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*NEWS* Photos Must See!
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Technical Diving and other interesting Articles (Article 2) The Pain Principal –‘Elements of extreme dive planning’ Oxygen
management Perhaps one of the most misunderstood constituents of all breathable gases, Oxygen has some weird, wonderful and deadly properties. Newspapers reporting fatal diving accidents with the usual ‘divers oxygen tank ran out’ headline only adds to the confusion. I know from experience that it comes as a shock to many students on their first ‘Open Water Diver’ that it’s only compressed air in the scuba tank. Let’s examine in this article the limits of Oxygen use from the deepest depths ever dived in the Ocean to Astronauts in outer space and appreciate our body’s requirements for this most useful of gases and it’s implications on diving methodology. That Oxygen(O2) can be both essential for human life and harmful to us in abnormally high amounts has been known since Paul Bert’s discoveries of the convulsive effects of elevated levels of O2 in 1878 and Lorrain Smith’s 1899 discoveries that long term exposures to relatively moderate levels of O2 causes lung damage. It’s not just human life that’s affected. Plants, insects and even bacteria are also adversely affected by high O2. This can be used to our advantage; badly infected wounds can be healed by hyperbaric oxygen treatments, but paradoxically high O2 levels in hospital incubators have been in indicted as the cause of blindness in premature babies.
It’s not the percentage of Oxygen in the breathing gas that is important. Our body needs the Oxygen partial pressure to be within an acceptable range as shown above. Our requirements and tolerances to Oxygen as divers are best summarized by stating that for saturation exposure (many days) the Oxygen partial pressure must be kept within quite narrow limits 0.20-0.5 bar, whereas when diving for short durations (45mins) we can tolerate a much wider range 0.20-1.6 bar. Oxygen
Tolerance (wet exposure) ppO2 Max Exposure 0
0.1
death
0.2
indefinite 0.3
indefinite 0.4
indefinite 0.5
indefinite 0.6
720 mins 0.7
570 mins 0.8
450 mins 0.9
360 mins 1.0
300 mins 1.1
240 mins 1.2
210 mins 1.3
180 mins 1.4
150 mins 1.5
120 mins 1.6
45 mins 1.7
10 mins
(unreliable) 1.8
2 mins
(unreliable) Some interesting observations result
Our ability to tolerate O2 is further complicated by the body’s reaction to the mild exercise involved in diving activities, cold water, drugs, tiredness and other poorly understood effects. That we now have impressive Nitrox diving tables detailing our apparent precise build up of CNS percentage during recreational dives, came at great cost in human life during WWII. The desire to use closed circuit rebreathers to place limpet mines on Battleships at harbor pushed the British Admiralty in one classic experiment to expose 100 men underwater at a depth of 50ft (15 meters) on pure O2 for 30mins. 26 divers convulsed, 24 had other symptoms. With their underwater convulsions(often coming on without warning) taking the form of grand mal epilepsy, drowning from loss of the mouthpiece or head injuries from contact with the inside of the brass diving helmet assured an unhappy outcome. Current recommendations for technical diving use would only allow pure O2 for decompression stops at a maximum depth of only 20ft (6 meters). For extreme dives beyond 330ft (100 meters) depth, many different mixtures have to be breathed by the diver on the way down, and on the way up to keep the ppO2 within the critical range. Overall dive CNS may be 250% or more. As the dive duration increases the continual use of high ppO2 is not possible and decompression becomes very inefficient. Additionally, there may be so little Oxygen in the bottom mix, that accidentally placing this regulator in the mouth on the surface or in shallow water during the descent would result in black out within a minute as the gas attempts to drive the blood ppO2 below that required for consciousness. Oxygen
Content comparisons for recreation, technical & extreme dives 13m Air Max saturation depth for air pp02 =0.5bars 56m Air Maximum recommended depth ppO2 =1.4 bars 60m Trimix 20/25 20% O2. Can breathe on the surface 90m Trimix 14/45 14% O2. Must not breathe shallower than 5m 313m Trimix 5/76 5% O2. Must not breathe shallower than 30m ppO2=1.615 bars. Bounce. Deepest Ocean dive 701m Hydreliox 0.7%O2 pp02=0.5bars. Saturation. Deepest chamber dive Finally, for an extreme diver suffering from DCS, attempts at treatment by exposure to additional hyperbaric oxygen such as that given by USN treatment table 6 on top of the already massive O2 dose received during the dive will complicate or delay the repetitive treatments necessary in attempt to give a full resolution of symptoms. Oxygen management on extreme dives is a very serious business.
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