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*NEWS* Photos Must See!
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The Pain Principal --- Elements of extreme dive planning Breathing Gas Selection during ascent and descent (Article #4) In the last article in this series we looked at the dark side of Helium and Nitrogen when breathed at great depth, and how their opposing toxic Psychotic properties could be carefully blended into the divers breathing mixtures so as to cancel each other out at the desired depth. But this is only part of the picture. The body’s requirements for Oxygen also demands that the breathing gas provide a 0.2-1.6bar ppO2 and that the overall dive does not exceed the pulmonary toxicity tolerance of the lungs. On extremely long dives with 12 hours of deco, the ppO2 may need to be kept way below the value required for optimum decompression so as not to irritate the lungs or trigger a potentially fatal oxygen toxicity attack. Finally there is one more poorly understood phenomena concerning breathing gas selection that has likely been responsible for many unexplained deaths of divers decompressing breathing Trimix. Narcotic shock or Isobaric Counter Diffusion (ICD) This is a thoroughly debilitating effect where simply changing the breathing gas from one supposedly breathable gas to another breathing gas both with acceptable ppO2/N2/He levels can trigger severe DCS within a few breaths. Narcotic shock can happen at constant depth hence the term ‘isobaric’ in it’s description, and the key to preventing it, is to ensure that a gas switch does not create a sudden jump of ppN2 at anytime during the main decompression portion of the dive. Many world depth record holders such as Mark Ellyatt and the late John Bennett have suffered from the debilitating effects of ICD hits during deep Trimix dives. Without the help of support divers during these dives, the symptoms of complete loss of balance plus vomiting on every breath would have lead to loss of decompression discipline followed by an inevitably fatal ascent to the surface. We can best understand these effects by looking at the gas choices for an extreme Tri-mix dive made by Mark Ellyatt, and examining the key survival attributes of each breathing gas switch. Breathing Gas Selections and Critical Physiological parameters for a 313m/1,026ft Trimix Dive.
Notes: ppO2 Limits 0.18-1.60bars 250%CNS max ‘explorer’ CNS dive limits, 850 OTU max pulmonary limit ppN2 Limits 0.00-3.95bars(recreational), 0.00-5.20(Technical), 0.00-6.50(HPNS compensation) ppHe Limits 0.00-10.00(Heliox limits), 0.00-50.00(with ppN2 or ppH2 compensation) Observations:
The Breathing gas selection process and execution of an extreme open-circuit Trimix dive of this nature is an incredibly complex process. As more technical divers investigate previously un-dived shipwrecks in the 150-250meter/490-820ft level, the trend is for them to use a computer controlled rebreather with onboard gas mixing and integrated dive computer to reduce the task loading and risk of diver error at depth. By using computer controlled closed circuit re-breathers instead of open-circuit techniques and by the use of better decompression models that take into account all critical gas selection parameters, the door is now open for un-tethered divers to descend past 300m/1,000ft on towards the 701m/2,300ft world record saturation diving depth limit established by mythical diver Theo Mavrostomos during the COMEX Hydra 10 experiments in 1992. +++++
Steve
Burton teaches Advanced Trimix Diving and the ‘Dive Industry Technician
Course’ at
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