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A place to ask general spud cannon related questions.
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Unread postAuthor: warhead052 » Wed Apr 25, 2012 10:55 am

Does anyone know of a way to calculate the amount of air in a chamber from the pressure? Or is that something obvious like, chamber size x pressure = amount of air. I need to find out so I can start planning something.
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 25, 2012 10:56 am

warhead052 wrote:Does anyone know of a way to calculate the amount of air in a chamber from the pressure? Or is that something obvious like, chamber size x pressure = amount of air. I need to find out so I can start planning something.


http://en.wikipedia.org/wiki/Boyle%27s_law
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Unread postAuthor: warhead052 » Wed Apr 25, 2012 11:19 am

Thanks Jack. Appreciate it.
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Unread postAuthor: DYI » Wed Apr 25, 2012 11:19 am

Does anyone know of a way to calculate the amount of air in a chamber from the pressure?


What do you mean by "amount"? Mass? Number of moles?

Either way, look into equations of state. Put quite simply, they're a relation between state variables. In your case, you need to find density from temperature and pressure.
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Unread postAuthor: warhead052 » Wed Apr 25, 2012 11:41 am

Amount as in cubic inch/feet of air in a given container. Say like a 1 cubic inch chamber with 4,000 psi in it, I would like to know how much air that would be stored up.
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Unread postAuthor: Technician1002 » Wed Apr 25, 2012 1:10 pm

Convert PSI to Bar. Each increase of one bar adds one volume to the container.

Example,
One container of 1 cubic inch contains one cubic of air at zero bar. It is not under pressure except atmospheric pressure.
Increase the pressure to one Bar (about 15 PSI) places 2 cubic inches of air into the one cubic inch volume. Heat of compression is ignored in this simplistic example.

Your example is about 276 cubic inches at STP.
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Unread postAuthor: DYI » Wed Apr 25, 2012 1:21 pm

Well, it's easy enough to tell how many cubic inches of air are in a chamber at a given pressure: that would be equal to the volume of the chamber, regardless of the pressure :wink:
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Unread postAuthor: kydavies » Wed Apr 25, 2012 3:44 pm

does anyone have a rough idea of what the maximum chamber volume that can be ported by a schrader valve in a coaxial piston (MY PISTON WILL BE LIGHT WEIGHT so don't worry))?
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Unread postAuthor: Labtecpower » Wed Apr 25, 2012 4:03 pm

Depends on the ratio of your valve, airtightness of the piston, and some other things.
more info would help!
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Unread postAuthor: kydavies » Thu Apr 26, 2012 11:17 am

I figured there was a rule of thumb so to speak. but with all the varriables, i guess not.

3/4" copper coaxial with a 1/2" copper barrel (i live in a small town so it was the best i could do for a small barrel). Unfortunatly its a low ratio but I'm making the piston servicable so I can quickley swap pistons. non-the-less I am keeping the pilot volume down by filling excess space with epoxy. when I'm done the pilot volume shouldnt be more than a cubic inch.
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Last edited by kydavies on Thu Apr 26, 2012 11:21 am, edited 1 time in total.
mikemurph wrote:yea, the ammo has been changed to a more aero dynamic projectile. it consists of the back half of a dart, and a hot glue ball
AHahahahah!!
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Unread postAuthor: warhead052 » Thu Apr 26, 2012 11:19 am

Technician1002 wrote:Convert PSI to Bar. Each increase of one bar adds one volume to the container.

Example,
One container of 1 cubic inch contains one cubic of air at zero bar. It is not under pressure except atmospheric pressure.
Increase the pressure to one Bar (about 15 PSI) places 2 cubic inches of air into the one cubic inch volume. Heat of compression is ignored in this simplistic example.

Your example is about 276 cubic inches at STP.


Thanks Tech. That helps alot!
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Unread postAuthor: Fnord » Thu Apr 26, 2012 5:22 pm

does anyone have a rough idea of what the maximum chamber volume that can be ported by a schrader valve in a coaxial piston (MY PISTON WILL BE LIGHT WEIGHT so don't worry))?


Maximum pilot volume?

If you have an o-ring piston with a perfect seal, a port-to-piston diameter ratio that is not too large, and a sealing face that is very malleable, it may be possible to pilot a 3" piston with a schrader.

Actually, you could theoretically pilot any valve with one. In reality, your sealing face will start to leak chamber pressure into the barrel right before it opens. Your pilot valve must be able to "out-run" the resulting drop in chamber pressure by some margin, otherwise you'll just have a very slow opening time.

The margin will depend on how tightly your projectile fits in the barrel, and how much air leaks past it.

Personally I've been able to pilot an o-ringed 2" piston with a 1" port (no projectile).
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Unread postAuthor: kydavies » Thu Apr 26, 2012 8:56 pm

I have an alternate plan for a pilot valve, I'm switching to a blow gun so I will have a higher tollerance when it comes to my piston (I'm new on the block when it comes to machining pistons). Also a blow gun will be easier to fire.

Because of the design of my cane gun I will be able to test both the schrader and the blow gun as pilot valves.

Anyways, thanks Fnord; I guess it's time I stop being piston shy and finish my cane gun.
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mikemurph wrote:yea, the ammo has been changed to a more aero dynamic projectile. it consists of the back half of a dart, and a hot glue ball
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Unread postAuthor: wyz2285 » Mon Apr 30, 2012 2:10 pm

I have asked this before, but nobody answered :?
So I´m going to ask again: is there anyway of knowing the flow coefficient of a homemade valve?
I´m working on a hammer valve, but if there are some general ways to calculate/find the flow coefficient of a valve, it would be nice :)
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Unread postAuthor: al-xg » Mon Apr 30, 2012 2:20 pm

Well one is to use a chronograph to work out projectile speed at different pressures and then run it though the flow coefficient finder on GGDT.
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