Analysis of a 47m deco proifile.

[Mark Chase]

Originally Posted by Howard Payne

Hi Mark
I sorted started all this with a light hearted comment on your Seaford Ferry trip report and it's grown into quite an interesting thread - so I'm not feeling quite as guilty as I would have done otherwise
I really hate "peer pressure deco" mate and you've done vastly more deep mix diving than I have so please don't feel any pressure from anyone, least of all me, to do anything other than what you feel is appropriate for the dive YOU are doing.
If you were feeling a bit crap and having a few PPO2 problems then under similar circumstances - I would probably have erred on the side of caution as well.
You're sitting at home writing a thread about the dive rather than sitting in the pot bored off your tits - so in that sense - you did the right amount of deco for the dive.
It seemed like a lot of deco so me - but I wasn't doing it and I wasn't there.
Besides any dive below 30m with the Dude where you live to tell the tell has got to be "doing it right" hasn't it

I don't feel pressure from any one about any of my diving (unless I am on a course then it feels like big brother is watching) so never concern your self with that M8. I was in no way offended by your origional comment.

However I am never too old to learn and I am always interested in other peoples experience and opinions. Especialy the opinins of people who the selves think outside the box a bit.

I will continue to do it my way at all times unless a better idea crops up in which event ill consider it

I read all this information with a view that all the contributionrs are younger or fitter (or both) than I am

ATB


Mark Chase

[RichW]

Originally Posted by And

As for the CCR thing, I am no expert, but just because the po2 stays at 1.5 (not 1.6, if you look at your chart) then does that not just mean that the o2 being metabolised is the same volume as that being injected into the loop? I am unsure what signicance it has in terms of inerts. It could I guess tell you that your blood has been saturated with o2 at a pressure of 1.5 and you could then say that the 'o2 window' is at its optimum, because at the beginning of the stop it will take a few minutes for the inspired po2 to match the rest of your bloodstream (in this case 5 mins perhaps?), but it isn't really an indication of what the state of play is with the inerts is it?

This is the type of question where we need the likes of Rich Walker, as it is too scientific for the likes of me.

Andy

You flatter me mate

I presume thast what Mark is talking abut is that if you are sat at 6m, with a set-point of 1.6ata ppO2, then the you should be breathing pure O2. If that ppO2 drops over a period of time (assuming the thing is working properly) then the reason for this must be that inert gasses are being expelled from the lungs into the loop. This would drop the pO2, but then I guess the machine would try to inject O2 to compensate. I'm not sure that it's physically possible for it to do it, without an O2 loop flush. I could easily be wrong on rebreathers though

I would have thought that the first noticable effect would be a buoyancy shift, as the loop volume would increase, first due to the inerts ending up in the loop, and then the machine trying to inject more gas to bring the pO2 into line.

Maybe you could run your deco based on buoyancy shifts. i.e. no more buoyancy changes at a given depth implies no more inerts coming out. So you can move on up to the next stop, and wait again. Anyone want to try?

Rich

[Andy Kerslake]

Looking at the various options you presented I suspect they would all work. I think this dive along with many others of a similar time/depth fall into the miniumum deco range or in other words there is a certain amount that you must do.

As Clare pointed out it would appear to be the intermediate stops that make a big difference (21m - 9M), and if these are not long enough then it may be too late by the time you get to 6m.

The ratio deco of 1:1 was designed to only use 1 deco gas eg 50% and not to use O2 at 6. So in theory we could do a bit less at 6m (and up).

If we beleive the Oxygen window works (whole seperate discussion), then deco on O2 at 6m is far more efficient than at 3m. This is not really reflected in any of the models. But then who says the "models" are right :-)

If I was using O2 on this dive then all I would really be prepared to do is shorten the 6m and up.

Regards

[Mark Chase]

Originally Posted by RichW

You flatter me mate

I presume thast what Mark is talking abut is that if you are sat at 6m, with a set-point of 1.6ata ppO2, then the you should be breathing pure O2. If that ppO2 drops over a period of time (assuming the thing is working properly) then the reason for this must be that inert gasses are being expelled from the lungs into the loop. This would drop the pO2, but then I guess the machine would try to inject O2 to compensate. I'm not sure that it's physically possible for it to do it, without an O2 loop flush. I could easily be wrong on rebreathers though

I would have thought that the first noticable effect would be a buoyancy shift, as the loop volume would increase, first due to the inerts ending up in the loop, and then the machine trying to inject more gas to bring the pO2 into line.

Maybe you could run your deco based on buoyancy shifts. i.e. no more buoyancy changes at a given depth implies no more inerts coming out. So you can move on up to the next stop, and wait again. Anyone want to try?

Rich

You could run it on your PP02 read out but not on boyancy. And once at 6m and on 02 it dosent matter if the unit is working or not. Only a total flood would render it useles as an 02 rebreather.

Obviously the inerts have no where to go unless you personally vent them. The 02 inject will only replace the 02 that you metabolize anyway so basically your PP02 just drops until you finish off gassing.

On Pure 02 CCR you metabolize the 02 and the counter lung volume drops so you have to inject 02 to compensate. However before buoyancy becomes an issue you should feel the counter lungs bottoming out on the end of the breath. This is called running minimum loop. IE the lungs hold no more and no less than one breath.


Buoyancy on a CCR is a pig because you cant use your own lungs to compensate for minor fluctuations in buoyancy. Breath in on OC you go up breath in on CCR nothing happens same with breathing out.

It takes a lot of getting used to.

Sinking on the stops due to metabolizing the 02 indicates that you arnt running minimum loop. QED its a newbie problem.

ATB

Mark Chase

[Janos]

On the whole loop thing.

Remember the KISS is a MCCR so doesn't adjust it's addition of O2 automatically. If I am working hard on the surface, I can feel the loop getting tighter as it shrinks, due to me metabolising the O2. Not a problem I just touch the O2 add button to top it up.

At 6m, breathing pure O2, the only way the ppO2 can change is if the percentage of O2 in the loop changes. And the only way it can do this is if inerts are off-gassed into the loop.

This will be picked up by the cells (assuming they are working of course).

The bouyancy idea is a nice idea, but I personally find it much harder to control bouyancy on the rebreather than open circuit, despite not much OC diving these days. Although I'm told others find it easier to control their bouyancy on a rebreather than OC.

Janos

[RichW]

Originally Posted by Janos

At 6m, breathing pure O2, the only way the ppO2 can change is if the percentage of O2 in the loop changes. And the only way it can do this is if inerts are off-gassed into the loop.

For sure. But I was wondering if this was a noticable effect. Is it? Any guess as to how much comes out?

Rich

[Janos]

Originally Posted by RichW

For sure. But I was wondering if this was a noticable effect. Is it? Any guess as to how much comes out?

I seem to remember NigelH posting a while back that He absorption is measured in litres but Nitrogen is many times less than that. However I couldn't tell you his source so although I have no reason to doubt him I wouldn't bet on the result.

Looking at the drop in ppO2 on Mark's profile then I think a few litres is about right (total loop volume is roughly 15 litres and so a drop from 1.6 to 1.5 is around one litre of inerts off-gassed.

But it's still speculation on my part. I wouldn't be surprised if I was an order of magnitude out.

Janos

[Garf]

This thread has been great, that's something I never thought about with breathers and a really interesting fact.

[Mark Chase]

Originally Posted by RichW

For sure. But I was wondering if this was a noticable effect. Is it? Any guess as to how much comes out?

Rich

Its very noticeable. When you first hit 6m after a longish deep ish dive on 45-50% mix you flush to 1.6 and you can watch the numbers go down as you breath.

ATB

Mark Chase

[ibbo]

thanks