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Equivalent Narcotic Depth October 13, 2009

Posted by Chris Sullivan in Technical Diving.
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I’m glad to see that the Wikipedia definition of Equivalent Narcotic Depth uses (1-FHe) instead of FN2 as the basis of the calculation. The difference between these two is that the older way of using the partial pressure of Nitrogen leaves out the current theory that Oxygen is more narcotic than Nitrogen, and the current practice of assuming that their narcotic potential is equivalent, as it is believed that in practice Oxygen is less narcotic than the theory would suggest due to partial metabolism by the body.

So how does this change the END? So let’s use 20/40 trimix like the Wikipedia example. With this mix, 60 metres should feel like 32 metres. Using the old formula, where Oxygen is treated as equivalent to Helium rather than Nitrogen, the END would be .40/.79*70-10, or just over 25 metres. That’s quite a difference.

I also found a web site that took into account the theoretical narcotic properties of Helium in one method of calculation, which is considerably less than Nitrogen. I’m not convinced that is valid as at some of the depths people are diving at now (over 200 metres), the Narcosis would be intolerable.

For instance, in the Deon Dreyer accident at Bushman’s Hole, he was diving at 270 metres with 4/80 trimix. If Helium was counted at 23% of the Narcotic effect of Nitrogen, his END would have been 97.5 metres! That’s almost double the value if Helium isn’t counted.



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