DEFINE the partial pressure P of a gas in a mixture of gases as the product of the total pressure of the mixture (measured in ata) and the proportion that gas represents to the total mixture. Symbolically, P(r,d) = [1 + d/33] * r. Note that this applies only for salt water, where the depth d is measured in feet.
Example: the partial pressure of oxygen in a 40% Nitrox mix at 66 fsw is [1 + 66/33] * 0.4 = 1.2 ata.
Example: the partial pressure of nitrogen in a 32% Nitrox mix at 99 fsw is [1 + 99/33] * 0.68 = 2.72 ata.
Example: the partial pressure of nitrogen in air at 80.6 fsw is [1 + 80.6/33] * 0.79 = 2.72 ata.