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Alan Wright writes:
>David Giddy writes:
>
>> In the recreational Nitrox community I believe that the most common mixture
>> being promoted is NOAA Nitrox I (32% O2). This mixture reaches a PPO2 of 1.6
>> at 40m which is the recreational depth limit.
>
>I know I keep saying this but this isn't true world-wide. 40m is the
>recreational limit only for certain organisations. The assumption that
>most dives are done on NNI may be valid but there are a significant
>number done on 36% O2. You can't just ignore them.
I guess what I was suggesting, but didn't state, was that maybe 'recreational'
Nitrox SHOULD be limited to NOAA Nitrox I (32% O2).
>> At this PPO2, the O2 toxicity limit for CNS toxicity is 45 minutes (DCIEM
>> manual). As the recreational use of nitrox is strictly no-decompression,
>> then the NDL will be far more restrictive than the O2 toxicity limit.
>> Hence, an O2 hit with Nitrox I in recreational profiles is extremely
unlikely.
>
>No it's not. It is for a pulmonary hit, which for all but the most aggressive
>recreational divers isn't a concern. And again only certain organisations
>define recreational diving as being no-decompression. Many divers in the UK
>regularly do decompression dives sanctioned by their organisation.
Here we come back to the age old argument of where the boundary lies between
recreational and technical diving. My comments were based on the USA dive
organizations definitions (40m, no decompression) - for these, I believe that
Nitrox I is still reasonably unlikely to produce a CNS hit.
If BSAC and SSA sanction deeper limits and decompression diving, then presumably
divers are taught how to do these safely. The additional knowledge and
equipment required would, I believe, also indicate a diver who could be taught
to safely use Nitrox II and take responsibility for O2 toxicity calculations.
This probably reflects the more committed nature of divers in the UK compared
with those in other (warmer water!) countries. [From my limited knowledge
of BSAC training (mostly through the net), I believe the divers produced
probably ARE better trained. However, this is necessitated by the diving
environment in the UK. The result is simply a lower participation rate in
diving in the UK.]
My concern over technical diving instruction in general is based on my
personal observations of the lowest-common-denominator recreational diver as
turned out by the major US dive agency programmes. I don't believe the
majority could be taught technical diving safely.
The mainstream US agencies have delibrately lowered the required performance
standards to a level where as many people as possible can become divers safely.
I believe that their limits are reasonable for the type of training they
provide.
In this context I think NOAA Nitrox I could be incorporated in recreational
diving.
I believe that technical diver training should cater to those who have the
capabilities to extend past these limits.
To me, Nitrox other than NOAA Nitrox I, other mixed gases and decompression
diving is technical diving.
>
>> If the only gasses being used
>> are Air and Nitrox I, then the consequences of a swapped tank aren't quite
>> so serious - well, DCS (which may occur if one is diving Nitrox tables with
air)
>> IS serious, but not as bad as an O2 hit!.
>
>Only valid if your limit is 40m and you only have NNI around. We regularly
>have mixes from 30% to 38% O2 around. Everyone should test their mix prior to
>entering the water. [Easy to preach. Is there anyone who can honestly say they
>have done this?]
I think I have addressed these above.
Back to the original suggestion: If ONLY Nitrox I was provided for recreational
divers (using premixing in the fill station), and if it was used purely for
added safety (as opposed to longer bottom times), then a mix up with air
cylinders would not matter (Reason: still diving Air tables, within CNS O2
toxicity limits). In addition, testing of individual tanks before diving
would not be required in this scenario.
[No doubt the counter argument to this is that people WOULD apply EAD
calculations to the AIR tables and take longer bottom times. Hence, the
risk of DCI if the wrong tank was used. Strict tank colour coding should
avoid most of the problems in this area.]
Enough from me I think!
David.
Disclaimer: Please don't plan any dives based on my comments - I may be
wrong!!!
______________________________________________________________________________
David Giddy, | Voice: +61 3 253 6388
Telstra Corporation, | Fax: +61 3 253 6144
P.O. Box 249, Clayton, Victoria 3168, AUSTRALIA | Net: d.giddy@tr*.oz*.au*
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