What is the maximum operating depth (MOD)?
Oxygen becomes toxic when its partial pressure gets too high. The partial pressure (ppO₂) is the fraction of oxygen in your gas multiplied by the absolute pressure around you, so it rises as you go deeper.
The maximum operating depth is the depth at which the ppO₂ of your mix reaches the limit you have chosen. Nitrox mixes with more oxygen give longer no-decompression times, but a shallower MOD.
The formula is: MOD = (ppO₂ limit ÷ oxygen fraction − 1) × 10 m, or × 33 ft in feet.
What is the best mix for a planned depth?
The best mix is the richest nitrox blend you can breathe at your planned depth without going over the limit. Richer means longer no-decompression times.
The formula is: oxygen fraction = ppO₂ limit ÷ (depth ÷ 10 + 1) for metres, or ÷ (depth ÷ 33 + 1) for feet.
The result is rarely a whole number, and gas is blended to whole percentages. The calculator rounds down, because a slightly richer blend would go over the limit at your planned depth.
Why 1.4 bar and 1.6 bar?
Most recreational training uses a working limit of 1.4 bar for the dive itself. A limit of 1.6 bar is treated as a contingency limit, for example for a short time during decompression or in an emergency, and not for planned bottom time.
A lower limit such as 1.2 bar adds margin, which some divers choose for long dives, hard work or cold water. Your training agency and your instructor tell you the limits that apply to you.
Worked example
You have an EAN32 cylinder and use the 1.4 bar limit. The oxygen fraction is 0.32, so 1.4 ÷ 0.32 = 4.375 bar absolute. Subtract 1 bar for the surface and multiply by 10 m per bar: 33.75 m. The calculator rounds down and shows 33.7 m (111 ft).
Going the other way: you plan a dive to 30 m. Absolute pressure is 30 ÷ 10 + 1 = 4 bar, so the best mix is 1.4 ÷ 4 = 35% oxygen. EAN35 has a MOD of exactly 30 m at 1.4 bar.
MOD of common nitrox mixes
These figures use the same calculation as the calculator above, rounded down.
| Mix | MOD at 1.4 bar | MOD at 1.6 bar |
|---|---|---|
| EAN28 | 40 m / 132 ft | 47.1 m / 155 ft |
| EAN30 | 36.6 m / 121 ft | 43.3 m / 143 ft |
| EAN32 | 33.7 m / 111 ft | 40 m / 132 ft |
| EAN36 | 28.8 m / 95 ft | 34.4 m / 113 ft |
| EAN40 | 25 m / 82 ft | 30 m / 99 ft |
Frequently asked questions
What does MOD stand for?
MOD stands for maximum operating depth: the deepest depth at which the oxygen in your gas stays within your partial pressure limit.
Is 1.6 bar safe?
It is accepted as a contingency limit, for example during decompression stops or an emergency, but not for the planned bottom part of a dive. The usual working limit is 1.4 bar.
Why is the best mix rounded down?
Gas is blended to whole percentages. A blend with more oxygen than the exact answer would go over your limit at the planned depth, so the calculator always rounds down.
Does this work at altitude or in fresh water?
No. The figures assume sea level and salt water. Fresh water and altitude change the pressure at a given depth, so ask your instructor or use a dive computer set for those conditions.
Can I use this for trimix or rebreathers?
No. This calculator covers nitrox only, which is oxygen and nitrogen. Helium mixes and rebreather set-points need different tools and training.