What is oxygen partial pressure (pO₂)?
Oxygen partial pressure (pO₂) describes oxygen's contribution to the pressure of a gas mix at depth. It is a key factor when diving with Nitrox.
The formula for oxygen partial pressure is:
pO₂ = (FO₂ / 100) × (depth in metres + 10) / 10
This simplified formula assumes 1 bar at the surface and an increase of 1 bar per 10 metres of depth. The results are calculation examples, not dive plans. Never round a MOD upwards when setting a depth limit.
Where:
- FO₂ = oxygen percentage (e.g. 32 for EAN32)
- Depth = current depth in metres
Example: Using EAN32 at 30 metres:
pO₂ = (32 / 100) × (30 + 10) / 10 = 0.32 × 4 = 1.28 bar
Oxygen toxicity – a critical safety limit
As oxygen partial pressure rises, so does the risk of oxygen toxicity (also known as oxygen poisoning). Excessive oxygen exposure can cause serious physiological reactions.
There are two types of oxygen toxicity:
- Central nervous system (CNS) toxicity: Risk depends on factors including oxygen partial pressure and exposure time. A planning limit is not a sharp boundary between safe and dangerous. There is a risk of oxygen-induced seizures underwater — a potentially fatal situation.
- Chronic toxicity: Prolonged exposure to elevated oxygen levels can irritate the lungs and cause long-term damage.
Training organisations such as PADI and ProNRC therefore set strict maximum oxygen partial pressure limits.
Maximum operating depth (MOD)
Maximum operating depth (MOD) is the calculated depth at which a specified oxygen partial pressure is reached. It is not a guarantee of safety and does not replace other training, equipment or planning limits.
The following two values illustrate the formula. The limit for an actual dive depends on your training and the conditions:
- pO₂ = 1.4 bar: Value used for the following calculations; not individual authorisation to dive
- pO₂ = 1.6 bar: Additional comparison value; not a recommendation for a planned recreational dive
MOD is calculated as follows:
MOD = (pO₂max / (FO₂ / 100)) × 10 − 10
Example calculations for common Nitrox mixes
EAN32 (32% oxygen):
- MOD at pO₂ = 1.4 bar: (1.4 / 0.32) × 10 − 10 = 33.75 metres
- MOD at pO₂ = 1.6 bar: (1.6 / 0.32) × 10 − 10 = 40 metres
EAN36 (36% oxygen):
- MOD at pO₂ = 1.4 bar: (1.4 / 0.36) × 10 − 10 = 28.89 metres (rounded)
- MOD at pO₂ = 1.6 bar: (1.6 / 0.36) × 10 − 10 = 34.44 metres (rounded)
These calculations show that increasing oxygen content makes maximum operating depth shallower, not deeper.
Why Nitrox doesn't let you dive deeper
A common misconception: Nitrox is not a deep-diving gas. In fact, the opposite is true:
- EAN32 and EAN36 have shallower MODs than air when using a pO₂ limit of 1.4 bar.
- Air diving is also subject to other depth and training limits. A depth calculated from oxygen partial pressure alone is not an allowable diving depth.
- Nitrox is used at shallow to moderate depths to benefit from potentially longer no-decompression limits.
Dives beyond your training require separate qualifications and planning; this article does not authorise them.
Putting it into practice – planning with MOD
When planning a Nitrox dive, you must:
- Know your planned depth — what will your maximum depth be?
- Respect your gas mix's MOD — do not dive below the MOD.
- Set your dive computer correctly — enter the correct gas mix.
- Analyse your gas — measure the actual mix before diving.
- Allow a safety margin — plan shallower than the absolute MOD limit to leave a margin.
Key safety guidelines
The following rules apply when using Nitrox:
- Never exceed the MOD — this is a critical safety rule.
- Analyse your gas before every dive — the actual oxygen percentage may differ.
- Check the cylinder label — confirm that the cylinder contains the right mix.
- Program your dive computer correctly — use the correct gas composition data.
- Know your emergency procedures — the response depends on the situation and your training; a general instruction to ascend does not replace emergency training.
Training and certification
Recognised training is essential for safe Nitrox diving. In our PADI Nitrox course or ProNRC Nitrox course you will learn:
- Detailed MOD calculations and their practical application
- Gas analysis and cylinder handling
- Dive planning and safety procedures
- Emergency situations and responses
Conclusion
Oxygen partial pressure and MOD are not arbitrary restrictions: they protect you from serious risks. Understanding and respecting these limits is fundamental to safe Nitrox diving.
For more on practical dive planning, read our article on Planning Nitrox dives.