Jozef

My last dive was 50m for a 44 minute ABT for a total in water time of 93
minutes (including an extra 8 minutes waiting for my buddy) and I carried 50%
and O2.  I would be far happier to do this dive or a shorter ABT on 50% alone
rather than O2 alone.  The reasons for this include the following:

1. I clear most of my deco earlier than would be the case on O2 alone.  If I
need to go to the surface, I'll be safer doing this having performed more
effective deco earlier.

2. Published stuff from the WKPP shows that the intermediate stops are
critical.  I listen to what they say.

3. I run the same tables whether I use 50+O2 or 50% alone.  If I dive 50%
alone, then I do half my "O2" 6m stop at 6m and what was the 6m stop at 3m. 
E.g. tables for 20 minutes on O2 at 6m, become 10 minutes at 6m on 50% and 20
minutes at 3m on 50%.   This keeps the dive planning nice and simple.

On shallower dives, say 35m on 31%, I would only carry O2.  Here I'd be doing
60 minutes ABT but may not bother with the O2 if the ABT was significantly
shorter.

In your example, 55m for 15 min ABT, I would definitely only carry a 50% since
I'd have 6 minutes on O2, easier to do 3+6 minutes at 6m and 3m on 50% and keep
the dive simple and more enjoyable.

For the shallower dives, you have to make your own assessment of this and
decide for yourself when O2 only is appropriate for a given depth/time. 
Consider the back gas you're using and how effective the deco is in the 21-9m
range.  If it can be significantly reduced then there is merit in carrying 50%.

David Shimell

-----Original Message-----
From:	Gliviak, Jozef [SMTP:Jozef.Gliviak@co*.co*]
Sent:	Thursday, August 23, 2001 4:04 PM
To:	techdiver@aquanaut.com
Subject:	one deco gas - N50 or O2, when?

Hello guys,

Recently we had a disscussion with a few friends where some of them
advocated usage of N50 instead of O2 typically for dives like 55m/15min. My
opinion would be to choose O2 rather then N50.
Can you please answer me if there are a situations when it is more
appropriate to use just N50 instead of oxygen. Please add a few examples of
dives where would you choose O2 and where N50 as the only deco gas.

Best regards

Jozef Gliviak
Slovakia

-----------------------------------
The answer I received from George is too general for me to see where is the
effective border
Trey:
Depends on the bottom gas. Oxygen if the dive is shallow, 50% if it is
deeper . The idea is to get good deco at all the intermediate steps,
relieving the need for oxygen up high. Go by the bottom gas and the time.
Add gases as needed.
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