Nitrox MOD explained: how to work out your maximum depth
The maximum operating depth, or MOD, is the deepest you should take a nitrox mix. The sum is short enough to do in your head; this guide shows it, with real numbers.
· 2 min read
What MOD is
Oxygen is only safe to breathe up to a limit of partial pressure, ppO₂. The deeper you go, the higher the pressure of the oxygen in your mix. The MOD is the depth at which that partial pressure reaches your limit. Below it, the risk of oxygen toxicity rises quickly.
Most divers use 1.4 bar as the limit for the working part of a dive and 1.6 bar at most for a brief contingency. Your training sets your own limits; use theirs.
The sum
In metres of seawater, with pressure rising about one bar every 10 m:
MOD = (ppO₂ limit ÷ oxygen fraction − 1) × 10
Take EAN32, which is 32 % oxygen, so the fraction is 0.32, at 1.4 bar. 1.4 ÷ 0.32 = 4.375 bar of total pressure. Subtract the one bar at the surface and multiply by 10: 33.75 m. At 1.6 bar the same mix gives 1.6 ÷ 0.32 = 5.0, so 40 m. Round down when you write it on the cylinder label, never up.
Two more examples are enough. EAN36 at 1.4 bar: 1.4 ÷ 0.36 = 3.889, minus one, times ten gives about 28.9 m. Air, at 21 %, at 1.4 bar: 1.4 ÷ 0.21 = 6.667, which is 56.7 m, deeper than any recreational dive.
The best mix for a depth
Turn the sum around. For a planned depth of 30 m the pressure is 4 bar, so the richest mix that keeps ppO₂ at 1.4 is 1.4 ÷ 4 = 0.35, which is EAN35. More oxygen means less nitrogen, which is why the richest mix within your limit is the one people mean by best.
Equivalent air depth
Equivalent air depth, or EAD, is the depth at which plain air would give you the same nitrogen pressure. For EAN32 at 30 m it comes to about 24.4 m. It is the depth you read an air table at. A dive computer set to the right mix does this for you.
What the number does not tell you
- It does not tell you what is in your cylinder. Analyse your gas yourself and write the result on the cylinder.
- It does not replace your training, your dive computer or tables, or an instructor.
- It does not cover decompression or the time you can stay at a ppO₂.
The nitrox calculator does the sum for any mix and any limit, using the same code as the Loggerfish app.
Plan your nitrox dives in Loggerfish
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