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Eric d'Souza wrote:
>
> Hi George,
>
> I've been reading your recent posts with great interest. They have been
> very informative. I'd be grateful if you could explain some points to a
> non techie.
>
> 1) Is the main reason for a 40m (130 ft) limit on aed due mostly
> to Nitrogen or O2 ?
REPLY -
If we relate everything to "air equivalent", that is nitrogen at
79% and oxygen at 21%, we find 130 to be an easy demarcation that has
been recognized for a long time as the beginning of the "trouble zone",
or the area beyond which the ratio of accidents to dives shows a marked
icrease. We use it as a MAXIMUM, and recognize that less is best is
nitrogen. Personally, I usually dive a 90-100 foot aed. The presence of
helium in any mix also reduces the micrcirculatory damage associated
with nitrogen tension, the subject of a whole other debate relating to
dcs , bone necrosis, and other situations.
The same applies to oxygen: the 1.4 partial pressure is that line
beyond which the seizure incidence rises markedly. There are more
considerations to this, and time of exposure is the big one. I use 1.4
MAXIMUM, and reduce my ppo2 according to time. For my dives, I like 1.0
ppo2.
The perceived savings of higher ppo2's on decompression is
illusory within the differential between safe and unsafe ppo2's, and is
a classic example of risk misidentification, for which the dive industry
is famous, and clearly has painted intself into several real squirley
corners. The damage due to pulmonary toxicity in nitrox must be weighed
against the few minutes of saved deco, and proper proceedure for
breaking from oxygen and nitrox must be employed to prevent this and cns
toxicity.
When is doubt, leave the nitrox out. Personally, I use it for deco
from extreme dives, for multiple short exposures ( like using 35% to
dive lobsters at 100 feet with 35 minute bottom times on repeats), and
for shallow cave dives where the bottom time would pump the air deco way
up. I would not do it just to do it.
Within a few short years you will be able to buy proper dive gas
mixtures at most dive stores, as PADI and NAUI get into tech, and you
will find that those mixtures will all be a trimix of helium, nitrogen,
and oxygen. This is just about all I use anymore, if I can help it.
I will be over at your BSAC conference this fall with Dr. Bill
Hamilton, and we will be armed with every piece of information you guys
could ever want. However, let me warn you that I got most of it from you
, the British. - G
> 2) Is Nitrox more likely to cause narcs at a given depth than air ?
>
> 3) Are the ppO2 's involved in diving with Nitrox 32 and 36 undesirable ?
>
> 4) Since Nitrox can only be used at shallow depths without ppO2
> problems, is there much of a case for it other than a stage deco mix ?
>
> cheers
>
> Eric
>
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