Equivalent air depth 974967 209717201 2008-05-02T16:05:33Z Gene Hobbs 3121896 add ref In [[technical diving]], the '''equivalent air depth (EAD)''' is a way of expressing the [[nitrogen narcosis|narcotic]] effect of specific [[breathing gas]] mixtures that contain [[nitrogen]], for example [[nitrox]] and [[trimix]].<ref>{{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 }}</ref><ref>{{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}}</ref><ref>{{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 }}</ref> 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. == Oxygen Narcosis == Since there is evidence that oxygen plays a part in the narcotic effects of a gas mixture,<ref name="pmid734806">{{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}}</ref> 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 ([[Outdated_theories_of_anaesthetic_action#Lipid_solubility|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 ==References== {{Reflist}} {{med-stub}} {{diving-stub}} [[Category:Underwater diving]] [[da:Ækvivalent luft dybde]] [[no:Ekvivalent luftdybde]]