French exception

[steinbil]

I'm no physiology expert either, but my understanding is that the "workaround" for narcosis is a progressive habituation to depth. It is agreed in France that one shouldn't dive to 60m after a long break and resume diving with progressive increments of 5m to the maximum depth.

As far as I can tell, the most serious danger at 60m on air is gas density. I seriously doubt that doing a few work up dives is going to change your physiology enough to improve the gas exchange in your lungs in any meaningful way.

 

[steinbil]

Each diver does what he sees fit for him. I might dive with those guys : I was trained to do the same : deep air, on a single tank, even to 60 m. Too be on the safe side, you're compelled to rather restrictive bottom times.

I've not done so for quite a while though, if Trimix is not available, I have my pony for deco (3L filled with either Nx60 or 70 depending of what I can get).

Good luck if you have any sort of failures leading to loss of gas at 60m. CESA from 60m?

 

[berndo]

As far as I can tell, the most serious danger at 60m on air is gas density. I seriously doubt that doing a few work up dives is going to change your physiology enough to improve the gas exchange in your lungs in any meaningful way.

Huh? And why is density the most serious danger? As you found out yourself, there is a psychological component to getting narced. That's what the work up dives are for.

 

[berndo]

The real scandal according to me lies in the difficulty of getting O2 for deco. It ought to be easier but in fact, is almost as complicated as getting helium.

Where do you dive? No shop around has O2? You could just get a 50l 300bar tanks and fill o2 yourself or with some friends. O2 is pretty cheap.

 

[JMBL]

Where do you dive? No shop around has O2? You could just get a 50l 300bar tanks and fill o2 yourself or with some friends. O2 is pretty cheap.

Basically I'm a nomad/tourist diver. Travelling with family, a 50L tank in the car already loaded for a 3 weeks for the 3 people + a dog is a big no no. Try geting 02 in the Alpes Maritimes, in August, and you'll see my point, believe you me.

 

[JMBL]

Good luck if you have any sort of failures leading to loss of gas at 60m. CESA from 60m?

Already happened, but I'm still here : that's why a dive is planned and why people train.

 

[steinbil]

Huh? And why is density the most serious danger? As you found out yourself, there is a psychological component to getting narced. That's what the work up dives are for.

Yes, I can see that the psychological factor plays a role and would be helped by progressive exposure, however your mental capacity and coordination will still be impaired by narcosis. I consider gas density to be more dangerous because it inhibits effective gas exchange in your lungs leading to CO2 build-up. If anything goes slightly wrong (which again is more likely when you're impaired) leading to a higher breathing rate, or increased effort, how much would it take for the CO2 levels to go beyond what you can recover from? At best you'll get even more narced from the CO2, possibly leading to panic, loss of consciousness etc. Please point out the flaws in my reasoning.

 

[steinbil]

Already happened, but I'm still here : that's why a dive is planned and why people train.

Please share the story, if you haven't already, or provide a link if you have. I would love to read about it. I am also curious about what goes into planning a single tank 60m dive on air with a 3L deco pony tank in terms of how you deal with failures.

 

[berndo]

Yes, I can see that the psychological factor plays a role and would be helped by progressive exposure, however your mental capacity and coordination will still be impaired by narcosis. I consider gas density to be more dangerous because it inhibits effective gas exchange in your lungs leading to CO2 build-up.

You consider based on what? You are more prone to getting narced than other people. Your experience had nothing to do with co2. You where clearly narced and didn't recognize what happened and now you're trying to lecture on people that have much more experience than you, on co2 issues?
Diving below 40m is much more common than you think it is and not only in France. (And I guess you might be a gue who's gotten very one-sided story.) All the online stuff doesn't give you a realistic picture about real world diving. It's fairly common but not advertised on the internet. CO2 build up doesn't just happen out of nowhere and these dive essentially are bounces... I've given you specific examples in your CO2 thread for cases in which co2 build up actually was an issue. Claiming it's an issue for experienced divers to do bounce dives in good conditions is kinda far fetched.
Having said that, somebody with little experience and that is more prone to getting narced really shouldn't dive deeper than 40m on air. Most people should not dive below 40m... vacation divers, inexperienced divers, people that don't dive alot in general.
What they are discussing here is only for people that dive a lot and have experience on their home turf. The French and Italian who live close to the Med have gread diving on their doorstep.

Please point out the flaws in my reasoning.

I already tried to explain it in your 'co2 hit thread'. You had a theory there too but it was not realistic.

Instead of making up theories, maybe ask questions as a new diver?

 

[steinbil]

You consider based on what? You are more prone to getting narced than other people. Your experience had nothing to do with co2. You where clearly narced and didn't recognize what happened and now you're trying to lecture on people that have much more experience than you, on co2 issues?
Diving below 40m is much more common than you think it is and not only in France. (And I guess you might be a gue who's gotten very one-sided story.) All the online stuff doesn't give you a realistic picture about real world diving. It's fairly common but not advertised on the internet. CO2 build up doesn't just happen out of nowhere and these dive essentially are bounces... I've given you specific examples in your CO2 thread for cases in which co2 build up actually was an issue. Claiming it's an issue for experienced divers to do bounce dives in good conditions is kinda far fetched.
Having said that, somebody with little experience and that is more prone to getting narced really shouldn't dive deeper than 40m on air. Most people should not dive below 40m... vacation divers, inexperienced divers, people that don't dive alot in general.
What they are discussing here is only for people that dive a lot and have experience on their home turf. The French and Italian who live close to the Med have gread diving on their doorstep.

I already tried to explain it in your 'co2 hit thread'. You had a theory there too but it was not realistic.

Instead of making up theories, maybe ask questions as a new diver?

Of course you begin with the straw man arguments. My opinions stated in this thread is completely unrelated to my incident, and is not based on my personal experience. If you actually read that other thread, you will see that I simply asked questions, I did not presume to know the answers.

I am not making up any theories, or making any assumptions based on anecdotal evidence, I'm simply rehashing what the actual experts in the field are saying based on their own scientific research.

Let me quote Reilly Fogarty from Dan, summarizing research by Simon Mitchell and Gavin Anthony (my emphasis):

Anthony and Mitchell found that both rebreather and open-circuit divers retained a dangerous level of CO2 when their respired gas density reached 6.0g/L (normal air is approximately 1.293 g/L at 1 ATA) (Figure 1, Table 1). Previous studies2 found that CO2 retention during exercise at 6.8 ATA (8.79 g/L) resulted in two subjects requiring rescue from a wet pot (a hyperbaric chamber filled with water and pressurized to simulate a particular depth) due to CO2-induced incapacitation without any self-awareness of the symptoms. Further complicating the issue, Christian Lambertsen and his colleagues found that at just 4 ATA — the pressure we would experience at 99 feet of seawater (fsw), where the density of air is about 5.17 g/L — maximum expiratory gas flow and ventilation rate were nearly half of what they were at 1 ATA.3 The evidence appears to indicate that gas density has a profound effect on not just WOB and CO2 evolution and elimination but also our ability to effectively respire and exchange gas.