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can any body explain to me what....

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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can any body explain to me what....

Unread postAuthor: far_cry » Sun Sep 14, 2008 11:32 am

the meaning of this option in ggdt
in the red square
i have been in the ggdt site but i did not understand what they mean by seat diam
and why if i make the seat diam biger than the barrel bore in the black square i get choking flow and the muzzle velocity increase ??
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Unread postAuthor: Hotwired » Sun Sep 14, 2008 11:55 am

It is the internal diameter of the opening that is uncovered when the piston or diaphragm moves back on piloting.

If the seat diameter is larger than the barrel and it tells you that flow is being choked... well that's because it is.

The seat diameter is the opening that the chamber air is dumped out through, if you then make the air go through a smaller diameter passage (like the barrel) then you aren't using the valve to its full potential and a smaller valve with the same size seat as the barrel diameter could have done as well.

It's not a problem, the cannon will work fine with a valve seat larger than the barrel, GGDT is just saying that you could have used a wider barrel there.
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Unread postAuthor: far_cry » Sun Sep 14, 2008 12:05 pm

i see
and why when the seat diameter is bigger than the barrel bore the muzzle velocit increase? that must be good thing. NO??
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Unread postAuthor: Hotwired » Sun Sep 14, 2008 1:29 pm

Up to a point yes.

The same way if you increase barrel length or chamber size you will get a few more fps.

Air passing through a piston valve has to do at least one 90 degree turn to get out, more likely 180 degrees so it loses some pressure and through that, loses energy which then doesn't get transferred to the projectile.

If the valve seat is exactly that of the barrel then the valve will be the part holding back the performance of the cannon as it is not supplying the maximum flow that could go through the barrel.

By increasing the size of the seat and therefore the valve you compensate for the energy loss by having a greater flow through the valve and you can then get maximum (or as close as no matter) flow through the barrel which translates into more energy in the projectile.

Really depends on your priorities on how much you care for those few extra fps.
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Unread postAuthor: psycix » Sun Sep 14, 2008 3:47 pm

Also the opening time can be cut with the larger seat.
Sounds weird, but heres the explanation:

When using a valve the same size as the barrel, the flow to the barrel depends on how far the valve is open. After X time, the valve is fully open and the air can flow without constriction from the valve. (Neglecting all energy losses stated by hotwired)

If you have that same barrel, but this time you have a valve that has twice the seat surface area (twice as much flow) the valve practicably only needs to be half open to grant the barrel full flow, any further opening doesnt matter, as the barrel is the narrowest diameter now, and the flow cannot go faster then the "weakest link" in this case, the lowest cross-sectional surface area.
If this larger valve opens up in the same X time, only 0,5X time is needed for the valve to give the barrel full flow.
Now larger valves tend to open slower, so this will not always apply, but opening time is just all about the point of the piston moving, until the moment of the flow being maxed.
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Unread postAuthor: far_cry » Sun Sep 14, 2008 5:30 pm

thank you all
so which piston valve is better
http://www.spudfiles.com/forums/piston- ... t8157.html
Coaxial Piston Valve or Barrel Sealing Tee Valve
i read that the Coaxial Piston Valve have no loses (dead volume)
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Unread postAuthor: D_Hall » Sun Sep 14, 2008 5:43 pm

far_cry wrote:i read that the Coaxial Piston Valve have no loses (dead volume)

A coaxial will indeed have zero dead volume, but that doesn't mean it has no losses.

ALL valves have losses unless they're pushing a superfluid through them (but I don't think many of us deal with liquid helium so I'm betting against that).
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Unread postAuthor: Hotwired » Sun Sep 14, 2008 5:55 pm

Well a coaxial has as much dead volume as any other.

If you ram the projectile all the way to the back of the barrel so it's basically touching the piston there's still dead volume and it can't be avoided.

When the piston moves back it leaves a space behind that is filled as the compressed air moves out.

Anyway it's splitting hairs as to which is "the best".

A barrel sealer is pretty much identical to a coaxial and the only worthwhile difference between them and a chamber sealing valve is that a chamber sealer needs two sealing points instead of a barrel sealer/coaxials one sealing point.

That's why a chamber sealer is a little harder to make, in fact requiring a sliding seal using o-rings or similar, thus making it a very rare choice for the average poorly tooled spudder.

The most practical is usually a barrel sealer or coaxial.
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Unread postAuthor: far_cry » Sun Sep 14, 2008 6:02 pm

thank you guys thats help
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Unread postAuthor: LikimysCrotchus5 » Sun Sep 14, 2008 10:54 pm

D_Hall wrote:
far_cry wrote:i read that the Coaxial Piston Valve have no loses (dead volume)

A coaxial will indeed have zero dead volume, but that doesn't mean it has no losses.

ALL valves have losses unless they're pushing a superfluid through them (but I don't think many of us deal with liquid helium so I'm betting against that).


How about Bose Einstein Condensates?

(Well we we would freeze to death by touching one)

But anyway, in my perspective, i like barrel sealers, only because its easier to get more volume out of one, which can mean more power, but not all the time.

In coaxials, the barrel is pushed in, and that takes up volume which can deprive you of some power.

And even though a barrel sealer is not usually inline, its still pretty powerful. (unless its a toolies, thats inline, or any other inline piston valve)

So although we cant bypass any restrictions on power, they still satisfy my needs of destruction :twisted:
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Unread postAuthor: psycix » Mon Sep 15, 2008 8:07 am

In a coaxial barrel sealing valve, the air needs to do an 180 degree turn.
In a barrel sealer inside a tee, you could construct it in such a way, that the air only needs a 90 degree turn.
So in theory barrel sealing tee valves have a better flow then a coaxial.
Advantages of coaxials are the compact package and they are easy to build.

You can also construct a chamber sealing coaxial valve, though you would only be able to fire donut shaped projectiles in that case. :D
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Unread postAuthor: Hotwired » Mon Sep 15, 2008 8:20 am

psycix wrote:In a barrel sealer inside a tee, you could construct it in such a way, that the air only needs a 90 degree turn.


Mmm, an L shaped cannon :wink:
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