Partial Pressure (ppO2) — What It Is and Why It Matters
Partial pressure is the pressure exerted by a single gas in a mixture, calculated by multiplying the fraction of that gas by the total ambient pressure. In diving, ppO2 (partial pressure of oxygen) is the critical value for managing oxygen toxicity. For example, breathing EAN32 (32% oxygen) at 30 meters (4 ATA) gives a ppO2 of 0.32 × 4 = 1.28 bar.
The recreational ppO2 limit is 1.4 bar during the active dive phase and 1.6 bar during decompression stops. Dive computers with nitrox support continuously display or monitor ppO2 based on your set gas mix and current depth, alerting you as you approach limits.
Frequently Asked Questions
How is partial pressure calculated?
Partial pressure equals the fraction of the gas multiplied by the total ambient pressure. At 20 meters (3 ATA) on air (21% O2), ppO2 = 0.21 × 3 = 0.63 bar. The same depth on EAN32 gives ppO2 = 0.32 × 3 = 0.96 bar.
Why does ppO2 matter more than just oxygen percentage?
Because oxygen toxicity depends on the absolute pressure of oxygen in your lungs, not just its percentage. 21% oxygen is perfectly safe at 10 meters but produces a dangerous ppO2 at extreme depths. Partial pressure accounts for the depth-dependent increase in gas pressure that percentage alone does not.
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