deco myths

[Angelo Farina]

In the case of a single inert gaz, I'm not convinced by your argument. I've not make the computations be to be sure (I fear that's the kind of maths I'm no more used to do), but think of how Thévenin's theorem in electricity shows that any passive linear circuit is equivalent to one resistor, one inductance and one capacitor. Having a more detailed circuit could help to map compartments to physical organs, but I don't think it would give more meaningful dependency on the dive profile when you consider that the current one have already to take into account the variability of the sensitivity of any diver at different time, and between divers (for which GF is providing us a way to tune). My intuition is that you'd need more than cross-coupling factors, for instance some non-linearity in the DE, between compartments to get something new.

I'm less familiar with how multiple inert gaz is handled. My intuition is that even the little I've been exposed to would have dive-profile dependent effects in a more complex circuit (with several compartments having the same time constant but different cross-coupling).

As said, I perceive there is a problem of using many independent compartments when there are strong pressure gradients between them, neglecting any gas transfer directly from one compartment to another.
But I have not enough knolwedge for proposing a proper lattice model, where gas transfer from one compartment to another is possible.
As we treat these gases as perfect gases, the fact there is only one gas, two, or more is substantially irrelevant, as in perfect gas theory each gas behaves and moves only based on its own partial pressure, and the partial pressures of the other gases in a mixture is irrelevant.

 

[rob.mwpropane]

I always thought the same. One of the main reasons I never pursued technical diving.

Would rather make a couple of shorter dives separated by a warm, sunny surface interval while eating snacks and drinking ice cold water than doing one longer dive and then hanging on a line trying to stay awake for an hour or more.

You just don't know what you don't know. We do the same dives I would argue probably safer though. Going tech means you learned more about what's really going on when you dive, planning better, and being safer. You really don't have to do hour long deco, but you don't have to be scared of this voodoo thing called NDL either.

I am way more conservative then I ever was before because I know what's past that imaginary line and have a good (or maybe I should say better) understanding of how to handle it. There is TONS of freedom in that.

 

[L13]

As we treat these gases as perfect gases, the fact there is only one gas, two, or more is substantially irrelevant, as in perfect gas theory each gas behaves and moves only based on its own partial pressure, and the partial pressures of the other gases in a mixture is irrelevant.

For the purposes of gas transport (diffusion, perfusion, and solution) each gas can be treated separately. For the purposes of bubble formation with supersaturation, the total partial pressures of all gasses are used. This is why adding Helium to displace Nitrogen makes NDL times shorter, and deco times longer (unless it is a really big dive then it goes the other way, there are subtle details).

 

[rob.mwpropane]

For the purposes of gas transport (diffusion, perfusion, and solution) each gas can be treated separately. For the purposes of bubble formation with supersaturation, the total partial pressures of all gasses are used. This is why adding Helium to displace Nitrogen makes NDL times shorter, and deco times longer (unless it is a really big dive then it goes the other way, there are subtle details).

I was under the impression (and going out on a limb here because I don't have any He certs) that the Helium penalty was going away? Like forefront science was finding out that it really wasn't needed?

Maybe I read that wrong (again, no experience with He, lol... don't burn me at the stake)

 

[L13]

I was under the impression (and going out on a limb here because I don't have any He certs) that the Helium penalty was going away? Like forefront science was finding out that it really wasn't needed?

Maybe I read that wrong (again, no experience with He, lol... don't burn me at the stake)

It depends on the algorithmic details as to whether there is a penalty. My statement was based on stock B-16C with GF75/80 (I actually ran it in SubSurface to verify my memory). If it wasn't for the fact that supersaturation bubble formation depends on the total partial pressures, there would actually be a benefit of shorter dives in all the major algorithms.

 

[harvalen]

I was under the impression (and going out on a limb here because I don't have any He certs) that the Helium penalty was going away? Like forefront science was finding out that it really wasn't needed?

Robert Helling, who's responsible for most the deco code in subsurface, has written a long blog post on it.

N2 vs. He, what’s the difference? – The Theoretical Diver

thetheoreticaldiver.org

 

[Tom_Ivan]

No shame, but at the same time: why are you commenting on a thread about deco?

The thread is in the "New divers and those considering diving" forum. Additionally the OP was a video mostly targeted at recreational divers.
@LI-er is more than welcome to chip into the conversation.

I may have misread your tone but maybe you should apologize?

 

[Angelo Farina]

Going tech means you learned more about what's really going on when you dive, planning better, and being safer. You really don't have to do hour long deco, but you don't have to be scared of this voodoo thing called NDL either.

I am way more conservative then I ever was before because I know what's past that imaginary line and have a good (or maybe I should say better) understanding of how to handle it. There is TONS of freedom in that.

Although I agree on the general concept, there is NO need of "going tech".
A fully recreational (not technical) diver should know enough about decompression theory and practice.
There is no need to undertake technical training for planning and conducting recreational dives which include deco stops.
This is normal practice for recreational divers here in Europe and in tropical locations frequented by EU recreational divers.
Technical diving is not for everyone, only people with a very stable capability of reacting to danger should enter the tech field.
Oppositely, in my opinion, every recreational diver at "advanced" level should master deco stop procedures, both theoretically and pratically, and be able to practice them within standard recreational limits (40 meters, EAN at 22% to 36% max).
The fact that some agencies release "advanced" scuba certification without teaching properly how to dive with deco stop is wrong, in my opinion.
There is nothing "technical" in doing deco stops, it is how people did dive recreationally between 1950 and 2000....
Unfortunately here I see some technical divers promoting the idea that only people with their outstanding level of knowledge and competence, fully trained and certified for truly technical diving, should do this kind of basic dives with deco stops in open water.
Here we are in the "basic scuba and those considering diving" forum: so the message must be clear!
Every dive is a deco dive.
Deco stops are not evil.
Millions of recreational divers perform dives with deco stops.
All recreational divers, when stepping up to an "advanced" (more than basic) level, should study the theory of decompression and become able to plan and conduct dives involving mandatory deco stops.
If your agency requires to spend more money for providing this knowledge and capability after an "advanced" certification, just do it: a complete recreational diver should know enough about deco procedures and not be scared of planning and executing dives exceeding the NDL.

 

[Lorenzoid]

@Angelo Farina makes a good point. Even in the US, where the training agencies still draw a bright line between "recreational" and "technical," even a diver who considers himself purely recreational could benefit from taking a deco course. If nothing else, it could help prevent divers from freaking out upon seeing their computers "going into deco."

 

[rob.mwpropane]

Although I agree on the general concept, there is NO need of "going tech".
A fully recreational (not technical) diver should know enough about decompression theory and practice.
There is no need to undertake technical training for planning and conducting recreational dives which include deco stops.
This is normal practice for recreational divers here in Europe and in tropical locations frequented by EU recreational divers.
Technical diving is not for everyone, only people with a very stable capability of reacting to danger should enter the tech field.
Oppositely, in my opinion, every recreational diver at "advanced" level should master deco stop procedures, both theoretically and pratically, and be able to practice them within standard recreational limits (40 meters, EAN at 22% to 36% max).
The fact that some agencies release "advanced" scuba certification without teaching properly how to dive with deco stop is wrong, in my opinion.
There is nothing "technical" in doing deco stops, it is how people did dive recreationally between 1950 and 2000....
Unfortunately here I see some technical divers promoting the idea that only people with their outstanding level of knowledge and competence, fully trained and certified for truly technical diving, should do this kind of basic dives with deco stops in open water.
Here we are in the "basic scuba and those considering diving" forum: so the message must be clear!
Every dive is a deco dive.
Deco stops are not evil.
Millions of recreational divers perform dives with deco stops.
All recreational divers, when stepping up to an "advanced" (more than basic) level, should study the theory of decompression and become able to plan and conduct dives involving mandatory deco stops.
If your agency requires to spend more money for providing this knowledge and capability after an "advanced" certification, just do it: a complete recreational diver should know enough about deco procedures and not be scared of planning and executing dives exceeding the NDL.

I agree that you don't have to go tech, I just wanted to maybe enlighten some on what it is and what it's not.

@Angelo Farina makes a good point. Even in the US, where the training agencies still draw a bright line between "recreational" and "technical," even a diver who considers himself purely recreational could benefit from taking a deco course. If nothing else, it could help prevent divers from freaking out upon seeing their computers "going into deco."

I think you hit the nail on the head. There's such a huge divide between rec and tech here in the states where as elsewhere it seems theres a much more gradual shift. I think it would be better to be that way. A lot if the stuff taught in OW doesn't even crossover here. For a lot of people it's basically start all over again.

Personally I read Mark's book Deco for Divers way before I took a tech class. I thought it was really well written, even for someone who didn't have any prior knowledge of decompression stops. It just gave me a better understanding of what was going on.