Rebreather World - View Single Post - A guide about setpoint selection for deep dives

Sutty · 16th April 2006, 13:21

Nice article cedric.

Firstly I'll make it clear that I am a relative newbie at all this, and possibly the least "techie" rebreather diver I know - I'm only MOD1 qualified and dive (mostly) within the limits of that so don't do long deco stops etc. So please consider my coments in that context.
However I do have an interest in the physics and physiology/pathology!

I think the best argument you have put forward for a lower setpoint at depth is the reduction in O2 exposure/toxicity. However this only seems to become a problem with long or many repetitive dives, having said that I would agree that you never know your CNS O2 tox threshold at any given moment.

The argument that hypoxia problems take longer to happen than hyperoxia is more difficult - it depends whether you are timing from malfunction (eg O2 flow stops, or O2 reg freeflows), or from onset of diver-detectable symptoms. Certainly hyperoxic CNS toxicity symptom onset is often without warning symptoms, and hypoxia will take a while from O2 flow stopping, but many people will have no symptoms they notice from hypoxia before blackout occurs. Ascents add another dimension to this too of course, where a higher PO2 at the start of an ascent gives a better safety margin. An un-noticed stopping of the O2 flow followed by an ascent a while later is a nasty one, and worse if PO2 starts lower! (I know it shouldn't happen but...)

Conversely (ignoring descents for the moment) a mechanical failure which causes hyperoxia (O2 free flow) will happen very quickly, how much difference it makes starting from PO2=1 or PO2=1.3 I don't know (and don't fancy testing), but suspect very little. We are dependant on recognising the situation and correcting it (or at least getting onto bailout) very quickly, and avoiding more than a few breaths at very high PO2.

With all these things of course it is an individual assessment of which risks an individual diver wants to offset most in their individual situation - abilities, rebreather, dive profile, etc, etc.

Nice article to get people to think about it all, rather than using 1.3 automatically.

16th April 2006, 13:21	#4 (permalink)
Sutty Classic Kiss diver Current Rebreather/s: Classic Kiss Other Rebreather/s: Join Date: Jun 2005 Location: Glossop, Derbyshire, UK Posts: 815	Re: A guide about setpoint selection for deep dives Nice article cedric. Firstly I'll make it clear that I am a relative newbie at all this, and possibly the least "techie" rebreather diver I know - I'm only MOD1 qualified and dive (mostly) within the limits of that so don't do long deco stops etc. So please consider my coments in that context. However I do have an interest in the physics and physiology/pathology! I think the best argument you have put forward for a lower setpoint at depth is the reduction in O2 exposure/toxicity. However this only seems to become a problem with long or many repetitive dives, having said that I would agree that you never know your CNS O2 tox threshold at any given moment. The argument that hypoxia problems take longer to happen than hyperoxia is more difficult - it depends whether you are timing from malfunction (eg O2 flow stops, or O2 reg freeflows), or from onset of diver-detectable symptoms. Certainly hyperoxic CNS toxicity symptom onset is often without warning symptoms, and hypoxia will take a while from O2 flow stopping, but many people will have no symptoms they notice from hypoxia before blackout occurs. Ascents add another dimension to this too of course, where a higher PO2 at the start of an ascent gives a better safety margin. An un-noticed stopping of the O2 flow followed by an ascent a while later is a nasty one, and worse if PO2 starts lower! (I know it shouldn't happen but...) Conversely (ignoring descents for the moment) a mechanical failure which causes hyperoxia (O2 free flow) will happen very quickly, how much difference it makes starting from PO2=1 or PO2=1.3 I don't know (and don't fancy testing), but suspect very little. We are dependant on recognising the situation and correcting it (or at least getting onto bailout) very quickly, and avoiding more than a few breaths at very high PO2. With all these things of course it is an individual assessment of which risks an individual diver wants to offset most in their individual situation - abilities, rebreather, dive profile, etc, etc. Nice article to get people to think about it all, rather than using 1.3 automatically. __________________ Never forget that life is a finite resource.
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