Help me to understand what happened

[Angelo Farina]

I would NEVER advice anybody to do deep air dives. There are just better ways.

I agree entirely on this.
Deep air is very dangerous and not worth the risk, there are much better ways, much safer, to go deep...
Only problem is to set the limit, where air starts to be "deep".
In my opinion, "deep" for air starts at 50m. Until 50m, diving with air and light deco is NOT technical diving, it is fully recreational, and safe enough for anyone who has got proper training for it.
This was the rec limit when I was a student, and for decades people were certified for rec diving down to 50m and with light deco.
I think that in organizations such as BSAC this is still the limit for rec diving.
Of course, other agencies decided to set the limit for rec diving in air at shallower depth, and to avoid deco, and of course this is safe to do. Particularly when training was sped up to short courses instead of a 6-months long OW course, as it was at my time...
But I know of tech divers who claim that even at 30m it is better to use normoxic trimix, and who use deco stages with 100% oxygen for a dive down to just 40m. These things can certainly be done, but I find them overkilling and entirely unnecessary for dive profiles which are perfectly safely done breathing normal air.
Then there is a limit (set you where you think more appropriate) where plain air is not anymore safe enough, and there tech diving starts. I prefer to not exceed that limit, I am not a technical diver and I have no need or desire to go there...
Coming back to the original post, it looked to me one of these cases where a perfectly recreational diving profile was transformed to tech diving without any need, just for sake of making things unnecessarily complex. As one of the divers suffered problems with gas switching, which was entirely unneeded for such a profile, my suggestion was simply to avoid to use gas switching for such shallow dives.
Some people love to make the things as complex as possible. I, instead, prefer to keep them as simple as possible...

 

[beester]

I agree entirely on this.
Deep air is very dangerous and not worth the risk, there are much better ways, much safer, to go deep...
Only problem is to set the limit, where air starts to be "deep".
In my opinion, "deep" for air starts at 50m. Until 50m, diving with air and light deco is NOT technical diving, it is fully recreational, and safe enough for anyone who has got proper training for it.
This was the rec limit when I was a student, and for decades people were certified for rec diving down to 50m and with light deco.
I think that in organizations such as BSAC this is still the limit for rec diving.

The point is I know where you come from, because I came from the same place (started diving in 1990 using still 1 stage regulators, advancing until 1 * CMAS instructor, doing loads of deeper dives on air... even as far as 2007-2008 I was doing deeper air dives, penetrating wrecks with insufficient gas, etc etc). But you don't know where I come from (technical/cave diving using trimix). Still you evaluate your "way" as the best without ever have been in touch with another way (using staged decompression and trimix). Isn't that a bit strange. It would be like critisizing an academic paper, without ever having read it, or without it being in your field of expertise.

I agree entirely on this.
But I know of tech divers who claim that even at 30m it is better to use normoxic trimix, and who use deco stages with 100% oxygen for a dive down to just 40m.

I've seen as many deco accidents on 30-40m dives than on 80-90m dives. The problem is slow tissues. I'm always very aware of this when I plan a "shallow" but longer dive (for example 45' at 30m).

Coming back to the original post, it looked to me one of these cases where a perfectly recreational diving profile was transformed to tech diving without any need, just for sake of making things unnecessarily complex. As one of the divers suffered problems with gas switching, which was entirely unneeded for such a profile, my suggestion was simply to avoid to use gas switching for such shallow dives.
Some people love to make the things as complex as possible. I, instead, prefer to keep them as simple as possible...

The original post was describing an incident during a technical diving course, so it doesn't matter if you find it unnecessary complex, gas switching and accelerated decompression are part of such a course ;-)

Anyway...I'm sure I would like drinking a cappucino with you somewhere talking about diving, you must have some really nice stories to tell. But I feel obligated to give another point of view, not for your sake (you will not change your mind) but for lesser experienced divers.

 

[Angelo Farina]

I see your point.
However let's me explain better. The guy who taught me diving in 1975, named Fabio, is a technical instructor and commercial diver, and one of my two best friends.
When he started his tech and commercial training, 20 years ago, he became a strong supporter of accelerated deco with pure oxygen or other oxygenated mixtures.
The other my best friend, Marco, is a CMAS instructor but also a doctor who studied hyperbaric medicine.
So the three of us started doing bibliographic research on the topic of accelerated deco.
Of course we also made some experiments on ourself.
Our findings, and particular Marco's opinion, is that doing accelerated deco in highly oxygenated mixtures is somethong not well proven scientifically, as it is based on over-simplified models of human physiology.
In practice the standard deco models used for computers and diving tables are used.
These models describe the human body as a set of independent compartments, each of them exchanging inert gasses directly with the lung contents, and each characterised by a different hemi-saturation time and by a different oversaturation limit factor.
Albeit such a model, after proper tuning, revealed to be succesfull for shaping DCS-free diving profiles when breathing air, Nitrox or Trimix, the model fails short describing what happens when you suddenly switch the gas mixture.
This particularly with reference to the slowest tissues (modelled by compartments with long hemisaturation times).
These tissues are poorly irrorated by the blood flow, and it is simply untrue that they exchange gasses directly with the content of your lungs.
During accelerated deco in pure oxygen, these slow tissues continue releasing the excess of inert gas with substantially the same rate as if you were breathing normal air.
They are more affected by ambient pressure than by the p.pressure of the inert gas in your lungs.
So you shorten the deco time thrusting an unrealistic model, and you risk that your slow tissues did not have time enough for degassing properly.
In this field I tend to be conservative, and I have great esteem of the medical opinions of my friend Marco.
Hence, whenever highly oxygenated deco gas was available, I often accepted it. But I did always wait for my air-based deco times. Never shortened the deco stops relying on availability of pure oxygen or EAN50...
Better to be safe than sorry.
If other people thrust these accelerated deco methods, good for them. But the info and the opinions I received on this topic are such that I do not risk a DCS in my slow tissues based on yet-perfectible models of the human body.
Developing numerical models of reality is my main academic job (I teach applied physics at the uni).
I even developed a couple of commercial computer programs for numerical simulations.
Exactly because I know how these physical models of reality are built and tuned I will never thrust them too much...

 

[beester]

Ciao Angelo, I'm leaving for Croatia to do some deeper wreck diving (I live in Veneto), but I wanted to answer you before I go.

First of all thanks for sharing, it gives more insight in where you are coming from.

I could reply that I've done many dives in depth ranges that would be impossible to do without accelarating your decompression. Running a typical profile through a planner decompressing on air would give 16-20 hours of decompression, while using decompression gases will let you get out of the water after 2 hours of deco. There are many many divers who have done the same.

Of course this is all anecdotal, and is no scientific evidence. Decompression remains a very complex physical/phisiological/chemical problem, and divers having done such dives, doesn't proof that it's safe, we could just be lucky.

Do you have any sources for below?

This particularly with reference to the slowest tissues (modelled by compartments with long hemisaturation times).
These tissues are poorly irrorated by the blood flow, and it is simply untrue that they exchange gasses directly with the content of your lungs.
During accelerated deco in pure oxygen, these slow tissues continue releasing the excess of inert gas with substantially the same rate as if you were breathing normal air.


I'm asking, because I wonder when you were doing your research on this, there has been quite a lot of evolution in our knowledge in the last 10 years. I suggest looking for and reading up on papers by Dr. David J. Doolette and Dr. Simon J. Mitchell. They are at the cutting edge of decompression research, and have regularly published research that has broken open established decompression theory (how deep you start doing decompression stops, the influence of gas density, the use of oxygen for decompression). Next there is research done by NEDU and by DAN, which is also very interesting.

Anyway I'm off to dive ;-)

 

[rob.mwpropane]

Anyway...I'm sure I would like drinking a cappucino with you somewhere...

I thought Italy was known for espresso?

 

[Angelo Farina]

Ciao Angelo, I'm leaving for Croatia to do some deeper wreck diving (I live in Veneto), but I wanted to answer you before I go.

First of all thanks for sharing, it gives more insight in where you are coming from.

I could reply that I've done many dives in depth ranges that would be impossible to do without accelarating your decompression. Running a typical profile through a planner decompressing on air would give 16-20 hours of decompression, while using decompression gases will let you get out of the water after 2 hours of deco. There are many many divers who have done the same.

Of course this is all anecdotal, and is no scientific evidence. Decompression remains a very complex physical/phisiological/chemical problem, and divers having done such dives, doesn't proof that it's safe, we could just be lucky.

Do you have any sources for below?

This particularly with reference to the slowest tissues (modelled by compartments with long hemisaturation times).
These tissues are poorly irrorated by the blood flow, and it is simply untrue that they exchange gasses directly with the content of your lungs.
During accelerated deco in pure oxygen, these slow tissues continue releasing the excess of inert gas with substantially the same rate as if you were breathing normal air.


I'm asking, because I wonder when you were doing your research on this, there has been quite a lot of evolution in our knowledge in the last 10 years. I suggest looking for and reading up on papers by Dr. David J. Doolette and Dr. Simon J. Mitchell. They are at the cutting edge of decompression research, and have regularly published research that has broken open established decompression theory (how deep you start doing decompression stops, the influence of gas density, the use of oxygen for decompression). Next there is research done by NEDU and by DAN, which is also very interesting.

Anyway I'm off to dive ;-)

As said, our research is dating back almost 20 years. I know of these "modern" developments in deco theory, but, as said, I am old school and very prudent and suspicious. Particularly when talking abut physical or physiological computer models.
Something which cuts deco times from 20 hours to 2 hours is "too nice to be true", in some sense.
I know that many tech divers are systematically running these accelerated deco plans in the last years. But DCS in slow tissues is not something you perceive immediately, You will start having problems to your joints and legaments only after 15-20 years of exposure.
I respect people taking these risks, but sorry, it is not for me. Not at this age, perhaps if this technology had been available when I was 20, and I was fearing nothing, I had jumped on the waggon of accelerated deco.
I am too old and too savvy for accepting these unknown risks now...
What I fear more, is people thrusting so much computer models. I see them failing every day...
The more advanced is the research on which they are based, the more risky is their usage in the real world.
The fact that they are based on research recently published on top-level peer-reviewed scientific journals is usually a trigger about technology which is yet far to be proven to be fail-safe for day-by-day engineering usage.

 

[Angelo Farina]

I thought Italy was known for espresso?

I also usually prefer cappuccino over espresso, or cappuccino's smaller variant, marocchino...
However I am from Parma, and usually we are most famous for the Parmesan cheese and the Parma's ham...

 

[JonG1]

@beester don't want to derail the thread but be interested in your comments about long shallow dives and DCI stress, happy to take it to a new thread if needs.

Are you thinking that existing models don't cater for this profile well.

 

[fsardone]

I also usually prefer cappuccino over espresso, or cappuccino's smaller variant, marocchino...
However I am from Parma, and usually we are most famous for the Parmesan cheese and the Parma's ham...

Me, you and @beester should meet sometime for an apericena or a spritz.
I come at least twice a year in Brescia to fly ...

Italy is known for a variety of great food!

 

[Angelo Farina]

@beester don't want to derail the thread but be interested in your comments about long shallow dives and DCI stress, happy to take it to a new thread if needs.

Are you thinking that existing models don't cater for this profile well.

Current models are based on an equivalent circuit made of a number of independent RC circuits, all driven by the pp of the inert gas in your lungs. No transfer among compartments is taken into account. And also the ambient pressure is considered irrelevant.
Such models, after proper tuning of their parameters, revealed to work very well for "normal" diving profiles, when all the compartments are ongassing first, and degassing during ascent.
The model reveal his weakness only under peculiar conditions, when some tissues are degassing and some other are still ongassing. And this weakness mostly affects the slower tissues, as they are not directly in contact with the main blood stream (which reflect closely the pp of gases in the lungs).
Said that, I have not enough knowledge and expertise for evaluating which diving profiles are correctly modelled, and which are not. What I see, instead, is that many tech divers and instructors simply "trust too much" current deco models.
Their high percentage of success avoiding DCS for millions of divers is no proof of the fact that the model is correct, it is just proof that the parameters were tuned conservatively enough for covering the model discrepancies.
Developing mathematical models of physical phenomena is my job at the university, and I see this happening in any field, every day. Wrong models which are successfully used only thanks to conservative tuning of their parameters.
For me, there is still need of developing more realistic models, which take into account tissue-to-tissue gas transfer, and the independent effect of pressure difference between the gas trapped in a tissue, the ambient pressure and the p.p. of the same gas in the blood. Hence, I do not thrust the usage of existing models for "extreme" diving profiles, such as those highly-accelerated deco methods. They are fine, instead, for doing NDL dives on my Cressi Leonardo computer.
I am not in favour of abrupt gas switching, too. I have seen too many cases of convulsions using military rebreathers (such as the Drager or the OMS Caimano IV SC) when the system is switched from SC nitrox to CC pure oxygen...