A method and apparatus for a self contained underwater breathing apparatus in which a breathing gas is supplied to a flow loop from two separate gas sources each having a different oxygen fraction, and each controlled by separate mass flow controllers having variable flow rate. The mass controller flow rates are adaptively adjustable to deliver gas at variable flow rates which depend solely on a function of depth. An algorithm determines these specific flow rates from each of the tanks at particular depths, such that the gas flow from an oxygen rich gas source decreases as a function of depth, while the gas flow from a diluent gas source increases as a function of depth, so as to maintain the oxygen partial pressure in the flow loop within a specific pre-determined range. The algorithm allows calculation of an optimum oxygen partial pressure, for a particular dive, which allows construction of a dive profile which maximizes bottom time while taking into account no-decompression time at depth, tank capacity limited time, and single-dive and daily pulmonary oxygen toxicity limits.