>> > > I suspect that exertion during a dive is as much and maybe more
>> > > important to post dive fatigue than the gas used.
>> >
>> > Possibly. I'm sure it has an impact on it.
>>
>> I don't believe the gas will matter much unless you're starting to
>> talk about a "deep" dive - say approaching 100' or more.  The reason
>> is that nitrogen loading at the shallower dives plays a progressively
>> smaller part.
>>
>How so? The greatest change in pressure is in the shallow parts and
>the fast tissues are loading up big time here.

Not true.  The greatest PERCENTAGE change in pressure is near the
surface, but the change in pressure as we descend is a constant
roughly equal to 14.7 psi / 33 fsw or .445 psi/ft.

If you believe the models and since we're having this discussion you
must...
then the amount of rate of N2 absorption is proportional to the
DIFFERENCE
between the PPN2 in the aspirated gas and the PPN2 in the tissues.

Another way to look at it...
We are "saturated" at sea level (or whatever altitude you normally live
at) and
those "fast" tissues will load up MUCH quicker if you go deeper than if
you
stay shallow.  IF you disagree with this then you do not accept the
premesis
of the models themselves and we can end this discussion right here and
now.

Just look at the tables.  You can stay at 20 feet just about forever and
surface immediately.  As you descend your NDL times become shorter and
shorter.
For a "first dive" a great many of these limits are being imposed by the
"fast tissues", it's not until we start talking about multiple dives
that
the slower tissue groups come into play.  But, since you singled out the
"fast" tissues alone I'm not going to get into a more detailed
discussion about the slower tissue groups.

Carl