Experience or stupidity may get you there...
inspired training will get you back!



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



copyright Mark Ellyatt