At what depth does oxygen partial pressure reach 1.4 atm for recreational diving?

1. The claim everybody repeats — “air hits 1.4 ATA at ~56–57 m” and why the math is unavoidable

Multiple independent analyses use the same physical law—PO₂ = FO₂ × ambient pressure—and identical ambient pressure conventions (1 ATA at surface, ≈0.1 ATA per metre seawater or 33 ft per ATA). Plugging FO₂ = 0.21 for air into PO₂ = 1.4 ATA gives total ambient pressure ≈6.667 ATA, subtracting 1 ATA surface pressure leaves ≈5.667 ATA of water column, which converts to ≈56.7 m (≈186 ft). Several sources present this exact numeric result and frame it as beyond regular recreational depths, noting that this is a calculation, not an opinion ^{[2] [1] [5]}. Even sources that present tabulated MOD formulas show the same end point when back‑solved for air ^[6].

2. Where Nitrox changes the picture — shallower MODs for higher O₂ fractions

Calculations for Nitrox demonstrate how increasing FO₂ reduces the depth at which pO₂ hits 1.4 ATA. For Nitrox 32, setting FO₂ = 0.32 yields a total pressure ≈4.375 ATA, so net water column ≈3.375 ATA and a depth around 33.8 m (≈111 ft)—a result repeatedly cited in recreational diving guidance ^{[3] [4]}. For Nitrox 36 the same algebra gives roughly 28.9 m, and Nitrox 40 would be still shallower; the pattern is linear and predictable. Several analysis entries emphasize these specific Nitrox numbers and the underlying constraints on gas selection for dives that approach agency pO₂ limits ^{[4] [3]}.

3. Practical limits vs pure calculation — agency limits and recreational practice diverge

Although the pure physics places air’s 1.4 ATA at ~57 m, nearly all recreational training organizations set operational depth limits well shallower—commonly 30–40 m or 130 ft maximum for recreational certifications—because of narcosis, decompression risk and oxygen toxicity exposure profiles ^{[5] [3]}. Sources note that 1.4 ATA is an operational PO₂ ceiling for working phases of a dive, while 1.6 ATA is used for short exposures or contingency situations; agencies and NOAA materials referenced in the compendium echo these operational constraints even while the raw calculation stands ^[3]. Thus, the calculated MOD is not a recommendation to dive to that depth on air.

4. Conflicting numeric claims in community discussions and why they arise

Some community posts and secondary summaries report different depth numbers (e.g., claims around 127 ft or 184 ft) because of mixed unit bases (ft vs m), rounding, or misapplied conversion factors; nonetheless, when recalculated consistently those claims collapse to the same answer near 56–57 m for air or ~34 m for Nitrox 32 ^{[7] [8] [1]}. One outlier analytic entry gave an implausible figure of 1122 fsw (≈340 m) for air based on a misinterpretation or transcription error in a table—this is clearly inconsistent with Dalton’s law and other analyses and should be treated as erroneous ^[6]. The community variance thus reflects calculation mistakes or unit confusion, not disagreement about the underlying physics.

5. What divers should take away — calculations, training limits, and safe practice

The bottom line for operational planning is that PO₂ thresholds are a calculable physical fact but must be applied within agency depth limits, exposure tables, and conservative practice. For mission planning, use the simple formula PO₂ = FO₂ × ((depth/10 m)+1) or its feet equivalent, and apply the accepted working limit of 1.4 ATA (with 1.6 ATA for contingency) to compute MODs; for air that yields ~57 m and for Nitrox mixes the MOD will be shallower proportional to FO₂ ^{[2] [3] [5]}. Divers must also account for local policy, certification limits, gas analysis, and decompression planning—the arithmetic is necessary but not sufficient for a safe decision ^[3].