Quote:
| Originally Posted by Abyss Divers When we do our Deco Stops after dives to 60 meters on air we use EANx 50% from the first stop on 9 meters still staying the computer Deco time on air this to be on the safe side! If you start with accellerated deco with Nitrox after a deep air dive you will for sure be to close to a Bend! Better to be conservative and knowing that you are safe! Even running up some stairs could cause a Bend after accellerated deco! |
This makes no sense but perhaps I have misunderstood
Air is a rubbish decompression gas because it has far too much nitrogen in it and that’s the stuff you’re trying to get rid of. 79% is bad. The best gas for off gassing Nitrogen is 100% 02. Off gassing is where the nitrogen is passed safely into the exhaled gas. In order to force the transfer of nitrogen it is better to have a higher pressure of oxygen to push it out. In order to achieve this we use high partial pressures of 02. 1.6 is used in 'normal' decompression diving and 2.0 in advanced decompression practice. The decompression chambers will use up to 2.8PP02.
Using 50% it is best to get on the gas at the highest safe PP02. Most divers would consider this as 1.6PP02. To achieve a PP02 of 1.6 with 50% you switch at a depth of 21m. Oxygen takes approximately 2mins to circulate through the system and provide benefit. As a result it is recommended that the 21m stop is extended to 5mins to open up the oxygen window and kick start the off gassing.
Using a 21m stop for 50% you could reduce the decompression for a 60m dive for 30 mins to 69mins. If you deco'd on air it would take 125mins. Starting your decompression on 50% at the 9m stop would take 86mins. (all figures created using decoplanner on a 20/85GF
What this demonstrates is that to utilise the 50% deco gas you should get on it as early as possible. The Hyperbaric docters recommend a minimum PP02 of 1.0 for oxygen for medicinal use. The PP02 of 50% at 9m is 0.96.
50% is a good single gas deco choice. However getting on it at 9.0m is not taking advantage of its full potential. Deep air using air for deco is bad. Using Nitrox is good. Personally on a 60m dive I would use two decompression gasses to keep the PP02 even higher. I use 32% switching at 40m and 80% switching at 10m. Using this I would require 52 mins of deco on a 60m dive for 30mins.
Modern thinking is that even better more effective deco can be achieved using Helium in the mix to further reduce the nasty Nitrogen. Helium is a fast gas, fast into the body, fast out. The up side is it is not toxic like O2. As a result the hyper baric community are looking carefully at using 50% 02 and 50% Helium to treat bends. This is not yet accepted practice but the DIR/ GUE boys and Mark Elliot are already recommending using Helium in the decompression mix to get rid of the nitrogen and allow effective off gassing to start deeper.
ATB
Mark Chase