Calculating MOD, PO₂ and EAD When Diving Nitrox

What MOD, PO₂, and EAD Mean

Maximum Operating Depth (MOD) is the depth where a gas reaches the oxygen partial-pressure limit selected for the dive. It is a planning boundary, not a depth where risk suddenly switches on. Going deeper than the planned MOD raises PO₂ and increases concern about central nervous system oxygen toxicity, including the possibility of a convulsion underwater.

Partial Pressure of Oxygen (PO₂) is oxygen's share of the ambient pressure at your actual depth. The oxygen percentage in the cylinder stays the same, but ambient pressure and PO₂ rise as you descend.

Equivalent Air Depth (EAD) is the depth on air that would produce the same nitrogen partial pressure as the Nitrox mix produces at the actual depth. It does not mean your body has absorbed a particular amount of nitrogen, and it does not change your actual depth, gas consumption, or oxygen exposure. Modern Nitrox-capable computers make EAD largely a conceptual reference for recreational diving.

• Fraction of Oxygen (FO₂) is the oxygen portion of the gas expressed as a decimal: EAN32 has an FO₂ of 0.32.
• Fraction of Nitrogen (FN₂) is the remaining nitrogen portion for an oxygen-and-nitrogen mix: EAN32 has an FN₂ of 0.68.

A richer Nitrox mix has less nitrogen, producing a shallower EAD at the same actual depth. It also has more oxygen, so PO₂ rises and the MOD becomes shallower. MOD and EAD answer different questions; neither replaces planning around your real depth.

For EAN32 with a 1.4 ATA limit, the calculated MOD is about 111 feet and this tool rounds it down to 110 feet. At an actual depth of 80 feet, the calculated EAD is about 64 feet and the table rounds it up to 65 feet. The dive is still an 80-foot dive.

Analyze the cylinder you will breathe, set your dive computer to that measured mix, and compare its MOD with the planned maximum depth before entering the water. This table is a visual reference; it does not calculate exposure-time limits or replace a dive plan.