isn't how it works... Remember, you are trying to exhaust volume under pressure to barometric pressure... The more volume you have, the slower you will reach barometric pressure and the slower the piston will pilot... So, all pilot chamber volume and air hose or vent pipe volume, or whatever it happens to be, behind the piston adds up to total pressurized volume... The pressure throughout this volume continues to equalize until a given volume and pressure is exhausted... It's all about fluid dynamics...
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Unread postAuthor: chaos » Sat Apr 21, 2007 8:20 pm

^^^^^ yer i agree with that. less hose = quicker vent.
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Unread postAuthor: scatdawg » Sat Apr 21, 2007 8:21 pm

rad14701, im not saying that you are wrong but if this is the case, how come when i was testing my last pneumatic cannon, i vented the pilot with my modded blowgun attached to about 25 feet of airline and then shortened to about 1ft and i didnt see any change in performance?

EDIT- i dont have a chrony to tell exactly the difference in the performance so maybe it does make a small difference. 23 feet of airline adds a lot of volume so t think it should have made it very niticable but it didnt. i just dont understand.
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Unread postAuthor: rad14701 » Sat Apr 21, 2007 8:41 pm

@scatdawg

You most likely exhausted more air than just what was in the pilot chamber...

Look at it this way... If you had a two cubic inch pilot volume and also had a 25 foot air hose, or an additional 15 cubic inches of volume, you had approximately 17 cubic inches of pressurized volume to exhaust... If you, as you have theorized, only exhausted the original 2 cubic inches then you only reduced the volume by approximately 8%... But that would be 2 cubic inches at pressure which equates to a much larger volume at ambient barometric pressure...

All that being said, piloting would not be instantaneous given the added volume required to be exhausted for proper piloting to take place... So the smaller the volume behind the piston, or total pilot volume, the faster and more efficiently the gun will pilot and fire...
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Unread postAuthor: Arborman » Sun Apr 22, 2007 1:52 pm

I think my original question has been answered. I now have a degree in pilot volumes :) .

Anyway....

anyone know where to get sch 40 4" end cap things without dropping a 20 on mcmaster.
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Unread postAuthor: pyro6314 » Sun Apr 22, 2007 2:38 pm

Threaded End caps? I get them where I live for like 14 bucks I think. Not that much difference.
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Unread postAuthor: Velocity » Sun Apr 22, 2007 2:55 pm

<a href="http://www.plumbingsupply.com/pvc.html#plug">Link finder to the rescue!</a>... they cost $10.27 for the SCH 40 ones.
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