Air is a mixture of gases. And CO2 is also a gas. So why are they filled in different cylinders? Let’s figure this out.
Air tanks are available for 3000 and 4500 psi internal pressures, but CO2 tanks can handle only 800 psi maximum. And no matter what the temperature is outside, air will regularly come from a cylinder with a fixed pressure because air cylinders are equipped with a reducer. However, the CO2 tank is sensible to low temperatures.
Due to the operation of the air tank at high pressure, it must be tested for professional suitability every three years. Refueling with air is more expensive, and few places where you can refuel it to the required pressure.
Carbon dioxide tank
The boiling point of carbon dioxide is -70oС, it is extracted by cooling under pressure. And even if it is + 30C outside, it remains liquid in the cylinder. The fact is that the pressure formed by the vapors above the liquid does not allow it to evaporate further. The carbon dioxide cylinder is filled with liquid CO2 to about 40%, the remaining 60% of the volume is occupied by vapors that create pressure for the marker to work. However, CO2, like any liquid, is sensitive to temperature changes.
And as you can see, CO2 cylinders cannot be filled with air! They are not designed for this high pressure.