Nitrox MOD and ppO2 Calculator

Why Oxygen Becomes Dangerous at Depth

At the surface, breathing air (21% oxygen) produces an oxygen partial pressure of about 0.21 bar. As you descend, pressure increases by 1 bar per 10 meters of depth, and the partial pressure of each gas rises proportionally. Pure oxygen at 6 meters is as dangerous as air at 30 meters, because both produce about the same ppO2.

Oxygen toxicity can cause convulsions underwater with no warning signs. The WKOF (working) limit for most dive agencies is 1.4 bar ppO2, and the absolute limit is 1.6 bar ppO2. Most sport divers stay at or below 1.4 bar.

The Formula

Oxygen partial pressure at depth: ppO2 = fraction_O2 x (depth/10 + 1)

Maximum operating depth (meters): MOD = (ppO2_limit / fraction_O2 - 1) x 10

For EAN32 (32% oxygen) with a 1.4 bar limit: MOD = (1.4 / 0.32 - 1) x 10 = (4.375 - 1) x 10 = 33.75 meters.

EAN32 vs EAN36

EAN32 (32% O2) is the most common nitrox blend. It extends no-decompression limits compared to air but limits maximum depth to about 34m at the working limit. EAN36 further extends NDL at shallow depths but limits depth to about 29m. No significant benefit using nitrox below its MOD versus just using air.

Oxygen Analyser

Always analyze your own tank before diving. Never dive based on what the label says. Blending errors happen. A 1% error in blend changes your MOD by roughly 1 to 2 meters.