Question

Why not only oxygen used in diving apparatus in deep sea diving?

Answer 1

To breathe in deep sea or underwater a diver is supplied with air at a pressure equal to that of the water surrounding the diver. The pure oxygen is not used for this purpose because there is an upper limit of oxygen pressure above which the oxygen becomes biochemically toxic. Therefore, it is necessary to maintain the optimum limit of oxygen pressure in a diver's breathing gas. To satisfy normal breathing requirements at high pressures, it is necessary to supply oxygen between 0.2-1.5 atm pressure with the the balance made up by a non-toxic diluent (such as nitrogen). Air is a suitable breathing mixture for a diver.