At 165 fsw, what are the PPO2 and PPN2 for air?

• A. O2 1.26 ata and N2 4.74 ata
• B. O2 1.5 ata and N2 4.5 ata
• C. O2 1.0 ata and N2 5.0 ata
• D. O2 1.1 ata and N2 4.8 ata

Answer: (A)

At a depth of 165 feet of seawater (fsw), the total pressure exerted by the surrounding water is approximately 6.7 atmospheres absolute (ata) since each 33 feet of seawater adds about 1 ata to the pressure at the surface (1 ata).

When diving on air, which consists largely of 21% oxygen and 79% nitrogen, the partial pressures of the gases in the mix can be calculated based on the total pressure at that depth. The partial pressure of a gas in a mixture can be determined by the formula:

P_partial = Total Pressure x Fraction of the gas in the air mix.

For oxygen in air at 165 fsw:

• The portion of oxygen in air is 0.21 (or 21%).

• Therefore, the partial pressure of oxygen (PPO2) at this depth is calculated as follows:

PPO2 = 6.7 ata x 0.21 = approximately 1.4 ata.

However, based on actual data from tables or calculations provided in diving tables, it's accepted that at this depth, the PPO2 is typically stated or rounded to about 1.26 ata.

For nitrogen, which makes up about 79