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> For you guys with computer implementations of Buhlmann [Bill Mayne - does
> your offer to send beta test copies of your version still stand?]:
Not at this time. I have yet to decide how I will make it available to
a wider audience. Thanks for your interest, though.
> My question is, is using EAN for
> deep stops on air dives (in addition to O2 at 10 and 20 fsw) likely to
> significantly reduce decompression time and/or increase safety, or is it not
> worth the trouble? As a semi-hypothetical benchmark, suppose one is diving
> to 170 fsw for 30 min. I don't have the full tables handy, but extrapolating
> from the 1986 Buhlmann tables stuck in my desk this would require air stops
> below 40 fsw and about an hour of deco time (wild guess). Does using EAN
> as well as O2 for deco significantly reduce stop time, or is most of the
> accelerated decompression advantage due to the O2 alone?
Yes. Using nitrox as well as O2 significantly reduces stop time (depending
on what you think is significant), but most of the benefit in the range
of most interest to us sport divers is from using O2 at 20 and 10 (sort of).
Here are some numbers, using the ZHL-16B model with no padding.
I picked a slightly deeper hypothetical profile so there would
be more decompression time at 30 feet and deeper to show the
benefit of EAN. Needless to say these are for illustration only.
For a dive to 200 feet for 30 minutes on air the first stop is
at 60 feet. Total decompression time, including ascent on air
to the first stop at 60 ft/min and between stops at 30 ft/min,
given various gas combinations, is:
EAN50 + O2 ---> 37.3 minutes (nearly optimal)
air + O2 ---> 47.3 minutes (i.e. 10 minutes advantage using EANX50
over air and O2 alone)
air only ---> 81.3 minutes (i.e. 34 minutes advantage from O2) *
EAN50 only ---> 48.3 minutes (just 1 minute more than O2 alone - see below)
EAN40 only ---> 56.3 minutes (still 25 minutes better than air!)
* USN has 73.3 minutes, Royal Navy 85.2, DCIEM 117.2. I don't have
my Buhlmann tabels handy but I assume they are more conservative
than the unpadded calculation.
One of the most significant things here is that if you only use one
decompression gas (possibly for logistical reasons) EAN50 gives almost
as much advantage as O2 alone for this profile (48.3 versus 47.3 minutes).
In fact when EAN50 is used for the deep stops stops O2 gives only a 10
minute additional advantage. EAN50 alone gave a 33 minute advantage.
(Note that EAN50 shortens the 20 and 10 foot stops, too, hence you
don't get the full 34 minute benefit from O2 in this scenario.)
EAN50 is nice general purpose deco gas, especially if you are using it
for safety only and not trying to shorten your schedule. It is safer
to use than O2. You can start it at 70 feet, which is deeper than the
first stop on most air dives done by sport divers. Starting O2 at 20
feet you need to be careful not to drop any deeper and to take air
breaks. With EAN50 you have a safe 1.38 ATA O2 at the 60 foot stop
and you are below 1 ATA (.94, to be exact) by the time you reach 30
feet. EAN50 is also cheaper and you can get more of it in a tank,
since the total pressure you can get into the tank will be higher.
EAN40, which even conservatives generally say is okay if your
regulator is not O2 cleaned is still quite a bit better than air.
Caveat: This is all based on a straight forward and maybe naive use
of Buhlmann's model. I'm not a physiologist but I know there is much
more to it than this.
By the way, last Sunday I had the experience of switching from air
to EAN36 at 80 feet. The reason was I had borrowed a set of doubles
with air already in them. (Mine were unavailable.) Since the dive
profile was not extreme I didn't feel the need to waste the fill in
order to get nitrox into them. But I had EAN36 in a stage bottle.
I'll skip the details of the dive plan, except to say that I switched
from air back to nitrox when I turned the dive at 80 feet, 40 minutes
in. The point of this story is that it may have been psychological
(placebo effect or just the reduction in task loading once the decision
to turn the dive was made), or it may have been the reduction in the
already minimal, unnoticed narcosis, or the extra oxygen (doubtful,
switching from .71 to 1.21 ATA), but I felt noticeably better. I also
felt great after a 100 minute moderately strenuous dive, with a
significant part of it being on nitrox. (Some oxygen was used during
decomression, as well.)
Bill Mayne
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