What is the partial pressure of nitrogen in air at 99 fsw?

To determine the partial pressure of nitrogen in air at 99 feet of sea water (fsw), it is important to first understand how pressure changes underwater, particularly with the effects of depth and the composition of air.

At sea level, the total atmospheric pressure is approximately 1 atmosphere (ata), and air consists of about 78% nitrogen. When diving, for every 33 feet of seawater, the pressure increases by about 1 ata due to the weight of the water above.

At 99 fsw, the pressure can be calculated as follows:

1. Calculate the pressure due to depth:

• The water adds approximately 3 ata (since 99 fsw is roughly 99/33 = 3, hence 3 ata from water).

2. Add the atmospheric pressure at the surface to this:

• 1 ata (surface pressure) + 3 ata (water pressure) = 4 ata total pressure at 99 fsw.

Now, to find the partial pressure of nitrogen:

• Since nitrogen makes up approximately 78% of the air, the partial pressure of nitrogen can be calculated as:

• 0.78 (fraction of nitrogen) multiplied by 4 ata (total pressure)