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Old 03-04-2013, 05:37 PM   #14 (permalink)
Slim
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Join Date: Dec 2010
Location: Connecticut

I'd just like to say that the best advice I could give anyone who is looking for technical answers is to do as much searching and reading before asking any questions as possible. Unfortunately, searching for answers here on MCB might not give you the help you really need. I've noticed quite a few folks here giving out tips and advice on CO2 efficiency who obviously haven't bothered to familiarize themselves with the basic properties and principals of CO2. This well-intentioned advice is actually costing other players money on mods that may actually cost them efficiency. The bottom line is, it's simply impossible to discuss CO2 efficiency without understanding how CO2 operates.

Without getting into any advanced physics here, the following will provide some basic, basic, basic information on the principals and properties of CO2.

From CO2info.com...

Quote:
CO2 Properties: Carbon Dioxide can exist in three states; Gas, Liquid & Solid.

Gaseous Carbon Dioxide
CO2 gas is heavier than air with a specific gravity of 1.53 at 70 dF
At normal temperatures and pressures, CO2 is colorless with a slightly pungent odor at high concentrations. If compressed and cooled to proper temperatures the gas liquifies. Solid CO2, dry ice sublimates back to the natural gaseous state.

Liquid Carbon Dioxide
Liquid CO2 is produced by compressing and cooling CO2 gas. This liquid is a clear transparent fluid. Liquid CO2 cannot exist as a liquid at atmospheric pressure. It must be pressurized above 60.4 psi to remain as a liquid. At this pressure, Triple Point, CO2 can exist as liquid, gas and solid. Below this pressure it will flash to a gas and solid. CO2 above a temperature of 87.9 dF Critical Point cannot exist as a liquid.
Normally liquid CO2 is delivered and maintained at 0 dF and 300 psi.


Solid Carbon Dioxide
Solid CO2 is commonly referred to as Dry Ice. It is produced by allowing liquid CO2 to expand to atmospheric pressure which forms dry ice snow. This snow is then compressed to form blocks and pellets. Dry ice at atmospheric pressure is -109 dF.
From PyramidAir.com (please note that Pyramid Air is discussing the specific effects of CO2 in a pellet gun as opposed to a paintball gun. As the two are almost identical in operation, the PRINCIPAL is fully applicable to any discussion we have regarding CO2 efficiency)...

Quote:
CO2 and Pressure
CO2 is a gas at temperatures above -69.9 degrees F and 60.4 psig (pounds per square inch gauge). It is a very complex compound with the ability to sublimate (change directly from a solid to a gas without becoming a liquid) as just one of its unique properties.

At 70 degrees F, CO2 obtains a gas pressure of 852.8 psi when confined in a vessel. If there is more CO2 in the vessel, it will be have to be in liquid form. So, the state of CO2 in a pressure vessel, such as a powerlet at room temperature, is a pressurized gas above a liquid. If the gas is released, such as through the operation of an airgun valve, some of the remaining liquid flashes to gas until the pressure is equalized for that temperature.

It's important to understand that CO2 pressure is determined by temperature, not by mechanical compression. If you were to compress gaseous CO2 by mechanical means, it would turn into liquid when the right pressure was reached. The pressure in a 12-gram powerlet remains constant until all the liquid is gone. A powerlet has the same internal pressure as a 10-oz. bulk CO2 tank when both are at the same temperature. Therefore, CO2 guns do not lose velocity as you shoot them until all the liquid is gone and they start to run out of gas.

Also, keep in mind that CO2 is a refrigerant gas. That means it cools when it expands by flashing from liquid to gas. Therefore, when you shoot a CO2 gun rapidly, the gas will cool the gun parts considerably. Because CO2 pressure is based on temperature, the pressure in a CO2 gun will drop if a series of shots are fired in rapid succession. In practical testing, I've seen velocities decrease by more than 100 f.p.s. over a long string of shots. That will affect where the pellet strikes the target unless it's very close to the shooter. So, if you want to shoot accurately with a CO2 gun, do not shoot rapid-fire. With a target pistol, I like to allow at least 15 seconds between shots so the gun's temperature can cycle back to where it was before the shot. But, if you're just plinking, you can shoot faster than that.

DOT regulations require the use of a burst disk in pressure vessels larger than two inches in diameter. The brass nut with the hole in the side contains the burst disk in this bulk CO2 tank. If pressure inside the tank rises above the safety level, the disk ruptures, releasing all the gas inside. That keeps the entire bulk tank from exploding with the force of a bomb.

On a very hot day, CO2 pressure will climb rapidly into the danger region. Where that danger region is, depends on how much liquid is in the pressure vessel. Larger CO2 tanks have pressure-relief devices for safety; so, instead of the whole tank blowing apart like a hand grenade, the burst disk will rupture and exhaust all the gas. When this happens, it's very startling to anyone nearby, and the tank has to be repaired before it will hold CO2 again. Obviously, it's unsafe to leave a CO2 gun or a tank in a closed car on a hot day.
So, what have we learned here?

1) CO2 can exist as a gas, liquid or solid. CO2 gas turns into a liquid when it is compressed and cooled. It will sublimate ("boils", "flashes") back into a gas (or solid) when it reaches a pressure of just over 60 PSI, and/or temperatures over 70F.

2) CO2 pressure is determined by temperature, not by mechanical compression.

When Scharfschutze91 states that he's getting "around 40 - 45 shots off of a 12 gram shooting around 290-300 fps", is it because he's using a Freak-bored barrel and a red main spring? Or is it because he lives in Miami, Florida and was shooting in 90 heat with 80% humidity?

If Pauly took Scharfschutze91's Phantom from Miami to his house in Long Island New York today, and shoots it in 41 weather with 30% humidity, do you think he'll get 40 shots off of a 12-gram?

No, because the cold temperature in NY will decrease the overall pressure (i.e. efficiency) of the 12-gram. While the mods in Scharfschutze91's Phantom may garner an extra shot or two, it's the temperature that is making the real difference. Also, the temps and lower humidity in NY will keep the paint from swelling up slightly, which may allow "blow-by" even with inserts or a bore-matched barrel.

If Pauly waited till the dead of Summer when it's 90 out with high humidity on Long Island, then I'm sure he would see similar results as Scharfschutze91, but at 90, even an unmodified Phantom will see a noticeable increase in efficiency. Why? Because CO2 pressure is determined by temperature.

3) At 70 F, the pressure of an unopened 12-gram CO2 Cartridge is 852.8 psi.

4) CO2 is a cryogenic liquid (gas) which dramatically cools down the valve system in the Phantom. Therefore, the pressure will drop dramatically when a series of shots are fired in rapid succession.

Another critical un-answered question from the scenario above involving Scharfschutze91's 40 shots off of a 12-gram: how much time elapsed in between shots? Without any mods, a factory Phantom will seem comparatively efficient if a sufficient amount of time is given for the gun to warm up in between shots (thus allowing the CO2 to heat up and expand into a gas). Add all the efficiency mods you want. If you don't pause between shots to let the gun/CO2 warm up, then the mods will have little effect.

Now that we've gone over the most basic principals and properties of CO2, we can discuss Phantom efficiency with a far better understanding of how potential mods may, or may not work.
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Quote:
Originally Posted by Axel View Post
Slim doesn't know what he's talking about.

Last edited by Slim; 03-07-2013 at 11:45 AM.
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