At a depth of 26 msw (85 fsw), what is the oxygen partial pressure in an EAN35 mix?

To determine the oxygen partial pressure at a depth of 26 meters sea water (msw) using an EAN35 (Enriched Air Nitrox with a 35% oxygen content) mix, first, we need to understand how partial pressure is calculated in relation to depth.

At sea level, the pressure is 1 atmosphere absolute (ata). As you descend underwater, the pressure increases due to the water above you. The formula for calculating the total pressure at a specific depth in water is:

Total pressure (ata) = surface pressure + (depth in meters / 10)

In this case:

• Surface pressure is 1 ata.
• Depth of 26 msw adds approximately 2.6 ata (since there is roughly 1 ata for every 10 meters of seawater).

So, at 26 msw, the total pressure is:

Total pressure = 1 ata + 2.6 ata = 3.6 ata

Next, to find the partial pressure of oxygen, we multiply the total pressure by the fraction of oxygen in the EAN35 mix (0.35):

Oxygen partial pressure = Total pressure × Fraction of oxygen Oxygen partial pressure = 3.6 ata × 0.35