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xander
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Postby xander » Thu Feb 23, 2006 12:02 am

From Wendryn, who used to do a lot of SCUBA, including tech diving (very deep stuff). The question was "Do you ever use 100% O2 while diving?":
Wendryn wrote:Yes, sometimes, for decompression. Some people will
cut it with Helium to make it easier to breathe
(Helium doesn't cause bends problems for the most
part).

You have to be careful because oxygen toxicity shows
up at varied rates. The rule of thumb is to not
breathe 100% below 20 feet (adjust for altitude, of
course, if needed) which at sea level is 1.6
atmospheres. You can usually get away with maybe 5
minutes at a max of 50 feet or 15 minutes at a max of
40 feet (not in order - if you do the 50' stop on
100%, you'd better have a different deco gas for
further up). The variations depend on water
temperature (very high or low increases risk of
oxtox), health of the diver, how hard they have been
working (convulsions at 20' after hard work when
decompressing on pure O2 have happened) and other
variables that have not yet been pinned down.

The Navy does not use more than 1.3 ata O2 for their
rebreathers (which would be a long dive scenario)
because more than that runs a risk of convulsions that
is too high for them. They aren't...always...the most
conservative.

More than you needed to know, probably. I never had to
use pure O2 for deco, but I did run my mixes
reasonably high, enough that the tanks had to be
oxygen clean.

--W


xander

EDIT: In answer to my next question, about what gasses are used to cut the O2:
Wendryn wrote:Nitrogen, mostly. For tech divers helium is used for
what is called trimix - oxygen, nitrogen, helium - so
you can limit the risk of oxtox, nitrogen narcosis,
and the bends. Depends on your depth, though. You can
go down to 120' on air, but you get double the bottom
time if you are on 32% oxygen and the remainder
nitrogen. You can't dive 36% oxygen at that depth or
you run oxtox risk.

Air is the normal mix - 21% oxygen, 79% nitrogen.
There are various levels of what is called Nitrox, or
Enriched Air Mixture (EAN), where the number means the
percentage of oxygen, thus EAN32. You have trimix,
oxygen, nitrogen, and helium.

You also have heliox, which is bloody expensive and,
unless you are diving seriously deep in a suit that
actively keeps you warm, you start running the risk of
hypothermia - you get colder when helium is in the
mix. The weird thing is, if you are going down below
600' sea water, you can get what seems to be helium
narcosis, and adding a tiny bit of nitrogen in helps.
Nobody knows why yet. You also have a problem with
longer decompression than you do with nitrox or
trimix, because helium saturates *much* faster into
your body than nitrogen does.

The reason you cut 100% oxygen with helium instead of
nitrogen for decompression stops is that generally
what you are waiting for is for the nitrogen to get
out of your system. The less nitrogen you breathe, the
more you offgas. Adding helium to the mix doesn't
change the rate of the nitrogen offgassing but it is
much easier on your lungs than pure oxyge - your lungs
can get kind of "burned" by pure O2.

For the really serious commercial divers when
everything ought to be under control and the oxygen
content is *very* low, some people use hydrogen and
oxygen.

Not suggested, what with the explosive problem & all,
but commercial divers are a whole 'nother breed.

-W

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