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Monday, March 26, 2007

Accelerated decompression

One fringe benefit which comes with technical diving is accelerated decompression. To minimize the time spent in the water decompressing, the practice entails switching gas mixes during the ascent phase of the dive. Of course, the greater complexity involved carries with it increased risk, but scuba diving, like many other sports, such as mountaineering, rock climbing, auto racing, is all about risk management. Properly managed, risk may be acceptable. With proper training, procedure, and technique, much of the additional risk can be reduced.

Listed below are 3 custom generated decompression tables for a theoretical dive to 55 meters seawater (msw) for a bottom time of 22 minutes.

Nota bene: These tables are for illustrative purposes only. Technical diving requires proper training, equipment, and experience. Actual use of these tables may result in candidacy for the 2007 Darwin Awards. To further discourage yahoos, the tables are deliberately left incomplete.


Backgas = the gas carried on your back (i.e. your main breathing gas).

Decogas = the gas used for decompression (which may or may not be the backgas).

msw = meters sea water. Depth in meters in sea water.

Min = minutes spent at each depth.

Run = run time. The cumulative time period between the descent phase (the beginning) and the end of the ascent phase (the completion) of the dive.

Mix = the gas breathed at each depth.

Dive A
Backgas: air (21% Oxygen / 79% Nitrogen)
Decogas: air (21% Oxygen / 79% Nitrogen)

       Bottom Time:        22 minutes
       Decompression:  73 minutes
       Total Run Time:   95 minutes

Dive B
Backgas: air (21% Oxygen / 79% Nitrogen)
Decogas: Enriched Air Nitrox EAN50 (50% Oxygen / 50% Nitrogen)

       Bottom Time:        22 minutes
       Decompression:  37 minutes
       Total Run Time:   59 minutes

Dive C
Backgas: Trimix 20/24 (20% Oxygen / 24% Helium, 56% Nitrogen)
Decogas: (1) Enriched Air Nitrox EAN50 (50% Oxygen / 50% Nitrogen)
                 (1) 100% O2 (100% Oxygen)

       Bottom Time:        22 minutes
       Decompression:  27 minutes
       Total Run Time:   49 minutes

All 3 dives have the same bottom time. I.e. 22 minutes at 55 m, but note the difference in decompression times. Using air (21% Oxygen), Diver A requires a lengthy 73 minutes of decompression before surfacing. On the other hand, Diver B, who uses EAN50 (50% Oxygen, 50% Nitrogen) to accelerate his decompression, only needs 37 minutes. Diver C does it even faster; employing a combination of EAN50 and 100% Oxygen, he only requires 27 minutes of decompression.

What do these figures translate to in the real world?

Diver B and C are out of the water after a total of 59 and 49 minutes respectively, have showered, eaten, and are shooting the breeze on deck while Diver A, who faces a run time of 95 minutes, is still underwater battling currents, passing jellyfish, thirst, hunger, the urge to pee in his wetsuit, etc.

Of course, there are additional risks in using accelerated decompression. There are no gas switches for Diver A to mess with, so it's a no-brainer — he only needs to keep breathing (something even comatose patients, retards, and cell phone users can do).

EAN50 has a Maximum Operating Depth (MOD) of 22 m, so if Diver B and C switch to breathing it too early, or have poor control of their buoyancy and sink below 22 m after switching, they may suffer a central nervous system Oxygen toxicity seizure (CNS OTox "hit"), convulse and drown.

Diver C faces an additional risk: as he carries two different decompression gases, he could mix up the two. I.e. breathing 100% Oxygen (which has an MOD of 6 m) at 21 m instead of EAN50. This is invariably fatal as a CNS OTox seizure swiftly follows, resulting in death by drowning.

Why do Dive profile C then, since the difference in decompression time between B and C is only 10 minutes?

By using Helium as a substitute for some of the Nitrogen in his backgas, Diver C suffers less nitrogen narcosis than Diver A or B. At 55 meters on compressed air (79% Nitrogen), Diver A or B will discover their skills and training severely compromised by nitrogen narcosis. (This one time, at band camp on an air dive to 57 m, I was too narced to remember which was clockwise and which was counterclockwise on my isolation manifold.) Correspondingly, Diver C also remembers more of his/ her dive than Diver A or B.


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