Convo on Deco Gas Use and Deco Stops

[Rob Evans]

Aharr - George Irvine hates Inspirations with a ferocity bordering on the fanatical.  Trouble is, most of his beefs against the unit are very poorly thought out and he's far too arrogant ever to admit he's wrong on much of the shite he spouts about it.  So we all laugh at him, which pisses him off no end.

BTW, most of the CCR lads I know (inc me) flush the loop with 100% when we get to 6m and ramp the PO2 up to 1.6.  Doesn't make a whole lot of difference though.  I've done the maths on doing OC with 50% and 80% (85%, whatever) and 100% and running the box at 1.3 for the working phase of the dive and 1.5 for deco.  There's only minutes in it.  I've dived with OC guys who came out of the water at the same time or before me cos the back/deco gas they'd chosen was ideal for the dive we were on.
A few % here and there makes sod-all difference on deco times in real time, so why all the fuss?  
I don't subscribe to the mantra of oxygen windows, I want a well-developed, tried and trusted algorithm with a shitload of 'fudge factor' built in, so that I can do the dives I want to do without constant stress about how close to the envelope I am.  Blimey, fell of the fence again.

BTW.... LOVE the spawn of satin remark.  You're a mate, I love you dearly, I do understand your dyslexia and everything, but THAT is getting printed on the lid of my unit.  Spawn of Satin.  Everybody'll think I'm a pimp.  Wonder if I should redecorate the hoses with a bit of leopardskin.  I'LL PAINT IT PINK AND PUT FINS ON THE BACK!!

Yip yip
Rob

[Rob Evans]

Or not


Suddenly sobered up when I read that.

Yellow's fine.  

[Mark Chase]

Whoops {:0

Major embaresment factor 10 /10

Hey Mr Moderator

How bout a spell check on the new web site?

No?

Oh Bugger. Oh well at lest it gets the odd laugh.

All the best

Mark Chase

[Diving Dude]

Quote: from Rob Evans
Wonder if I should redecorate the hoses with a bit of leopardskin.  I'LL PAINT IT PINK AND PUT FINS ON THE BACK!!

Please do it after the Mull trip, we don`t want them up there to get the wrong impression of us down here.
Mind you it`ll match my daughter Becky`s dry suit.

[Rob Evans]

No, that was just a mad, babycham-and-curry inspired rant.  I'm far too sensible for that sort of nonsense.  However I do have lots of rufty tufty double hard gouges hacked out of the lid of the unit. I maintain this is all down to extreme-diving-type-stylee wreck penetration.  The guys I dive with reckon it's cos I spend lonely nights in front of the antiques roadshow wielding a rough file.  They are -of course- liars, fools and vagabonds.

A leopardskin drysuit you say?  Hmmm.. I might have to spend the trip strapped to the roof of your Discovery.  I'll only be a nuisance if left to my own devices inside the car.

[Rob Evans]

Blimey.  Three blobs now.  I'll be getting an avatar next.

[Paul Thomas]

<font color='#000080'>Does this make any sense?

George Irvine argues that decompression should be completed with 100% O2. His arguments, presented as a &quot;Baker's Dozen&quot;, boil down to just a half dozen points:

1. Decompression with mixes less than 100% is inefficient.

2. Decompression with 80/20 simply allows divers to compensate for poor buoyancy control and generates longer decompression times.

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 &quot;worthless for decompression.&quot;

5. In a diving emergency that causes decompression to be shortened, 80/20 does not provide as large a safety margin as 100% O2.

6. If buoyancy control is a problem at 6 M, why isn't it a problem at 9 M?

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.

I do not believe that there is a right answer to the question as posed. During decompression one must find a balance between efficent offgassing by minimising pN2 (by maximising pO2) and preventing bubble growth.

An ideal deco mix is one that gives a safe maximum pp O2 of around  1.6 bar with a different diluent from that used in the bottom gas, one with a heavier molecular weight. A 50:50 and 80:20 mix of Oxygen: Xenon have been proposed but Xenon is highly narcotic and causes unconsciousness at atmospheric pressure! Clearly Nitrox mixes are ideal for decompression following heliox dives because nitrogen diffuses much slower than helium so the problems of isometric counterdiffusion are absent and the nitrogen content of the deco gas is largely irrelevant following a dive where there has been no nitrogen loading from the bottom gas;- so get you cheque books out!

In the case of oxygen as a &quot;diluent&quot; there are toxicity concerns. Ideally one would breath as much O2 as possible throughout the decompression, and that is exactly what rebreather divers do. For the lung tissue concerns it isn't a question of the fraction of oxygen, but of the partial pressure of oxygen breathed - the limit is reached deeper for 80% than 100%.  

Either approach will work, but one point to consider is what happens if a diver were forced to breath his O2 on ascent after a catastrophic loss of breathing gas. At 20 M on pure O2 your pO2 is 3.0 bar, while on 80% it is 2.4. Both are dangerous but 3.0 is very dangerous, and almost guaranteed to cause fitting. Here a little inert gas goes a long way, which is why I always carried Nitrox 50 for the deeper stops when needed.

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.

Surface oxygen

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. Indeed I used 100% surface oxygen as an added &quot;hidden stop&quot; on many dives for this reason alone.

With surface 80% Nitrox, nitrogen is present with a counter pressure of 0.2 bar, while air has a counter pressure of 0.8 bar.

I firmly believe post-dive isometric exertion is a common cause of subsequent unexpected DCI and a sadly neglected phenomenon. &nbsp;

Decompression does not stop when you leave the water!

(with acknowledgements to Dr Micheal Powell)

[Andy the Coastie]

<font color='#0000FF'>hi all.


ermmmmmmmmmm what you lads mean when saying , &quot; HE IS A STROKE &quot; &nbsp; ?


ANdy

[Dominic]

Originally Posted by [b

Quote[/b] ]1. Decompression with mixes less than 100% is inefficient.

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?

Originally Posted by [b

Quote[/b] ]2. Decompression with 80/20 simply allows divers to compensate for poor buoyancy control and generates longer decompression times.

Isn't that the same as point 6?

Originally Posted by [b

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.

Isn't that why we keep Oxygen on the boats..?

P.S. A stroke originally mean a deliberately unsafe diver, ie one that had been shown a safer way to dive but stuck with the dangerous way as it was more &quot;macho&quot;. Original term was &quot;ego-stroker&quot; then just &quot;stroker&quot; and finally &quot;stroke&quot;.
These days, calling someone a stroke basically translates to &quot;You're not 100% DIR so you're a crap diver&quot;

[Mark Scott-Simons]

Originally Posted by [b

Quote[/b] ]1. Decompression with mixes less than 100% is inefficient.

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 &quot;air&quot; breaks need to be considered to avoid CNS &amp; Pulmonary toxicity effects that may impinge on efficient off gasing via diffusion and perfusion methods.

Originally Posted by [b

Quote[/b] ]2. Decompression with 80/20 simply allows divers to compensate for poor buoyancy control and generates longer decompression times.

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%. &nbsp;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.

Originally Posted by [b

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 &quot;worthless for decompression.&quot;

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.

This is why the deco gas FO2 &nbsp;are what they are:

&lt;6m 100%
21m - 9m 50% 1.6bar - 1bar
and then 35% for vary deep deco gas the switch is not made at 1.6 to minimise tox exposure and risk.

Originally Posted by [b

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.

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

Originally Posted by [b

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.

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 &nbsp;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.

Originally Posted by [b

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.

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.

Originally Posted by [b

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!

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.

Originally Posted by [b

Quote[/b] ]Decompression does not stop when you leave the water!

Worth repeating.

Scotty