It’s essential that the Dive Supervisor can rapidly calculate changes to depth, ppO₂ or percentage of oxygen in Nitrox.

The good thing is that there is a rapid solution to calculating one of these elements if you have two of the other values

To calculate depth:

Depth in bar + 1 bar = Absolute Pressure (AP) = ppO₂ x 100 ÷ %

(AP) = 1.4 x 100 ÷ 40 = 140 ÷ 40 = 3.5 therefore 3.5 bar x 10  = 35 msw – 10 msw = 25 msw

Answer Depth is 25 msw

The maximum depth that can be achieved to keep the ppO₂ within limits on a 40/60 nitrox mix is 25 msw

To Calculate ppO₂

ppO₂ = AP x % ÷ 100

ppO₂ = 3.5 x 32 ÷ 100 = 1.12 bar

Therefore:- on a 32/68 Nitrox mix at a depth of 25msw the ppO₂ is 1.12 bar or 1120 mbar

To calculate O₂ percentage

% = ppO₂ x 100 ÷ AP

% =  1.4 bar x 100 = 140 ÷ AP (28.88 ÷ 10 = 2.88 + 1 = 3.88) AP  =  3.88

% =  140 ÷ 3.88 = 36

Therefore:- at a depth of 28.8 msw with a ppO₂ of 1.4 bar or 1400 mbar you would be breathing a 36/64 Nitrox mix the O₂ would be 36%