Maximum Operating Depth (MOD): what it is and how to use it safely
MOD tells you the maximum depth for a given gas mix before oxygen exposure becomes unsafe. This guide explains the concept in simple terms, with clear examples.
- MOD defines the maximum safe depth for a gas mix.
- It depends on oxygen percentage and chosen ppO₂ limit.
- Used correctly, it helps prevent oxygen toxicity.
- What is MOD?
- Why MOD matters
- MOD formula explained
- Practical examples
- Nitrox MOD table
- Common mistakes
- Quick checklist
- Gas mix: EAN32
- ppO₂ limit: 1.4
- Resulting MOD: approximately 33–34 meters
- Example 1: EAN36 is suitable for shallower dives due to higher oxygen content.
- Example 2: Air allows deeper MOD but offers less decompression benefit.
- Example 3: Technical mixes require strict labeling and verification.
- Using the wrong ppO₂ value during planning.
- Assuming MOD is the same for all dives.
- Failing to label or verify gas mixes.
- ✅ Confirm gas mix
- ✅ Choose appropriate ppO₂ limit
- ✅ Calculate MOD
- ✅ Set dive computer accordingly
- ✅ Stay within planned depth
What is MOD?
Maximum Operating Depth (MOD) is the deepest depth at which a specific breathing gas can be safely used without exceeding a chosen oxygen partial pressure (ppO₂).
Why MOD matters
Exceeding MOD increases the risk of oxygen toxicity, which can have serious and immediate consequences underwater. MOD is a core concept in dive planning for both recreational and technical diving.
MOD formula explained
MOD is calculated using the oxygen fraction of the gas and a selected ppO₂ limit. Divers choose conservative limits based on training and dive conditions.
Metric (metres): MOD = 10 × (ppO₂ ÷ FO₂ − 1)
Imperial (feet): MOD = 33 × (ppO₂ ÷ FO₂ − 1)
ppO₂ = your chosen oxygen partial-pressure limit (1.4 bar working, 1.6 bar contingency); FO₂ = the oxygen fraction of the mix (for example 0.32 for EAN32).
Worked example − EAN32 at ppO₂ 1.4: 1.4 ÷ 0.32 = 4.375; − 1 = 3.375; × 10 = 33 m (rounded down, matching the table below).
Practical examples
Nitrox Maximum Depth Chart — MOD by mix (ppO₂ 1.4 / 1.6)
Ready-to-use nitrox depth limits — the maximum safe depth (MOD) for each EANx mix. For example, the maximum depth for 32% nitrox (EAN32) is 33 m at ppO₂ 1.4 and 40 m at 1.6. Depths are metres of seawater, rounded down to stay conservative.
| Gas mix | MOD @ ppO₂ 1.4 | MOD @ ppO₂ 1.6 |
|---|---|---|
| Air (21% O₂) | 56 m | 66 m |
| EAN28 | 40 m | 47 m |
| EAN30 | 36 m | 43 m |
| EAN32 | 33 m | 40 m |
| EAN34 | 31 m | 37 m |
| EAN36 | 28 m | 34 m |
| EAN38 | 26 m | 32 m |
| EAN40 | 25 m | 30 m |
| EAN50 | 18 m | 22 m |
| EAN80 | 7 m | 10 m |
| Oxygen (100% O₂) | 4 m | 6 m |
Common mistakes
Quick checklist
FAQ
Is MOD the same for every diver?
No. MOD depends on training standards, chosen safety margins, and dive objectives.
Can I exceed MOD briefly?
No. MOD should be treated as an absolute limit, not a guideline.
Does my dive computer calculate MOD automatically?
Most modern dive computers do, but the diver is still responsible for planning and verification.
What is the MOD of 32% nitrox?
The MOD of EAN32 (32% oxygen) is 33 m (108 ft) at the standard ppO₂ limit of 1.4, or 40 m (131 ft) at the contingency limit of 1.6. See the full nitrox MOD table above.
What is the MOD for 36% nitrox?
The MOD of EAN36 (36% oxygen) is 28 m (92 ft) at ppO₂ 1.4, or 34 m (112 ft) at ppO₂ 1.6.
How do you calculate MOD for nitrox?
MOD = 10 × (ppO₂ ÷ FO₂ − 1), in metres of seawater. Divide your chosen ppO₂ limit by the oxygen fraction of your gas mix, subtract 1, then multiply by 10. See the MOD formula explained section for worked examples.
Dive computers, slates, and nitrox analyzers can help you apply MOD principles in real dives.
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