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Definitive
Decompression - For Advanced Deep Diving (2003)
Safe
deep diving needs a sound understanding of decompression physiology more than
ever. A deep dive nowadays could mean 100m plus. Divers have tools for deep diving
now
that can extend depth and time often way beyond the individuals experience. The
widespread use of rebreather units allow divers to explore depths way deeper
than a scuba set would ever allow them. With the hardware easily available to exceed
common sense, it is glaringly obvious that academic training and practical deep
diving experience should be similarly advanced.
As divers explore wrecks and reefs in
deep water they are increasingly pushing decompression boundaries to the point
that many are injured or killed. Standard classes in deep diving techniques are
very antiquated, mostly being written over 20 years ago, by well meaning
individuals with a commercial or military background. These techniques have
evolved over time but are seldom collated into a lucid text. Many experts in the
field of technical diving are simply computer programmers that dive. When two
get together in a chat room, become leading gas physiologists and decompression
gurus...the non diving kind!
Trimix training courses, even to "Advanced levels", prepare the diver for at
best...60m diving. To prepare a student to safely dive as deep as 100m -
as many agencies offer, should take in my opinion at least 20 dives below 60m, not
2-4 dives as typically offered! A
customer expects to be able to dive deep unsupervised after training. Technical
dive students are becoming less and less capable after spending many hundreds of
pounds with their "cross over special" instructors with their typical 10 dives below
40m!
Diving to depths no deeper than 60m
and no longer than 25 minutes at this depth are within the realms of safety, but
only if the individual is adequately experienced and diving regularly.
Trimix
training classes typically contain techniques that include gas management
protocols like 1.6 po2 for deco and 1.4 po2 for bottom mix, also divers are
recommended to stop breathing trimix mixtures as early as possible during the
ascent phase and switch to nitrox mixes to speed off gassing. These techniques
have been used relatively safely for many years and thousands of dives.
But read on.
If the dive-plan goes beyond 70m to
80m for significant bottom times (25 minutes) then the above techniques are not
advisable and should be modified. Trimix gas mixtures with helium values
commensurate with the conditions and divers experience need careful
consideration. If conditions are not ideal than an END of 30-40m would be
appropriate. If the dive takes place in ideal conditions outside of an overhead
environments then a higher END would be a better choice (40-50m). Using more
helium is not a substitute for experience. Most deeper diving fatalities are
directly linked to buoyancy control. If a diver has a runaway ascent because of
dry suit complications or problematic surface marker buoy deployment, the use of helium
in the gas mixture will compound the problem exponentially. Helium is a far less
forgiving gas than nitrogen. Helium will bubble during a problem ascent far
sooner than nitrogen would, sometimes with disastrous effects. Helium is a faster gas to
decompress from, BUT, only if the ascent goes perfectly. The more helium in the
breathing gas, a perfect ascent needs to be guaranteed, therefore
lower Helium gives more room for buoyancy control flaws.
Helium should be the reward for truly
experienced deep divers, This new breed of "shallow", "entry
level" or "Triox" trimix divers
can
expect long term bone damage that their air diving counterparts are less likely
to experience. It is truly a scandal how training agencies create courses that
let inexperienced believe that diving helium shallow is somehow safe. While you
may be decompressing efficiently if all things go perfectly, one problem ascent
is all it takes to make your skeleton decay ahead of time!
Once the helium values are derived,
oxygen values are worthy of deliberation. While 1.4 po2's are common place for
bottom mixes, an advanced trimix diver should be thinking about po2's in the 1.2-1.6 range
depending on exposure times, not traditions. Helium has the ability to deflate nerve synapses and
this must have a negative affect on divers during descents and in the early
phase of the bottom time. Rapid descents are associated with deep scuba dives,
these are frought with danger. HPNS is a concern for dives below 150m but END's
around 50m will help minimise both tremors and pain associated with bone space
compromise (be assured that the body has plenty of void spaces that, if
compressed regularly will get bigger and bigger causing inflammation and pain
and even
decompression complications, then bone necrosis. Too much helium will ensure that this
happens in your lifetime. If a diver is not comfortable with 50m+ END's then the
diver has absolutely no place below 50m!
A descent speed of 10m-20m per minute
and slower for anchor line descents should be considered. Descending quickly on
trimix feels better than air what with its decreased density and it ventilates
the lungs nicely, lessens c02 build up giving some protection against narcosis
also.
Drysuits pose an interesting complication for decompression divers. If you
inflate drysuits with mixes containing helium then this will speed diver
cooling. The cooling is not due to heat loss because of helium’s presence in the
suit solely (a function of its poor insulating properties). As helium de
pressurises and expands (Joules Thompson effect) much faster than air or argon,
it enters the dry suit much colder and has a noticeable chilling effect. For the
non-believers…simply try it! The Trimix will feel much cooler than the argon or
air. Simply lowering the supplying first stage intermediate pressure may
sort this problem to some extent. Many technical
divers rant about the use of Argon in their drysuits. Because of Argons
increased density it travels slowly into the drysuit and this feels quite
neutral with regard to temperature. However, most of Argons 'benefits' are
imaginary - air is just as good. A
complication with Argon is, as you are unlikely to be breathing it, there is a
large pressure gradient between inert gases in both your skin and other tissue.
This may even cause skin bends on larger dives.
If
divers have not been experiencing this, its most likely that they haven’t got
much argon in their suit anyway! Remember
that the Cold just hurts, it doesn’t kill. But getting cold may worsen your
chances during decompression, before you pull huge deco’s, practice the same
time period to see how cold you will feel at the end. Back gas or nitrox mix up
to 40% or air is a better alternative to argon. Also this avoids the ridiculous
argon bottle - usually filled with air :-)
With regard to the construction of drysuits…consider the following. Where will
you be spending most of the dive? Probably doing decompression…If this is not
the case…it doesn’t matter what suit you wear! If you do complete long
decompressions…6-8mm Neoprene will keep you warmest for longest. Compressed
neoprene or tri laminates are lighter and “possibly” more comfortable on the
surface, but these types of material have virtually no insulating properties and
rely heavily on under suit selection. You will not get any heavier at maximum
depth with a neoprene suit compared with a membrane suit. If you can dive a membrane or
crushed neoprene without an under suit, you probably don’t need the drysuit in
the first place. Undersuits at depth definitely compress as much as thick
neoprene!
Drysuits make very poor back up
buoyancy alternatives also, unless you are wearing the lightest of tanks in warm
water. Consider a dual bladder wing.
If you can, wear a wetsuit. This fixes
the urination/hydration issue also ;-)
For dives shallower than 60m for no
longer than 20minutes…then any training manual, and virtually any decompression
program will give you a fairly reliable ascent plan.
For deeper dives (sub 100m): Keep the Helium
as low as experience allows. When choosing decompression gases, when you make a
raise in oxygen, drop the helium by this amount. You must keep the nitrogen %
constant or dropping. Keep helium in the decompression gas right up to the
oxygen stop at 3-4m. By keeping your decompression Po2 less than 1.4 you can avoid making po2 breaks (air breaks.)
Decompression Table Design
Decompression table
designers used to work for large, often government backed organisations and
institutions. Names that deepened the well of decompression understanding are
well known and respected. Haldane, Workman , Buehlman and the many others that
have furthered our understanding spent many years analysing decompression data
taken from men working in hyperbaric environments both dry and wet, and then
refining the results to reflect what needed improving. Much of the research into
decompression theory has been driven by the oil industry and commercial diving
companies. Additionally many navies around the world have seen the tactical
benefits in understanding the inter action between man and the undersea
environment. These two large research entities have made big leaps with the
knowledge to descend a person deep underwater and return them safely and
undamaged.
Since the drive for
oil production has largely turned to remote and robotic means, and the navies
have realised that the risk/reward equation is unbalanced when sending human
beings without protection against the massive pressures found underwater, the
need for refined decompression data has dried up. Divers could be reasonably
assured by the research conducted in the past, and the fact that the tables were
validated during actual experiments on men and woman.
The dive profiles that
technical divers call routine are quite unlike the controlled dives that
commercial and military divers repeat everyday. Decompression theory is
advancing at a much slower speed than the desire to descend deeper and longer by
technical and recreational divers. In the last 10 years a new breed of
decompression “experts” are emerging, they offer un-tried and un-tested methods
that are propagated through the internet to a hungry audience.
Some of the new
decompression techniques are based around traditional decompression algorithms
that, with a few modifications can be reliable when used conservatively. Other
table designers have ventured completely away from historically validated
methods and have marketed radical decompression algorithms that received no
verification through human testing. Unfortunately the designers of decompression
software are very happy to let the customer base be the testers of the
product.
Decompression injuries
can occur instantly or manifest themselves over years. Divers wishing to dive
outside traditional methods of decompression should take time to understand the
implications, and attempt to analyse the ascent plan using knowledge from truly
experienced divers or software products designed to compare past human based
test data with newer math based alternatives. The decompression analyser ”DecoChek”
developed by Stephen Burton and Mark Ellyatt is an excellent tool for divers to obtain
decompression plan comparisons and analysis data.
There
are many versions of dive software on the market. Models based on dissolved gas
theories have many verification dives. The models based on free gas elimination
(bubble models), have often been tested in various net experts imaginations
only. Free gas models have slowly merged with traditional models over time,
coincidentally as their bends incidence increased. Free gas models are
associated with very deep stops that actually increase decompression loading
later that they don't compensate for adequately later.
Bubble models have been poorly tested by
users mainly during bounce dives between 60m & 90m, the user defined parameters
used to generate the profiles are very variable and consequently very difficult
to validate. While you might not get bent using them every time, your
bones and circulation will suffer in the long term, because of the repetitive
short deco's. This is the reason that the
developing scientists discontinued work on these models in the late 1970’s!
The I/T specialists that have reincarnated them recently, are interested
in your cash now, not your arthritis later…you have been warned ;-)
In the last couple of years
bubble models have evolved and generally predict far longer shallow stops than
earlier versions. The
latest VPM 3.60 model provides acceptable decompressions as deep as 100m
with bottom times up to 15 minutes, as long as level 4 conservatism is employed
table table generations.
RGBM models still lack
any useful development and must be avoided.
This text may contain errors or type
O’s…You should obtain training from an Instructor who has dived deeper than
you plan to.
The
DecoChek analysing software
is available
HERE and is free of charge for the
demonstrator version, and inexpensive for the explorer
model. The program will further any divers understanding of decompression
theory. It was developed by Stephen Burton (veteran Trimix instructor trainer and
electronics guru / actual rocket scientist!).
Mark Ellyatt (10 year Trimix instructor
trainer and deepest scuba diver) helped develop the counter diffusion algorithm.
I hope you find the above ramblings of
interest…The text will be modified often as time allows and on realising I've
typed some rubbish!
If
you would like some clarification on any of the above points, feel free to drop
me a line...I'll do my best to explain what i'm talking about
Dive
Educated…Dive Safe
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