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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]].

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 system|metric]] depths, as follows:

:EAD = ( ( (Fraction of N2 &times; ((Depth in metres / 10) + 1)) / 0.79 ) &minus; 1) &times; 10

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

:EAD = ((0.4 &times; (( 60 / 10 ) + 1) / 0.79 ) &minus; 1) &times; 10

:EAD = ((0.4 &times; 7 / 0.79 ) &minus; 1) &times; 10

:EAD = ((2.8 / 0.79 ) &minus; 1) &times; 10

:EAD = (3.544 &minus; 1) &times; 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. 


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[[Category: Chemistry]]

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