I like to dive deep, I like a challenge and it’s rewarding to do something
difficult and return safely. In February 2003, I almost dived my last dive, and
this was my first major diving incident in almost 3000 dives. This deep dive
was to 260m as a practice dive for a deeper one soon after. The ascent plan was
aggressive time wise, but I had built confidence in this particular
decompression algorithm and had dived it “deep for long” many times. How
mistaken could I have been? The decompression schedule proved woefully
inadequate and the injuries I sustained will probably take a lifetime to fully
recover from. During my rehabilitation I couldn’t do much but read books and
try to make the best of it. I went over my dive plan again and again, It was
not until after the dive that I discovered that although it was commercially
available dive software, It was not tested in any way, and had no place
suggesting it could provide an ascent solution from a depth well within its
stated specifications.
As time went by my health improved, and over the months I became proficient in
dive table design and had reworked a dive plan, which I felt covered all the
weaknesses of the February plan. My doctors had advised me against diving
again, but what had become a career for 10 years was proving very hard to
simply discard. If I had not been able to return to the sport I love, then the
depression that was sure to follow would be harder to endure than any physical
injuries I might suffer.
Anyways…now I can build my own dive plans incorporating years of deep dive
experiences and its not rocket science at all with all the information freely
available and man tested long before most of us were born. If its new (as I
found out to my cost) it has not been tested outside of a PC or Petri dish. The
dive plan software I worked on together with a colleague knowledgeable in
programming skills, reflects information actually tested by commercial divers
in the past, utilises data from large dives that have not worked recently and
includes matrixes to avoid counter diffusion problems, It has already received
interest from military and governmental academics. Even more so with its recent
success on predicting the ascent solution from the deepest solo dive ever, at
313m without DCI. I believe that now, safe, extreme scuba doesn’t need luck.
Right…less of the sobering stuff and more information on the dive…
(I think) A dive below 300m needs a rapid descent. This causes HPNS and this
can be minimised by using a high Equivalent Nitrogen depth (END) value. I used
an END over 70m. I kept the oxygen high on this dive also (Po2 was 1.6+). The
reasons for this are as follows. The exposure was short, so not problematic
(for me). Keeping the helium as low as possible in the bottom mix has many
benefits, and it makes it easier to derive the next Trimix decompression gas.
There will always be a step up in nitrogen on open circuit, unless you
have “yet another bottle”, too many bottles adds to the risk, and a support
diver bringing gas to 150m+ is not ideal either! On the dive my
nitrogen “spike” was down deep when critical tensions were not yet high. The
140m deco gas went up 10% nitrogen with a raise of 6% oxygen, this meant that
the rest of the ascent gases could keep the same or more helium and critically…
less nitrogen.
To counter the enormous dehydration due to immersion
diuresis on this almost 7 hour dive, I had to drink 2-3 litres every hour...
All the subsequent ascent gases kept the same helium content from 140m to 9m,
the only changes were to increase oxygen and decrease nitrogen. At 9m, Heliox
was used, it has no n2 to complicate matters and is fast to decompress with. I
managed oxygen toxicity by keeping PO2 low from 6m. No “air breaks” were needed
(call them what you want) Air breaks work OK in a o2/n2 (chamber)
environment…but are possibly suicidal on a Trimix / Heliox dive. Trimix “air
breaks” or Heliox “air breaks” are just as bad, for the same reasons. Managing
Po2’s to the 1.3 level is by far the safest way to deal with long
decompressions.
Having no “p02 breaks” (better term for air breaks) is hard from a pulmonary
toxicity point, also breathing heliox on open circuit for multiple hours is
difficult. It might be “easy” to breath but the body wastes energy heating it
(as it arrives in the lungs cooler than a typical nitrogen/oxygen mix and
overall heat loss can reach unsustainable levels. (Helium has a high capacity
to bleed heat from the body, which is strange for such a light gas)
I chose the heliox route, to avoid counter diffusion problems only (the cause
of all my previous dive problems) and knew that its use was troublesome but
less life threatening. To counter the enormous dehydration due to immersion
diuresis on this almost 7 hour dive, I had to drink 2-3 litres every hour and
this was difficult. A CCR would be the better alternative on the heliox deco
(warm and moist) A dropping set point would be my choice also to counter the
pulmonary toxicity and increased carbon dioxide complications. The longer times
on deco that this would have caused would be worth enduring.
The OTU and CNS count on this dive was going to be high and managed, by not
chasing the 1.6 Po2 mantra, If I had then…who knows, although a drop in vital
capacity was measured for 25days + after the dive.This may have been in fact,
lung fatigue from breathing un heated un hydrated gases for long periods (my
own view) or simply pulmonary toxicity
