<span =''>[b said:Quote[/b] ]Here is the so called "bakers dozen" reasons why not to use 80/20:
Anyone know enough about the topic to confirm or deny that, since Oxygen is a vasoconstrictor, breathing pure Oxygen will slow down offgassing and therefore NOT be as efficient as you might think?[b said:Quote[/b] ]1. Decompression with mixes less than 100% is inefficient.
Isn't that the same as point 6?[b said:Quote[/b] ]2. Decompression with 80/20 simply allows divers to compensate for poor buoyancy control and generates longer decompression times.
Isn't that why we keep Oxygen on the boats..?[b said:Quote[/b] ]5. In a diving emergency that causes decompression to be shortened, 80/20 does not provide as large a safety margin as 100% O2.
You need to consider the 100% question in context of the hole dive. Ie using O2 below 6 or 9m (habitat) is generally unsafe. Also "air" breaks need to be considered to avoid CNS & Pulmonary toxicity effects that may impinge on efficient off gasing via diffusion and perfusion methods.[b said:Quote[/b] ]1. Decompression with mixes less than 100% is inefficient.
Thats one of the reasons why the industry moved away from the 1.6 bar @ 6m stop. Afterall if you can only hold a stop to +/- 0.5m then at 6m you'll go from 1.65 to 1.55 bar ppO2. Might be enough to push you over the edge. So rather than turn away a cheque from a unready cash cow .. oops sorry meant diver... they came up with 80%. Shifts the higher exposure down to 9m where its easier to hold a stop and the effects of poor buoyancy are less significant regarding O2 spikes.[b said:Quote[/b] ]2. Decompression with 80/20 simply allows divers to compensate for poor buoyancy control and generates longer decompression times.
The oxygen window concept behind the WKPP deco model and its subsequent deco gas choices is that more gas is removed the higher the O2 pressure (not forgetting upper limits due to tox problems) 1.6 (1.9) bar is generally the best obtainable underwater. Once a gas switch is made and you ascend from the optimum depth the ppO2 will drop, the benefit from this high ppO2 will therefore drop. Once it drops to approx 1 bar the effect is minimal so a switch to a higher FO2 gas is made. The obvious corollary is that you should switch at every depth to a gas to maintain the ppO2 at 1.6 bar- this leads to the constant ppO2 RB argument.[b said:Quote[/b] ]3. The 80/20 mix is inadequate for depths above 9 M as it provides a maximum pO2 of 1.52 bar.
4. The 80/20 mix provides a pO2 of only 1.28 at 6 M and 1.04 at 3 M, which he claims is "worthless for decompression."
First off there is mounting evidence that adding He to deco mixes eg 50/25 increases deco efficieny allowing for shorter cleaner deco. Though these benefits have only been experienced on dives with significant BT. I mention that to show that life isn't simple and the Art of Deco is often a balence of conflicting forces. Using 100% does max out the pressure gradient to allow the max off gassing in both diffused and bubble forms. But cos you have to control your O2 exposure you have to balence the effect you want with protocols to minimise effects you don't wont. C'est la vie[b said:Quote[/b] ]In response to arguments that breathing pure O2 has measurable negative effects on the lungs one can take occasional breaks by breathing a little backgas. This argument appears to negate the principal argument as breathing backgas provides a return dose of nitrogen.
Do the maths at 9m on 80% ppO2 = 1.52 bar a +/-0.5m varience on stop depth will alter ppO2 from 1.56 - 1.48 bar cf 100% @ 6m where the range is 1.65 - 1.55 bar so at 9m the absolutes values are lower and the range is smaller. Thus allowing a diver who cant hold a stop a larger margin of error at 9m than at 6m. As for divers who cant hold a stop WTF should I alter the way I dive cos of the incompetance of others.[b said:Quote[/b] ]Also for point 6, Boyle's law is why buoyancy control is worse at 6 M than 9 M. I have seen divers gain buoyancy as their tanks empty, becoming uncontrollably buoyant when they reach the shallows leading to a runaway ascent.
Not sure that your right when you say that any micronuclei are composed soley of 100%. As there will be plenty of diluent still being diffused out of the body as well as coming out in bubble form. Afterall if breathing 100% would protect from post dive exercise triggered DCS then a lot of bends could be avoided.[b said:Quote[/b] ]A very important point to consider, and one that is almost always overlooked, is the ever present formation of micronuclei during the climb up the ladder onto a boat or over rocks. If 100% oxygen is used during this phase none of the resulatant newly formed micronuclei will contain any nitrogen so rapidly disappear due to metabolism. Not so if any nitrogen is present in the arterial blood.
Any rpt any form of exercise during deco causes a significant increase in risk. After all thats what DR Powell talke about at the recent workshop in Tampa.[b said:Quote[/b] ]I firmly believe post-dive isometric exertion is a common cause of subsequent unexpected DCI and a sadly neglected phenomenon.
Decompression does not stop when you leave the water!
Worth repeating.[b said:Quote[/b] ]Decompression does not stop when you leave the water!