In technical diving, the equivalent air depth (EAD) is a way of expressing the narcotic effect of specific breathing gas mixtures that contain nitrogen, for example nitrox and trimix.

The equivalent air depth is the depth of a dive when breathing air that would have the same partial pressure of nitrogen as the breathing gas in question, which has a different proportion of nitrogen and is being breathed at a different depth. So, for example, a gas mix containing 40% nitrogen being used at 60 metres (200 feet) has an EAD of 25 metres (83 feet).

The equivalent air depth can be calculated, for metric depths, as follows:

EAD = ( ( (Fraction of N2 × ((Depth in metres / 10) + 1)) / 0.79 ) − 1) × 10

Working the earlier example, for a gas mix containing 40% nitrogen being used at 60 metres, the EAD is:

EAD = ((0.4 × (( 60 / 10 ) + 1) / 0.79 ) − 1) × 10

EAD = ((0.4 × 7 / 0.79 ) − 1) × 10

EAD = ((2.8 / 0.79 ) − 1) × 10

EAD = (3.544 − 1) × 10

EAD = 25.44 metres

So at 60 metres on this mix, the diver would feel the same narcotic effect as a dive on air to 25 metres.

The severity of nitrogen narcosis depends on the proportion of nitrogen in the gas mix and the depth of the dive. Nitorgen narcosis is a major factor limiting the depth of dives where air is breathed. Air consists of 79% nitrogen. Other gas mixes, such as trimix, heliox and nitrox, contain different proportions of nitrogen.

Diving

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