From: mayne@ds1.scri.fsu.edu (Bill Mayne)
Newsgroups: rec.scuba
Subject: Nitrox (Was Re: Thermodynamic Decompression)
Date: 19 Aug 92 17:03:14 GMT
Reply-To: mayne@ds1.scri.fsu.edu (Bill Mayne)
Organization: SCRI, Florida State University

NOTICE: This followup discusses technical diving beyond the limits of
what most recreational divers are trained for or want to do. It isn't
that difficult but and will probably be of interest only to techies.
Maybe I should reconsider the difficulty considering considering
Dave (Duis) Story's article quoting a study that quite a few divers
evidently can't even use tables correctly for straight air, but I
figure there is no hope for them and I give the rec.scuba audience
more credit.

Non-techies may have already been warned off by the original subject
line anyway.

In article <1992Aug13.032632.20963@herboid.uucp> adb@herboid.uucp (Anthony DeBoer) writes:
>
>Couldn't you just do extra "safety" decompression up to the minimum amount
>of decompression the computer wants?  I find that even if I know I could
>get out of the water sooner, I have no trouble hanging on the line a bit
>longer, especially if it's reducing the already-remote possibility of a
>chamber trip.  From what I understood, the best safety factor comes from
>doing nitrox decompression on air tables/computers, especially as it
>lets you stick to the limits (ie. no extra safety factors or "stepping
>back" on the tables) without feeling guilty.

As a general principle for most recreational dives and divers, including
most of my own dives (so far, anyway), I absolutely agree with this. There
are exceptions, though. I bent my computer in class dive where the
instructor wanted us to get out of the water sooner, but still by a very
conservative schedule.

The extreme positions are (1) using nitrox just like air and (2) going to
the nitrox limits, or at least calculating what you do, including padding,
based on equivalent air depths. Obviously, as you say, the best safety
factor (short of using an air schedule and then padding that, which gets
ridiculous) comes from the first method. Sometimes at least a compromise
between the extremes may be appropriate. From what I understand there are
physiological and safety advantages to nitrox even if used with equivalent
air depth for calculations. These include:

(1) Slight reduction in narcosis - not likely to be a big factor at the
    relatively shallow depths where nitrox is used.

(2) Staying in safer regions of the tables, with better repetive groups
    for repetive diving, if the dive drives the decompression schedule
    rather than the other way around.

(3) Faster nitrogen elimination during decompression or safety stops,
    which is a benefit ignored by the air tables and the NOAA
    nitrox tables. (Correct me if I am wrong about the latter, but
    I'm pretty sure this is correct. I know this is true for some
    profiles.)

(4) Direct benefit to using higher percentage of oxygen as opposed to
    the primary benefit of reducing the percentage of nitrogen.

If you are only talking about a stop of 10 minutes versus a minimal
(say 3 minute) safety stop I'd say using an air schedule is the way to go.
If you are looking at a dive to 30 meters for 60 minutes with equivalent
air depth of 24 meters the alternatives, using Buhlmann tables, are:

(1) Air: 52 minutes to ascend and resulting repetive group H.
(2) Nitrox: 27 minutes to ascend and resulting group G.

(You could do this with a mix barely over 32% O_2 (NOAA I) and an O_2
partial pressure at depth barely over 1.3 ATA so you are nowhere near
O_2 exposure limits.)

In this case it could be tempting to shave something off the air
schedule, and the experts say you'd as safe - actually slightly
safer - as you would have you would have been on an air dive to an
actual depth of 24 meters and using air tables without padding, and
safer than if you had done the 30 meter dive on air and used the air
schedule for your actual depth, again without padding. And you can
still pad, such as using the 24 meter 80 minute schedule (slipping
two lines). In this example the nitrox schedule is 46 minutes - still
a bit shorter than air. I think anyone would agree that this is an
extremely conservative profile, given that you want or need 60 minutes
bottom time at 30 meters. Staying the extra 6 minutes to satisfy the air
tables (or computer) would cost little (mainly that much more discomfort)
but it would also be of little or no benefit.

(Note that we are already outside what nearly anyone without gills
could do on one tank, not even counting the decompression part. This is
not your average spear fishing or sight seeing kind of dive.)

Bill Mayne