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As a referee, I often get a lot of questions from new players about CO2. I also encounter a lot of confusion of the pressure that a tank holds, the output pressure, and its relation to the secondary regulator. Rather than provide an advanced guide full of conversion formulas and science I wanted to provide a simple and easy to understand guide for newer users.
The modern CO2 tank is the staple of recballers and fields across the United States. The extensive use of CO2 along with the increased appearance of fill stations are retail outlets, along with its reasonable cost, makes it a choice for thousands of players. Understanding CO2 is essential to understanding your marker, potential upgrades, and your own safety when playing.
Remember, CO2 is a liquid and a gas, and your tank might be filled with a combination of the two at any given time. Please play responsibly and use a chronograph for casual play. CO2's proneness to fluctuate in velocity makes it extremely important that you keep marker speeds in check!
When liquid CO2 enters the valve of your marker, it can cause erratic velocity spikes and damage o-rings. This is mostly characterized by pieces of CO2 "ice" coming out of the marker. This "ice" does not melt, but will sublimate (turn into a gaseous state immediately) and is very cold.
How do you avoid getting liquid into the marker? Many blowback guns come with angled ASAs to tip the marker backwards and help keep CO2 from going through the airline into the marker. You can take this even farther by holding your marker upside down, with the barrel pointing upwards. This keeps the liquid at the rear of the tank. An anti-siphon tube can be installed, which helps to only allow air into your marker. An expansion chamber or regulator can be purchased and installed on your marker as well.
What do you do if CO2 has entered the marker? Try holding it upside down and point your barrel toward the sky, at a sharp angle. Fire your marker around 10 times until the visible white jets decrease and you stop seeing "snow" exiting your barrel. Then just carry your marker upside down to prevent more CO2 from entering. You want to make this a practice, especially when playing "outlaw" ball. Since chronographs are (unfortunately) often overlooked, liquid CO2 usually raises the velocity of the ball substantially. A marker chronographed with air at around 290 Feet Per Second could chrono in at 315 when shooting liquid CO2.
Disposable Air Sources
Some types of tanks are pre-filled with air by a machine and sealed, packaged, and marketed. They can only be used once and cannot be refilled. These can be purchased for pump play, casual play in remote areas, or as backups to your main tank. There has been some talk about ways to refill these tanks on message boards - DO NOT ATTEMPT THIS. Not only will it probably not work, you could severely injure yourself in the process.
12 grams - Used primarly for pump action markers. 12 gram cartridges were the successor of the 8 gram CO2 "Sparklet" bulb used extensively in the 1950s for carbonation machines. After airguns used 8 grams running special adaptors, Crosman came up with the legendary 12 gram powerlet. Powerlets held more air and also included a bit of lubricant for the gun. Since then, a few different types of 12 grams have been produced. They differ mainly in the cap construction (for example, Brass Eagle cartridges seem to have a thinner cap and are easier to pierce, while Crosman powerlets seem to have a thicker cap that seals with the pierce pin). Essentially however, all 12 grams are created equal. Because most paintball players think in terms of ounces instead of grams, each cartridge holds about .40 ounces of CO2. Some markers come with built in adaptors for use of cartridges (most pistols, the Brass Eagle Blade/Blade Turbo). All 12 gram adaptors are standard ASA threaded and can be used in conjunction with expansion chambers and regulators. Off certain guns, a 12 gram changer can be mounted vertically.
Large Disposables - These tanks are essentially a 12g cartridge on steroids. Large disposable tanks include the Crosman Airsource (88 grams, about 3 ounces), the Pure Energy Spare Air (4 ounces), and 45 gram cartridges (primarily for use with the Real Action Marker series). The tank is pierced by a pierce pin and this may be connected to an ASA adaptor to mount to the marker. Make sure that the adaptor is properly set up before piercing the disposable bottle, or else all of the CO2 will escape and will leave you with a non-functional, cold cylinder of metal. Also keep in mind that these bottles do not hold much air, and so they should only be used with pump guns or in emergencies. The 4 ounce Spare Air, for example, may only get around 180 or less shots on a standard blowback marker.
Pin Valve Tanks
These are the mainstay of the paintball industry. They are simple, easy to maintain, quick to fill, and very common. They can be equipped with anti siphon tubes and on/off dials. They also come in a variety of shapes and sizes. Unlike HPA tanks whose size is measured on cubic inches of space and pounds per square inch the tank can hold, a CO2 tank's size is referred to by the amount of CO2 (the combination of liquid and air) that the tank is designed to hold. The smallest refillable CO2 cylinder available for paintball use holds 30 grams. The largest common tanks hold around 20 ounces. As a result, some tanks that hold the same amount of CO2 can be different sizes. For example, some 9 ounce CO2 tanks are long and narrow, while others look like smaller versions of 12 ounce tanks in the interest of comfort.
When you go to get your tank filled, remember that the fill operator cannot "top off" your tank - its entire contents need to be dumped so that the tank is chilled, and then it is filled. (Technically, a tank can be “topped off” if it can be frozen and if the exact weight of the tank is known but this would take time and is impractical) If you haven't played a lot of games with a tank, use up more of the air it contains before getting a fill. However if you're not sure, it's always best to fill the tank completely.**
Which type of tank you want to buy is up to you. Keep in mind the length, weight, and approximately how many shots you can get out of your tank when you look to purchase a tank. There's no way to tell how many shots you can get off any given tank, because of tons of factors (the type of gun, the amount the fill operator filled the tank to, outside temperature, secondary regulation) but here are a few LOOSE generalizations.
4 Ounce - 150 to 200 shots
9 Ounce - 450 to 500 shots
12 Ounce - 500 to 600 shots
20 ounce - 600 to 650 shots
Be extremely careful when buying used CO2 tanks! A tank might appear fine on the outside but the internal o-rings could be damaged. Certain CO2 tanks also must go through hydrotesting periodically, an essential safety procedure that tests the integrity of your tank. Using an old tank and trying to "risk it" might seem economical, but safety is not something that can be purchased.
Paintball is a very safe sport when the rules and regulations of the game and equipment are adhered to. A butcher's cleaver could maim or kill, but he knows how to responsibly use it for its purpose. You have the same responsibility to understand and exercise safety with your tank.
If your tank is larger than 9 ounces, it will need to be hydrotested periodically. Hydrotesting ensures that the integrity of your tank has stayed the same and that it can still be filled. However, this service is reasonably expensive and it might be more economical to simply buy a new tank. Do