[cedricverdier]

Hi all,

A lot of interesting discussions in the last few weeks about calibration and O2 sensors.

Another factor that could lead to potential oxygen problems is the selection of the right pO2 setpoint.

I just posted an article about this:

http://www.rebreatherworld.com/close...html#post43537

As usual, all your comments are welcome (or almost all ).

Cheers

[rogeringebo]

Thank you Cedric!
Very interesting.....

Well done

Rodge

[Genesis]

I like it... and think you're onto something.....

[Sutty]

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.

[Helium_diver]

Cedric,

Great article , a must read.

Helium_diver

[nigelh]

I'm having problems with some of the text and a few of the deductions.

Quote:

1. For Gas management, we should use the rule of 1/3rds. At the end of the dive, a CCR diver should still have at least 1/3 of the oxygen cylinder left.

This is a conceptual misunderstanding of rule of thirds. My reserve should not depend on my cylinder size but my gas usage plan. Also I would prefer to have a specific size of reserve on a CCR as my usage is so small.

Quote:

5. For the Oxygen exposure, a competent CCR diver should always stay within reasonable limits and should select a setpoint lower than 1.1 for the bottom part of the dive, maybe increasing this setpoint during the decompression phase.

This is complete twaddle contradicting what most agencies teach and negating much of the benefit of CCrs. We are not seeing divers dropping like flies on 1.3 bar ppO2s. There may be a case for reducing it on longer dives or in repetative dive situations but as a general rule it has a good track record with no reported problems.

[JonnyB]

Awsome article Cedric, have a green...

Quote: (Originally Posted by nigelh)

This is complete twaddle contradicting what most agencies teach and negating much of the benefit of CCrs. We are not seeing divers dropping like flies on 1.3 bar ppO2s. There may be a case for reducing it on longer dives or in repetative dive situations but as a general rule it has a good track record with no reported problems.

Nigel, i do not agree with you at all here, the article here is written for deep dives and i must say i agree with cedric on this, keep the setpoint low, kepp the cns clock low and pump it up on deco. Looking at the example cedric made, i have to agree that 12 minutes more deco on a 3 hour dive is worth it to keep the cns down...

/Jonny

[AD_ward9]

Quote: (Originally Posted by nigelh)

We are not seeing divers dropping like flies on 1.3 bar ppO2s. There may be a case for reducing it on longer dives or in repetative dive situations but as a general rule it has a good track record with no reported problems.

Nigel, your posts are generally very good, but on this one I think you may have overlooked something.

Divers do not have to die like flies for there to be a problem. Divers are dying from O2 convulsions on rebreathers, with a surprisingly high figures given the number of RBs in circulation. Just look over the accident reports. I must confess to agree with Cedric on this one.

With DCS, divers take a risk and when the 1 in 10,000/hour risk bites them, they get treatment.

With CNS, divers take a risk, and when the 1 in 100,000/hour risk bites them, they are dead.

It is a simple matter: look at the CNS probability curve, decide how many sigmas you want to keep safe, and dive that.

To save 10 or 20 minutes of deco, I for one reduce my max PPO2 to 1.1, though saying that, I use it throughout the dive.

What some do is say "Well, for medical treatment of the bends we whack in a PPO2 of 2.3 and they don't convulse, so it must be OK to go with 1.4", or even 1.6 I have seen a couple of times.

Again, just look at the accident figures. Divers are dying from O2 convulsions on rebreathers. Very experienced divers.

As regards agencies, I think they are wrong on this one and should be recommmending 1.1, or even 1.0.

Alex

[nigelh]

Quote: (Originally Posted by AD_ward9)

Divers do not have to die like flies for there to be a problem. Divers are dying from O2 convulsions on rebreathers, with a surprisingly high figures given the number of RBs in circulation. Just look over the accident reports.

I'm sorry but you are clearly reading different accident reports to mine but I confess I'm not following other areas than Inspiration.
Do you have an internet source you can point me too and I'll go away and read up.

[ScubaDadMiami]

I am quite the . However, given that that there have been a number of experienced CCR divers reported to have toxed, this makes me start to think that perhaps we do not know everything there is to know about this phenomenon. On the other hand, we do know that by limiting PPO2 exposures (within acceptable ranges), this seems to reduce the chance of toxing.

I just don't see why, in an abundance of caution, we do not maintain more conservative limits and extend deco a bit. There are circumstances do not allow for this luxury in all dive locations (local condition limitations, currents, etc.). However, I see no reason not to be a bit more conservative when circumstances allow.

Perphaps one could argue to reduce even more. However, there is a sweet spot that seems to be a reasonable balance of risk. That is what we all seek.

My plan is to run about 1.0 to 1.1 on the bottom, and then bump up to 1.3 or so depending on run time for deco. I am willing to spend some extra time on deco as a price to get myself back home.