Oxygen Percentage (FO2) — What It Is and Why It Matters
Oxygen percentage, technically called the fraction of oxygen (FO2), is the proportion of oxygen in a breathing gas, expressed as a percentage or decimal. Standard air has an FO2 of 0.21 (21%). Enriched air nitrox typically uses 0.32 (32%) or 0.36 (36%). Technical divers may use oxygen-lean mixes as low as 10% for deep bottom gases, or pure oxygen (100%) for shallow decompression stops.
Knowing your FO2 is critical for calculating your maximum operating depth and managing oxygen toxicity. Divers must personally analyze their gas mix before every dive and set the correct FO2 in their dive computer. Never assume the fill is what you requested — always analyze.
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Frequently Asked Questions
How do I check my oxygen percentage?
Use an oxygen analyzer before every dive. Place the analyzer's sensor over the tank valve and slowly crack the valve to flow gas across the sensor. The reading tells you the exact FO2. Many dive shops have analyzers available at the fill station.
What happens if I set the wrong FO2 in my computer?
If you set a lower FO2 than actual, your computer will underestimate oxygen toxicity risk, potentially allowing you to exceed safe ppO2 limits. If you set a higher FO2, your computer will underestimate nitrogen loading. Both errors are dangerous. Always analyze and double-check.
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