What is the partial pressure of oxygen in air at a depth of 21 msw (69 fsw)?

To determine the partial pressure of oxygen in air at a specific depth, it's essential to understand how pressure changes with depth underwater. The pressure increases by approximately one atmosphere (ata) for every 10 meters of seawater.

At a depth of 21 meters, the total ambient pressure can be calculated as follows:

1. The atmospheric pressure at sea level is approximately 1 ata.
2. The water pressure due to the depth can be calculated as 21 meters divided by 10 (resulting in 2.1 ata).
3. Therefore, the total pressure at 21 meters is approximately 1 ata (atmospheric pressure) + 2.1 ata (water pressure) = 3.1 ata.

Since air is composed of approximately 21% oxygen, the partial pressure of oxygen can be calculated by taking 21% of the total pressure at this depth:

Partial pressure of oxygen = Total pressure x Fraction of oxygen in air Partial pressure of oxygen = 3.1 ata x 0.21 ≈ 0.651 ata.

This result aligns closely with the choice provided. Therefore, the partial pressure of oxygen at 21 msw is approximately 0.65 ata, making it the correct answer