- Equivalent air depth
technical diving, the equivalent air depth (EAD) is a way of expressing the narcotic effect of specific breathing gasmixtures that contain nitrogen, for example nitroxand trimix. [cite journal |last=Logan |first=JA |title=An evaluation of the equivalent air depth theory |journal=US Naval Experimental Diving Unit Technical Report |volume=NEDU-RR-01-61 |date= 1961|url=http://archive.rubicon-foundation.org/3835 |accessdate=2008-05-01 ] [cite journal |author=Berghage TE, McCraken TM |title=Equivalent air depth: fact or fiction |journal=Undersea Biomed Res |volume=6 |issue=4 |pages=379–84 |year= 1979|month=December |pmid=538866 |url=http://archive.rubicon-foundation.org/2835 |accessdate=2008-05-01] [cite book |title=DAN Nitrox Workshop Proceedings |author=Lang, M.A. |year= 2001|publisher=Divers Alert Network |location=Durham, NC |pages=197 |url=http://archive.rubicon-foundation.org/4855 |accessdate=2008-05-02 ]
The equivalent air depth is the depth of a dive when breathing
airthat would have the same partial pressureof nitrogenas the breathing gas in question, which has a different proportion of nitrogenand 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 narcosisdepends 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, helioxand nitrox, contain different proportions of nitrogen.
Since there is evidence that oxygen plays a part in the narcotic effects of a gas mixture,cite journal |author=Hesser CM, Fagraeus L, Adolfson J |title=Roles of nitrogen, oxygen, and carbon dioxide in compressed-air narcosis |journal=Undersea Biomed Res |volume=5 |issue=4 |pages=391–400 |year=1978 |month=December |pmid=734806 |url=http://archive.rubicon-foundation.org/2810 |accessdate=2008-05-01] 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
trimixconsisting 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
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