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Hi all. I have experience with pneumatic and combustion cannons, but I'm interested in using 12 gram CO2 cartridges and I could use some clarifications. I want to copy this design that I found here:
I understand CO2 operates at extremely high pressures, so steel parts are needed for the construction. I'm concerned about a few things:
1) The video (and the detailed construction plan) has no mention or warning about pressure ratings on the parts used. Are these parts, especially the unspecified ball valve, really safe with 800-900 PSI? Do I need to find a specific kind of 'heavy duty' ball valve or 'heavy duty' teflon tape, or is the difference negligible?
2) What's the purpose of the expansion chamber? I assumed it was intended to lower the pressure, but according to the wiki and relevant sticky, CO2 pressure from a 12 gram cartridge won't decrease until it fills a much larger volume. How would the design perform with a shorter/longer chamber, or if the adapter were just screwed directly into the ball valve?
3) What would a catastrophic failure look like? I know PVC shatters and explodes, but what happens to metal pipe and metal ball valves?
The video has almost two million views and I can't find any complaints about it, so I have to assume it works as pictured. I'm just cautious because the author spends more time discussing his launcher's paint job than describing the mechanism.
Metal parts are only "required" if no form of regulator is used.
Connecting the co2 directly to your ball valve will give you full pressure from your co2 cartridge. The small expansion chamber a is like a regulator allowing the pressure the drop to a reasonable level for the materials you are using. The larger the chamber, the lower the pressure.
Metal pipe usually splits and from what I've seen ball valve leak before they blow. The only one I've seen crack under pressure was used in a hydraulic system it was not designed for.
Maybe no one survived to complain
Never assume when it can kill you
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Was it a nice paint job at least?
The purpose of an expansion chamber is to allow the liquid co2 to expand to a gas before it is fired, this increases performance.
As a quick visit to Wikipedia will confirm, once co2 is compressed beyond a certain point it will liquefy, so what you essentially have in a 12g co2 capsule is 12g of liquid co2.
When liquefied, the pressure above the liquid, or the 'vapour pressure' is always a constant, as long as there is still liquid in the capsule.
When the co2 capsule is allowed to expand (eg firing a launcher) the space is filled by the gaseous co2 still present above the liquid, and to make up the volume, co2 will 'boil off' to maintain the pressure, so constant pressure, less liquid.
The pressure will only drop after there is no more liquid co2 to boil off, hence it requires a reasonably large space.
The process of liquid co2 turning to a gas takes in so much heat that it often results in freezing, and it requires a portion of time to do so.
Without an expansion chamber the launcher is:
a) very susceptible to 'siphoning' or firing liquid co2 as opposed to gas, as it often doesn't have time to expand to a gas in the few milliseconds of firing.
b) Very likely to freeze, which causes both the pressure of the gas to decrease, and the launcher to become ice cold.
d) Very low on power, as it is co2 is not a 'fast gas' , coupled in with the time it takes for co2 to expand and the lowered pressures generated it will really reduce muzzle velocity.
So all in all, expansion chambers are quite important
Thanks, I was trying to figure out how to explain that better. I was reading about a similar set up a few months back( In fact it might have been the same one). It was used to launch Nerf Vortex footballs. When the guy first built it he did not have an expansion chamber, so the co2 only produced a low volume high-velocity jet. This jet had insufficient volume to pressurized the 2 inch PVC barrel. Once he added the small pressure chamber, when the valve was opened there was enough volume to launch the football.
You smart guys explain stuff so much better than I can.
Here are my findings after a quick build and some tests.
Maybe. My local hardware store has a few varieties of metal ball valve, but they're all rated to 600 PSI. I went with the cheapest one. I also had a galvanized 1/2" nipple (no apparent pressure rating) and a stainless coupling I had (McMaster # 45525K564) Every male thread was given 4 wraps of the cheapest Teflon tape and wrench-tightened (hooray for hexagons ). I took appropriate safety precautions, then pierced a cartridge and found that all the seals were airtight. It was at least 80 degrees outside, so the CO2's pressure could have been upwards of 1000 PSI. I dipped the launcher in a water bucket and couldn't see any gas escaping. I was especially impressed with the male ASA paintball thread screwed into the female 1/2" NPT coupling - they're technically incompatible but combining them caused no problems.
Opening the ball valve released a huge blast of CO2 that recoiled sharply even without a projectile. But after this initial release, the gas kept flowing out with a very loud hiss for 5-10 seconds. With the open valve held near the ground, I managed to form small piles of solid dry ice. This was an alarming and unpleasant experience. I recommend using an expansion chamber large enough to let all the liquid CO2 boil to gas before firing.
The ball valve kept springing a small but audible leak near the stem after firing each cartridge; the leak would disappear after being given a few minutes to warm up, and it would then be able to contain the pressure from a new cartridge. All other joints remained airtight. Overall, I'm impressed by and excited about the power potential of these cartridges.
This statement would be true if releasing a compressed gas into an expansion chamber. But this is boiling liquid.
This statement is only true if the expansion chamber is too big and no liquid remains, thus dropping the pressure. If liquid remains, the system pressure including the expansion chamber will be at the vapor pressure of the remaining liquid.
Where the power gain in achieved is when the expansion chamber warmes up the liquid/solid pressure rises. The pressure is directly related to the liquid temperature. Piercing the cartridge releases pressure and the liquid boils which cools it. The cooler liquid and gas will be at a lower temperature.. until it warms back up, returning the pressure to a higher level.
The amount of pressure rise is dependant on the chamber size. If it is too small to boil all the liquid (eg dry ice on the ground) then the pressure will return to the initial cartridge pressure when the temperature of the system returns to room temperature. If the chamber is larger, and all the gas turns to liquid, the pressure varies with chamber volume.
The highest possible performance is a chamber just large enough to boil all the liquid providing the highest possible volume at the highest possible pressure. This ideal volume will depend on temperature.
Consider adding a spring to open your ball valve rapidly and consistently, you will be impressed by the power.
Re: 12 gram CO2 adapter pressure & safety
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