PaintballDeals.com gives the lowdown on Co2 including, its history in paintball, compatible paintball guns, and proper usage with paintball products.
CO2 paintball tanks are filled with hyper cooled liquid CO2. The liquid CO2 converts to gas within the tank this creates the pressures required to operate a paintball gun. In typical natural conditions, CO2 is a gas. In order to use CO2 as a propellant source it must be super cooled so it condenses into a liquid. Since the CO2 is in liquid form CO2 paintball tanks capacities are described in ounces. For example the most common CO2 paintball tank capacity is 20 ounces. Comparing this to a HPA compressed air tank you will notice that the capacity is described in cubic inches/ pounds per square inch. The volume of the tank over the pressure limit. The capacity for HPA tanks are described in these terms because compressed air does not change physical state. It remains a gas throughout the process of being compressed into a high pressure air tank and expelling the paintball.
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CO2
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HPA
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Carbon Dioxide is bottled by distributors, from the bottling center, large tanks are shipped to businesses that use CO2, such as welders, sporting goods outlets, and paintball fields. These large tanks have been filled with liquid CO2. The only way to tell if a CO2 tank has any CO2 left inside it is to press on the pin valve located at the top of the tank. The pin valve is at the very center at the top, it is depressed when the tank is screwed into the air source adapter (ASA) of the paintball gun.
Sometimes the pin valve gets stuck open when the tank is removed from a paintball gun but is not empty. This is not a defect, and will reduce in frequency with use. It may cause the o-ring which helps to create an air tight seal between the CO2 tank and the ASA.
When filling CO2 tanks, a technician will weigh the tank before, and after filling. The change in weight should be the capacity of the tank. If the difference before and after filling is less than the capacity of the CO2 tank then the tank was partially filled prior to being refilled to full capacity. For safety and practicallity reasons, all CO2 tanks must be filled by a qualified technician. CO2 paintball tanks have a regulator that generally outputs 800-850 psi. This output pressure can be effected by extreme temperatures. In cold weather the CO2 condenses and is not readily converted from a liquid to a gas. This reduces the amount of pressure within the tanks. The majority of electro-pneumatic paintball guns should not use CO2. CO2 is recommended for use with mechanical paintball guns and specially designed electric paintball guns like the HK Army SABR which has been engineered as a beginner electric paintball marker that can use both CO2 and compressed air without the risk of damaging internal electric components. In the majority of electro-pneumatic paintball guns there are electrical internal components, using CO2 could break those components as liquid CO2 can . Any paintball gun that is completely mechanical, or pump action, can use CO2 without any risk to the internal components. The majority of electro-pneumatic paintball guns should use compressed air as a propellant source.