Can Old Scuba tanks be used for CO2?

Which gas is used in scuba diving tanks?

• Answer:

  For normal "sport" diving, scuba tanks are filled with just plain old air! That's just the mix of gases you're likely breathing right now, and it contains 78% Nitrogen, 21% Oxygen, and 1% other gases. But nitrogen can cause problems for divers ("bends" and "narcosis"). So for technical diving (below 100 feet), divers sometimes use Nitrox, less nitrogen than normal air, and more oxygen. But even pure oxygen can be dangerous when breathed under pressure. In fact, breathing pure oxygen while scuba diving below 25 feet can be fatal! So for very deep technical dives, the inert gas helium is often added into the gas mix; which can be heliox (oxygen & helium), or trimix (oxygen, helium, and nitrogen).