Maximum Operating Depth (MOD)

The deepest depth a gas mix can be breathed safely, set by its oxygen fraction and the chosen PO2 limit; 33.7 m for EAN32 at PO2 1.4.

Maximum operating depth, or MOD, is the deepest point at which a given breathing gas can be used without exceeding a chosen oxygen partial pressure limit. Because oxygen becomes toxic to the central nervous system at high partial pressure, every mix richer than air has a hard depth ceiling that matters far more than any preference written into the dive plan.

MOD is computed from the oxygen fraction and the PO2 limit. In meters, MOD equals the PO2 limit divided by the oxygen fraction, minus 1, times 10. At the standard working limit of PO2 1.4 bar, EAN32 has a MOD of 33.7 m (110 ft) and EAN36 has a MOD of 28.9 m (95 ft). A limit of 1.6 bar is generally reserved for resting decompression, which is why pure oxygen is used no deeper than 6 m as a deco gas.

Nitrox and technical divers write the MOD on the tank in large numbers after analyzing the mix, and dive computers alarm when you approach it. Small habits keep the limit honest: analyze every cylinder yourself, round MOD calculations down rather than up, and on multi gas dives confirm each switch depth with your teammate before anyone touches a regulator. It is a limit you plan around, never a target.