Equivalent air depth

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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.^[1]^[2]^[3]

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. Nitrogen 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.

[edit] Oxygen Narcosis

Since there is evidence that oxygen plays a part in the narcotic effects of a gas mixture,^[4] an alternative calculation of EAD may be preferred as it is more conservative than the above. In this analysis, it is assumed that the narcotic potentials of nitrogen and oxygen are similar. Although oxygen has greater lipid solubility than nitrogen and therefore should be more narcotic (Meyer-Overton correlation), it is likely that some of the oxygen is metabolised, thus reducing its effect to a level similar to that of nitrogen.

In this scenario, a trimix consisting of 40% helium, 20% oxygen, 40% nitrogen used at 60 metres would be considered as (20% O2) + (40% N2) = 60% of the narcotic potential of air. The pressure at 60 metres is 7 bar, so the trimix would have a narcotic effect equivalent to air at a depth of 32 metres. This is found by observing that 60% of 7 bar is 4.2 bar, which is the ambient pressure at 32 metres.

The general formula in metres may be expressed as:

EAD = (Depth + 10) x (1 - Fraction of helium) - 10