Nitrox Maximum Operating Depth, Best Mix, and Oxygen Limits

More oxygen buys less inert gas and longer no-decompression time, and it buys a hard depth ceiling in exchange. EAN32 has a maximum operating depth of 111 ft at the 1.4 ata working limit and 132 ft at the 1.6 contingency figure. At 80 ft that same mix is at 1.096 ata -- inside the working limit with 31 ft of depth margin still in hand. THE TWO LIMITS ARE NOT INTERCHANGEABLE. The 1.4 working limit covers the active portion of a dive and is chosen with margin because exertion, cold, and carbon dioxide retention all raise susceptibility, and because oxygen toxicity at depth presents as a seizure with no reliable prodrome. The 1.6 figure is a contingency and decompression value used at rest, and treating it as a working limit removes the margin that exists precisely because the failure mode underwater is drowning. BEST MIX IS WHAT A SUPERVISOR ACTUALLY USES WHEN BLENDING, and the rounding runs one way only. At 80 ft the exact answer is 40.9% oxygen, so the blend is EAN40 at 1.370 ata -- because EAN41 there is 1.404, past the very limit it was solving for. A blend figure rounds DOWN. Whatever it returns, the mix must be analysed before use: the number on the cylinder is a label and the analyser is the fact. On air the same arithmetic gives 187 ft, which is why air diving past that depth is a decompression and oxygen-exposure planning problem rather than a casual one. A partial-pressure calculation, not an oxygen exposure plan. It does not track cumulative oxygen exposure over a dive or a series, which is a separate limit with its own tables and units, and it does not address the pulmonary toxicity long exposures at lower partial pressures produce. It does not evaluate a decompression obligation, gas blending procedure, or oxygen service cleanliness. The operation diving safety manual, the applicable training agency or regulatory limits, a gas analysis before every dive, and the diving supervisor govern.

ppO2 = the oxygen fraction x the absolute pressure (1 + depth / 33 seawater); maximum operating depth = 33 x (the ppO2 limit / the oxygen fraction - 1); best mix for a depth = the ppO2 limit / the absolute pressure, FLOORED to a blendable whole percent.

The oxygen partial-pressure relations by name, with 1.4 ata as the customary working limit and 1.6 ata as a contingency and decompression figure used at rest. The operation's diving safety manual, the applicable training agency or regulatory limits, a gas analysis before every dive, and the diving supervisor govern.

Partial-pressure arithmetic on the user's own mix and depth; no agency table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: oxygen_fraction, ppo2_limit, contingency_ppo2_limit, planned_depth_ft, feet_per_atm, depth_ata, ppo2_at_depth, mod_working_ft, mod_contingency_ft, best_mix_fraction, best_mix_pct_floor, best_mix_ppo2, air_mod_ft

• 1.4 working, 1.6 contingency and treating the contingency figure as a working limit removes the marginnitrox practice
• A blend figure rounds DOWN rounding up puts the diver past the limit being solved forgas blending practice
• The analyser is the fact the number on the cylinder is a labelgas analysis practice