deco myths

[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.

 

[L13]

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?

This thread appears to have been moderated, and some (most?) of the posts that motivated that comment have been removed.

A person making authoritative comments on deco should at least understand GFs even if they don't use them or accept them, or at least have significant experience doing deco without them.

However, you are correct about my tone. I apologize for that, and for making a post that did not add usefully to the dialog.

 

[L13]

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

Rober Helling did not address one additional difference between He and N2. Our tissues start with 0.79 ATA partial preasure of N2, but with 0.0 ATA of He partial pressure. This effects the rate of change of the total inert gas loading in tissues. The effect is approximately equivalent to He on-gassing as if you were ~8 m (~25 feet) deeper than for N2.