What Is Nitrox?
Nitrox — also called enriched air nitrox (EANx) — is a breathing gas with a higher oxygen percentage than standard air. Regular air is approximately 21% oxygen and 79% nitrogen. The most common nitrox mixes are EAN32 (32% O2) and EAN36 (36% O2). By replacing some of the nitrogen with oxygen, nitrox reduces the amount of nitrogen your body absorbs at depth, which extends your no-decompression limits and shortens surface intervals.
Why Nitrox Matters for Your Computer
A dive computer set to air (21% O2) calculates your nitrogen loading based on the assumption that 79% of your breathing gas is nitrogen. If you are actually breathing EAN32, only 68% is nitrogen — your computer is overestimating your nitrogen absorption and giving you shorter NDLs than necessary. Conversely, if your computer is set to nitrox but you are breathing air, the computer underestimates your nitrogen load, which is dangerous.
Setting the correct O2 percentage is the single most important pre-dive configuration step when diving nitrox.
Setting Your Computer for Nitrox
Analyze your tank. Use an oxygen analyzer to measure the exact O2 percentage in your tank before every fill. Never assume the fill is what you ordered.
Enter the O2 percentage. Navigate to gas settings on your computer and set the analyzed percentage. Most computers accept values from 21% to 50% (recreational range) or 21% to 100% (for technical oxygen deco gas).
Verify the MOD (Maximum Operating Depth). Your computer will display the MOD for the entered O2 percentage. The MOD is the deepest depth at which you can safely breathe that mix before exceeding the recommended oxygen partial pressure limit (typically PO2 1.4 for working dives, 1.6 for decompression stops). For EAN32, the MOD at PO2 1.4 is approximately 33 meters (110 feet). For EAN36, it is approximately 29 meters (95 feet).
Do not exceed your MOD. Breathing a nitrox mix deeper than its MOD risks oxygen toxicity, which can cause sudden seizures underwater — a life-threatening emergency. Your computer will alarm if you approach or exceed the MOD.
How Computers Track Oxygen Exposure
Your computer monitors two oxygen-related limits:
PO2 (partial pressure of oxygen): The instantaneous oxygen pressure at your current depth. Your computer alarms when PO2 approaches 1.4 or your MOD.
CNS% (Central Nervous System oxygen toxicity): A cumulative tracker of your total oxygen exposure across a dive or dive day. CNS% is based on the NOAA oxygen exposure tables. Your computer accumulates CNS% during each dive and displays it. Exceeding 100% CNS% significantly increases the risk of oxygen toxicity. Most computers warn at 75% and alarm at 100%.
OTU (Oxygen Tolerance Units): A cumulative measure of pulmonary (lung) oxygen toxicity used for multi-day diving. Some computers display OTU; others only track CNS%. OTUs are primarily a concern for technical divers doing repetitive deep nitrox or oxygen-decompression dives.
Common Nitrox Mistakes
Forgetting to change the O2 setting between fills. If you dive EAN32 in the morning and get an air fill for the afternoon, you must reset your computer to 21%. Some computers auto-reset to air after 24 hours; others retain the last setting.
Not analyzing your own tank. Even if the dive shop analyzed it, verify with your own analyzer or at least witness the reading. A mis-labeled tank is a serious hazard.
Exceeding MOD. EAN32 has a shallower safe depth limit than air. Going deeper than your MOD on nitrox is more dangerous than accidentally going deep on air.
Diving nitrox without certification. Nitrox diving requires a separate certification (Enriched Air Diver / Nitrox Diver) from your training agency. The course covers gas analysis, MOD calculation, and oxygen toxicity management.