From: mayne@phi.cs.fsu.edu (William Mayne)
Subject: Re: Nitrox for safety?
Reply-To: mayne@cs.fsu.edu
Organization: Florida State University Computer Science Department
Date: Mon, 12 Oct 92 19:48:14 GMT

In article <1992Oct12.154323.17538@meiko.com> gordon@meiko.com (Gordon Henderson) writes:
>Just thinking about Nitrox ...
>
>Anyone out there using Nitrox for safety purposes, as opposed to using
>to extend bottom times or reduce deco. times? (Which would be hard for
>me to do anyway, as no one else in my club is even thinking of using
>Nitrox yet)

I think using nitrox whenever possible is a great idea. The safety
advantage is about 2 stops, just about the amount of padding you
might use if you are as conservative as I am. Doing this an air
schedule could be used, but going closer to the limit.

Being a poor student I don't use nitrox as much as I would like to,
and when I do it is usually to take some advantage of the reduced
decompression. Hence I use it mainly on dives which have reasonably
flat profiles around 100 feet. This limited use is definitely not
my preference. But I do prefer to be able to afford to dive often.
When I have more income I plan to use nitrox much more.

On multi-level profiles a computer schedule assuming air may be
shorter than a nitrox table schedule. But then I'd be willing to
not pad the computer schedule as much when I'm using nitrox.
By the way, I confess I also pad less on square profiles when using
a computer even if I'm breathing air, because I know my computer is
already very conservative on those kinds of profiles. Theoretically
it is equally conservative on multi-level dives, but I don't trust
it as much then.

I do get a little nitrox safety bonus when I have a lot of nitrox
left after a dive and top off with air for the next dive, treating
the resulting mix like air even though the O_2 content is higher (and
hence N_2 lower). I normally finish a dive with at least 1000 psi in
my double 104s (1/3 of an over fill to 3000), which is about 80 ft3 of
gas, so if the nitrox was on the rich side this will give me a
significant safety advantage. Don't worry or flame me. I wouldn't do
this for a deep dive where I might run into O_2 toxicity. In that case
I'd just have to waste the nitrox by draining the tanks before filling
with air. (I'm aware you can get into trouble using the same tanks
for nitrox and air, especially if you are the least bit careless.)

If diving tables and using an analyzed nitrox mix I figure the dive
based on equivalent air depth so I can do any padding I choose to do
deliberately. In some cases that may come out close to an air schedule.
However I believe in knowing what I am doing and not getting my safety
margins from simply ignoring factors. That is one reason I like using
computers, by the way.

>I was thinking of doing a course, so I could use it, then use it with
>existing tables and computer models. It's use being nothing more than
>to give me an extra margin of safety. (Can you be too safety conscious?)

If you are too safety conscious you may decide not to dive at all.

>My local shop has recently started selling Nitrox, but they are
>charging #5 (UK pounds) a fill for it (#2 for air). I'm not sure it it
>would be worth the extra charge, just for the added safety. (I'd
>probably need to get a 2nd tank too, as I usually do 2 dives a day when
>I dive.)

This of course depends on your financial situation.

>Anyone comments from anyone already doing this?

A dive shop operator from Key West who spoke at a workshop I attended
said he had instructors and dive masters who dive with customers use
nitrox just for safety, as you are considering.

I'd like to pose another question to nitrox divers and wanna bes:

Do you prefer to use nitrox tables or standard tables with EAD?

I strongly prefer air tables. EAD is not that hard to compute.
You can even write down a list of depths for each EAD in 10 foot
increments on your slate before starting a dive. Don't convert
each multiple of 10 in real depth to EAD. The tables do that,
and then round up to 10 foot increments when getting the NDL
or stops from the air table. Calculate the exact real depth for
each 10 feet EAD in the range of interest, then use the EAD
when reading the table. This is more accurate than nitrox tables.
The inaccuracy in nitrox tables is on the side of safety. I just
have this thing about being accurate when I can and then adding
safety factors. Using EAD means less tables to keep track of and
more flexibility in matching the mix to the dive plan, too.

Bill Mayne
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