One of the most common questions beginning players ask is the difference between using Compressed Air (also referred to as Nitro, Nitrogen, N2, HPA or High Pressure Air) and CO2 tanks. In a nutshell, both provide pressure for a paintball marker to cycle and propel the paintball. However, both work on a different prInciple and sometimes only one can be used.
CO2 (Carbon Dioxide) was the first propellant used in paintball and set the standard for many years. These are filled with liquid CO2 which expands to create the pressure used for the marker. This pressure fluctuates due to elevation, temperature and other variables but the benchmark is 850 psi (Pounds per Square Inch).
Using CO2 has some positive advantages. The tanks tend to be smaller and lighter than HPA while yielding the same or more shots per fill. Facilities to have the tanks filled are generally easier to find as many gas/welding supply and fire extinguisher shops have the means to fill your tanks as well as sporting goods stores and paintball pro shops. The number one reason for CO2's enduring popularity is it's low cost. The tanks are very inexpensive, so a player can easily own several tanks, thus having plenty of air for a full day of fun.