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Help me with the maths for a refill tank

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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Help me with the maths for a refill tank

Unread postAuthor: hyldgaard » Sun Apr 13, 2008 8:14 am

Hello.
Im considering making a portable tank for my upcoming pneumatic gun, and im looking for help in calculating how many shots i can get from a tank with a given volume and a chamber with another given volume, including pressure drop. Honestly i should be able to do this myself but i cant seem to get it right, so any help on which equations i should use wold be greatly appreciated :)
As a sidenote, im considering a large CO2 extinguisher as the portable tank, does anybody have experience with this? a search on the forums results in a lot of threads where people talk about using them as chamber on their launchers, but i cant seem to find someone that have actually used one :roll:
Thanks in advance.
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Unread postAuthor: DYI » Sun Apr 13, 2008 10:04 am

Well, if the refill tank is filled with a pressurised gas, all you have to do is find the volume of gas in the tank (volume of the tank * pressure in ATM)
and the volume of gas in the chamber of the gun (same as above), then divide the tank volume by the chamber volume to get number of shots (note that this assumes the storage tank is at relatively high pressure, as you obviously can't fill after the tank pressure drops to the chamber pressure in the launcher).

If the refill tank is filled with a liquified gas (which is usually sold by the pound or kilogram), just find the mass of gas that will be in the chamber after a fill, and divide the mass of the liquified gas in the chamber by that number.
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Unread postAuthor: hyldgaard » Sun Apr 13, 2008 10:14 am

Thank you for your reply DYI, but what im looking for is a bit more advanced than that. What im looking for is a way to calculate what pressure the first, second, third and so on fills will be at. An example close to reality of my upcomming project would be a chamber size of 76cubic cm and a refill tank size of about 5L. the refill tank will be pressurized to about 350psi.
I apologie for my bad explanation, but english isnt my first language ;) hope you understand what im trying to say anyway.
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Unread postAuthor: DYI » Sun Apr 13, 2008 10:30 am

Are you not using a regulator?

For simplicity's sake, we'll assume a 100cc chamber, and a 1000 cc refill tank.

So, the refill tank is at 350 psig, or, in more useful units for this calculation, 25 ATM, and the chamber is at 1 ATM. When we combine the volumes together we get 1100 cc. Now, at atmospheric pressure, the gas from the tank would occupy 25 000 cc, and the gas from the chamber will still occupy 100cc, since it isn't pressurised yet. When you combine the 2 volumes together, you have 25 100 cc of gas, compressed into an 1 100 cc volume. So, the pressure of the entire system drops to
(25 100 / 1 100) = ~22.82 ATM. Repeat this for any further shots.
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Unread postAuthor: hyldgaard » Sun Apr 13, 2008 10:40 am

No, im not using a regulator. these calculations were ment to determine if one was needed, and i can see now that it is. This was exactly what i needed, thank you very much for your help DYI :)
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Unread postAuthor: rcman50166 » Sun Apr 13, 2008 11:31 am

This has been discussed before. Don't know how much the link will help (didn't read the entire thread) http://www.spudfiles.com/forums/how-much-co2-do-i-need-t7133.html
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