MOD: maximum operating depth

Your mix and your ppO2 limit: the tool gives you the depth you must never exceed.

Your mix

The O2 % from your analysis. With trimix, only the O2 counts for the MOD.

Oxygento 0.1 %

% O2

Your ppO2 limit

The one from your training agency or club. 1.4 bar is the most common.

Other value

bar

Where you dive

In a lake, fresh water and altitude change the MOD slightly.

Nx32 · ppO2 1.4 · sea water

33m

Do not go below 33 m with this cylinder.

Absolute limit (1.6) 40 mExact value 33.8 mMax pressure 4.38 bar

Write it on your cylinder

Nx32 · MOD 33 m · ppO2 1.4.

Planned depthppO2 1.28: OK

How it is calculated

We look for the depth where the oxygen partial pressure reaches your limit.

P max = ppO2 limit ÷ FO2

MOD = (P max − surface P) × m per bar

FO2 = O2 % ÷ 100. P max in bar absolute.
At sea: 1 bar at the surface and 1 more bar every 10 m. This is the convention used in French tables and courses.
In a lake: 1 bar every 10.2 m (fresh water is lighter), and less than 1 bar at the surface at altitude (about 0.90 bar at 1000 m, 0.80 bar at 2000 m).
The absolute limit is the MOD at 1.6 bar: you never aim for it, it is the margin in case of error.
Result rounded down to the metre, to stay on the safe side.
Sea example: EAN32 at 1.4 → 1.4 ÷ 0.32 = 4.375 bar → (4.375 − 1) × 10 = 33.75 m → 33 m.

A helper tool: it replaces neither your training nor your computer. Set your computer to the same mix and the same ppO2. At altitude, decompression also has its own rules.

For every mix

Tap a row to use it.

Mix	1.2	1.4	1.5	1.6
Air	47	56	61	66
Nx28	32	40	43	47
Nx32	27	33	36	40
Nx36	23	28	31	34
Nx40	20	25	27	30
Nx50	14	18	20	22
Nx80	5	7	8	10
Pure O2	2	4	5	6

MOD in metres for each ppO2 (bar), rounded down to the metre, in sea water.

Understanding MOD

What is the MOD?

The MOD (Maximum Operating Depth) is the deepest you can use a breathing mix. The deeper you go, the higher the pressure: 1 bar at the surface, then 1 more bar every 10 m. The oxygen partial pressure (ppO2) you breathe rises with it. Beyond your chosen limit, often 1.4 bar on the bottom, oxygen becomes toxic to the nervous system: that is the risk of an oxygen seizure. The MOD is the depth where your mix reaches that limit.

Which ppO2 should you choose?

Most divers use 1.4 bar for the bottom phase. 1.6 bar is the absolute maximum: it is kept for stops, at rest, or as a margin for error. Some clubs use 1.5 bar, or a more cautious 1.2 bar. Use the one from your training agency, and set the same in your computer. To check the ppO2 at a given depth and at the stops, use the PpO2 tool.

Calculate from your analysis, not the mix you asked for

A cylinder filled as EAN32 may hold 31.5 % or 32.8 % oxygen. Always calculate the MOD from the percentage measured by the analyser, to one decimal. Then write the mix and the MOD on your cylinder with the nitrox label tool. To pick the ideal mix for a dive, use the best mix tool; for nitrogen, use the EAD tool.

Lakes and altitude

In fresh water it takes 10.2 m instead of 10 m to gain 1 bar, so the MOD is slightly deeper. At altitude, surface pressure drops (about 0.90 bar at 1000 m) and the MOD gets longer still. The tool uses the standard atmosphere but never counts more than 1 bar at the surface, so it never lengthens the MOD by mistake.

Frequently asked questions

What is the MOD of EAN32?

33 m at 1.4 bar and 40 m at 1.6 bar, in sea water. Calculation: 1.4 ÷ 0.32 = 4.375 bar absolute, i.e. (4.375 − 1) × 10 = 33.75 m, rounded down to 33 m.

What is the MOD of air?

With 21 % oxygen, air reaches a ppO2 of 1.4 bar at 56 m and 1.6 bar at 66 m. In practice, narcosis limits air diving well before that.

Why round the MOD down?

To stay on the safe side: rounding up would take you a few centimetres past your ppO2 limit. The tool also shows the exact value to one decimal.

What is a hypoxic mix?

A mix with too little oxygen to breathe at the surface: below a ppO2 of 0.16 bar you can black out without warning. It can only be breathed from a certain depth and requires trimix training.