For a dive to 27 msw (90 fsw) with an oxygen partial pressure limit of 1.6 ata, what is the best mix of oxygen not to exceed 40%?

To determine the best mix of oxygen for a dive to 27 meters seawater (msw) or 90 feet seawater (fsw) while not exceeding an oxygen partial pressure limit of 1.6 ata, it's essential to first calculate the maximum partial pressure of oxygen (PPO2) at that depth.

At 27 msw, the total pressure is approximately 3.7 ata (which is the atmospheric pressure at sea level - approximately 1 ata, plus 2.7 ata from the water column). To find the maximum allowable oxygen fraction, we can use the formula:

PPO2 = Fraction of O2 × Total Pressure

Rearranging gives us:

Fraction of O2 = PPO2 / Total Pressure

Substituting in our values:

Fraction of O2 = 1.6 ata / 3.7 ata ≈ 0.432 (or 43.2%)

In this context, we need to select a mix of Nitrox that does not exceed this oxygen fraction and is less than or equal to 40% oxygen as per the criteria given. Since EAN40 consists of 40% oxygen, it perfectly aligns with the calculated limit while remaining below the maximum allowable fraction when