Air or mixed gas diving is used at depths no greater than 75msw depending of the mix of gas used. At any greater depth than 50msw saturation diving is preferred to avoid the risks of decompression sickness and allow divers to work at depth for longer.
Air or mixed gas diving relies on the supply of breathing gas from the surface. Gases used may be oxygen, or a mixture of oxygen and nitrogen (Nitrox) or oxygen and helium (Heliox). Oxygen and nitrox is used a shallower depths (50msw) with heliox and provide a bottom time of no more than 50 minutes at such depth. Likewise, Heliox may be used down to 75msw however with only 30 minutes bottom time. Depending on the working depth divers may be lowered to depth in a basket or a wet bell.
Air or Nitrox diving requires a team of 5 (Diving Supervisor, working diver, stand-by diver, tender for working diver, tender for stand-by diver), whereas a team of 7 is required in Heliox dives (Diving Supervisor, working diver, stand-by diver, tender for working diver, tender for standby diver, short notice surface standby and tender)