What is the formula for calculating the Maximum Operating Depth (MOD) for a specific Nitrox mix?

The Maximum Operating Depth (MOD) is a critical concept for divers using Nitrox, as it helps ensure that the partial pressure of oxygen remains within safe limits to avoid toxicity. The correct calculation for MOD is derived from the maximum partial pressure of oxygen (PPO2) that a diver can safely experience, multiplied by the fraction of oxygen in the mix, all adjusted for the ambient pressure at depth.

The formula for MOD is given as:

MOD = (Maximum PPO2 / Fraction of Oxygen) - 1 atm pressure.

This formula stems from the fact that, in diving, the ambient pressure increases with depth. The partial pressure of a gas is determined by the formula: PPO2 = Depth Pressure x Fraction of that gas. By rearranging the terms, one can determine the depth at which the partial pressure of oxygen equals the maximum permissible level.

The correct answer focuses on using the fraction of oxygen in the mixture because it directly relates to the amount of oxygen being breathed at depth. For example, if a diver has a Nitrox mix of 32% oxygen, then the fraction of oxygen would be 0.32. Thus, if the maximum PPO2 is set at 1.4 ATA, the MOD calculation correctly determines how deep the