Nitrox Equivalent Air Depth and the Oxygen Check

EAN36 at 100 ft loads nitrogen like a 74.7 ft air dive -- 25 ft shallower, a large increase in allowable bottom time on an air table, and the clearest way to see what nitrox actually buys. Then run the oxygen check on the same mix at the same depth: 1.451 ata, past the 1.4 working limit. THIS MIX IS NOT USABLE AT 100 FT however attractive its equivalent air depth looks, and that pairing is the entire point. The same mix that extends the nitrogen clock brings the oxygen ceiling UP to meet you, and the two limits close on each other as the mix gets richer, so both are computed here on one set of inputs rather than left to a second calculation someone may not make. THE RICHEST MIX THIS DEPTH PERMITS IS EAN34, at 1.370 ata, whose equivalent air depth is 78.1 ft -- less extension, but usable. Note the rounding: the exact figure is 34.7% and a blend rounds DOWN, because EAN35 at 100 ft is 1.411 ata and over the limit. Holding the nitrogen partial pressure constant is the whole idea, and once the equivalent depth is known an air table or air algorithm applies without modification, which is why equivalent air depth came first historically. FOR TRIMIX THE RELATION DOES NOT APPLY as written, because helium is present and has its own kinetics; equivalent narcotic depth is a different calculation for a different purpose. A two-gas nitrox relation. It does not produce a no-decompression limit, model tissue compartments, or evaluate a decompression obligation, and it does not track cumulative oxygen exposure, which is a separate limit. It does not address gas blending, oxygen service cleanliness, or the analysis that must confirm the mix before every dive. The operation diving safety manual, the applicable tables, a gas analysis before every dive, and the diving supervisor govern.

EAD = ((1 - FO2) / 0.79) x (depth + 33) - 33 in seawater feet, which holds the nitrogen partial pressure equal to that of air at the equivalent depth; the oxygen check ppO2 = FO2 x (1 + depth / 33) is run on the same inputs, and the richest usable mix is the ppO2 limit / the absolute pressure, floored to a whole percent.

The equivalent air depth relation by name, for two-gas nitrox only; trimix needs a different treatment and equivalent narcotic depth is a different calculation. The operation's diving safety manual, the applicable tables, a gas analysis before every dive, and the diving supervisor govern.

Arithmetic on the user's own mix and depth; no dive table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: oxygen_fraction, depth_ft, target_ead_ft, ppo2_limit, feet_per_atm, nitrogen_fraction, equivalent_air_depth_ft, depth_reduction_ft, pn2_at_depth, ppo2_at_depth, deepest_usable_pct_floor, deepest_usable_ppo2, deepest_usable_ead_ft

• EAD and MOD are read together a plan built on one of them is not a plannitrox practice
• Two-gas nitrox only helium has its own kinetics and needs a different treatmenttrimix practice
• The usable mix is floored rounding up puts the diver past the oxygen limitgas blending practice