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Karen:
 
You're asking the wrong question.  If you are planning two dives to the same
depth, the question you should be asking is what is the correct gas to use at
that depth and then you should be filling all your tanks with that gas.  In
your example, two dives to 98', I would bring EAN32 in both my tanks.
Assuming of course you have a hard bottom and it's not the top of the wreck
that's at 98' with the sand at say 150'.  Point being, plan your gas for your
maximum contingency depth, especially wreck diving.  In any event, why on
earth would you chose to use air when you have nitrox available?  Given the
cost of your charter and dive gear I trust you can afford the extra $4 or so
for the second nitrox fill.
 
If you're planning two dives to different depths, you should be choosing the
correct gas for each dive based on your dive plan.
 
Oxygen cleaning is a whole different subject.  You can find megabytes on this
subject in the techdiver archives.  Personally I keep all my tanks clean and
only use an oil free compressor so I can put any gas into any tank.  But then
I fill my own tanks.  The only first stages I clean are the ones I use on 100%
O2 deco bottles.  I don't clean any second stages, even the ones I use for
100% O2 deco.  I've never seen anything to suggest that low pressure (140 psi)
oxygen is a safety issue.  And, I've never seen or heard any evidence to
suggest that nitrox below 40% requires  any additional cleaning of regulators.
 
-Kent-
 
 
 
-----Original Message-----
From: Scuba diving discussion list [mailto:[log in to unmask]]On
Behalf Of Karen Nakamura
Sent: Tuesday, August 31, 1999 6:20 AM
To: Multiple recipients of list SCUBA-L
Subject: Re: Mixed drinks (Nitrox and Air)
 
 
OK, I have a related question that deals with switching gases between dives.
 
I just got my nitrox certification last week (yeah!) but this discussion has
gotten me thinking.
 
---
 
I have one nitrox tank (EAN32)  and one air tank.
 
If I were to take a 2-dive boat charter to the same wreck (say 98'), would
it be better:
 
        A. Dive nitrox first dive, SI of 2 hours, then air the second
 
        B. Dive air the first, SI2hrs, nitrox the second
 
Which is the better dive plan?
 
---
Let's look at the SSI/Nitrox tables:
 
Plan A gives 30:00@EAD80 for the first dive and 6:00@100' for the second,
for a total of 36:00 NDL.
 
Plan B  gives less time,  20:00@100 for the first dive and 12:00@EAD80 for
the second for a total of 32:00 minutes NDL.
 
 
----
 
Now, physiologically is Plan A better because it gives more time, thus
indicating less nitrogen loading?
 
Or is Plan B better since the nyperoxic mix in the second dive helps offgass
N2, thus acting as a deco gas in some sorts, preventing less chance of DCS
after the repetitive dive?
 
---
 
My own intuition is that I'd go with Plan B. It gives less time overall, but
more time to the second dive, since 6:00 underwater isn't a whole lot.
 
Note that these are SSI tables, which are much more conservative time-wise
than PADI/NAUI.
 
Any thoughts?
 
(Or should I just get that second tank O2 cleaned as well?)
 
Karen Nakamura