My head hurts!!!!
more later .......
Okay it's later now, so if you're sitting comfortable I'll begin...
Tim and I (Simon) have just finished the IANTD Advanced Nitrox course at the Old
Harbour Dive School in Weymouth. Ian Fuller was the instructor, and most of
you out there I am sure know of him, his school, and now his views on
spearfishing !
'Why the Advanced Course?' I hear you shout and 'Why IANTD?' - Well the answer
to the first question is that the basic course doesn't allow accelerated deco,
and as we were eligible for the advanced, well.....
There's one heck a lot of theory on the course, but none of it is actually too difficult - there's just a lot of things to do. New tables, Equivalent Air Depths, CNS Oxygen Toxicity all rear their heads, but when you actually get down to it, all you do is pick the best mix, work out EAD and look at the appropriate deco table, and track your O2.
Buhlman Tables
These tables are actually what most dive computers use these days - certainly
the Aladins use them, so I guess it makes a lot of sense to use them too. They
are quite different to BSAC tables in look, but pretty similar in use. Dive
time is taken to the beginning of the ascent, and last stop is at 4.5 m - a
compromise between 6 and 3 as far as I can work out. Repeat dives are
calculated by using Residual Nitrogen Times (RNT) ie subtract the RNT from the
table times, and you get your second dive time.
Equivalent Air Depth
When you've got you best mix, there is less nitrogen in the gas than in air.
At depth therefore, the partial pressure of nitrogen is less than if you were
breathing air. This is what causes DCS. Therefore, the ppN2 you have in a
nitrox mix can be equated to the ppN2 in air at a shallower depth this
is why nitrox diving works, it is why it is safer, and why you can have a
longer dive for the same safety factor as air.
Oxygen Toxicity
Another way to die !!
Ok, you get this with air too, but we live with it don't we.
Ever wonder why there was a depth limit on air diving?
well, you get narced to hell o course, and your no stop time is less than zero,
but the absolute depth limit on air is because the O2 becomes toxic at around
66m. In fact what happens is the ppO2 reaches a limit, you get convulsions and
die.
In a nitrox mix you've got more oxygen right? So the ppO2 is higher than for
air at any given depth. This means that with a nitrox mix the depth limit is
less than for air. Nitrox is not a gas for deep diving
By calculating the ppO2 and tracking the exposure to
it over time you get a measure of the toxicity (ever wondered what that little
CNS% is on your nitrox computers?)
to be continued.....
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