FO₂ — Fraction of Oxygen (Nitrox notation)

The FO2 is the specific percentage of oxygen contained within your breathing gas mixture. This number tells you exactly how much oxygen you inhale during every breath. Divers use this value to calculate their maximum allowable depth because higher concentrations increase the risk of central nervous system oxygen toxicity. Most recreational Nitrox divers use a FO2 of 32% or 36% for standard dives. You will often see these values printed on dive computers after an oxygen analysis.

Always check your gas manually. I once saw a diver in Koh Tao assume his tank was 32% when it was actually 36% because he skipped the analyzer step. If you use a higher FO2, your depth limit drops significantly. You must verify the number before you descend into the water.