Why EAN32 and EAN36 Became Common Nitrox Mixes

EAN32 and EAN36 are common nitrox mixes because their oxygen percentages are convenient for many recreational profiles and became widely supported by tables, training and filling practice. They are not universally safe gases and NOAA’s use of standard mixes is not a guarantee for every depth or diver.

The higher the oxygen fraction, the lower the maximum operating depth for the same oxygen partial-pressure limit.

The MOD calculation

Maximum operating depth is calculated from the chosen oxygen partial pressure and the analysed oxygen fraction:MOD = (ppO₂ ÷ FO₂ − 1) × 10 metresThe result should be rounded conservatively and applied according to training procedures.

EAN32

For EAN32, FO₂ = 0.32. At a working ppO₂ of 1.4 bar:(1.4 ÷ 0.32 − 1) × 10 = 33.75 mA practical MOD is therefore about 34 metres. At 1.6 bar the mathematical value is 40 metres, but 1.6 is commonly treated as a contingency limit rather than a routine working target.

EAN36

For EAN36, FO₂ = 0.36. At 1.4 bar:(1.4 ÷ 0.36 − 1) × 10 = 28.9 mThe practical MOD is about 29 metres. At 1.6 bar the mathematical value is about 34 metres. EAN36 is therefore shallower than EAN32, not deeper.

Why the mixes are useful

EAN36 provides a larger reduction in nitrogen fraction at moderate depth but reaches the oxygen limit sooner. EAN32 has a broader depth range and is common for repetitive recreational diving. The best mix is selected for the planned maximum depth and exposure, not by choosing the highest oxygen percentage available.

The diver’s responsibilities

• Analyse the cylinder personally or witness the analysis.
• Confirm and label the oxygen percentage and MOD.
• Set the computer to the analysed mix.
• Track oxygen exposure and respect the planned ppO₂ limit.
• Use oxygen-clean equipment and filling procedures where required.

Sources and further reading

• NOAA Diving Tables
• DAN — Oxygen toxicity