IANTD Enriched Air (Nitrox) Practice Test

To determine the partial pressure of oxygen at a depth of 40 meters using a Nitrox mix of 36% oxygen, it is crucial to understand the relationship between depth, pressure, and gas composition.

At 40 meters of seawater, the ambient pressure is the sum of the atmospheric pressure plus the hydrostatic pressure due to the water column. Atmospheric pressure at the surface is approximately 1 ATA (atmospheric pressure at sea level). Underwater, for every 10 meters of depth, the pressure increases by an additional ATA. Therefore, at 40 meters, the total pressure is calculated as follows:

Total pressure = 1 ATA (surface pressure) + 4 ATA (pressure due to water column, as 40 meters divides by 10 meters/ATA gives 4)

Total pressure at 40 meters = 1 ATA + 4 ATA = 5 ATA.

Given that the Nitrox mix contains 36% oxygen, the partial pressure of oxygen can be calculated by multiplying the total pressure by the percentage of oxygen:

Partial pressure of oxygen = Total pressure × Fraction of oxygen

= 5 ATA × 0.36

= 1.8 ATA.

Thus, at a depth of 40 meters