Nitrogen loading and DCS risk based of depth

[MarineResearch]

According to Dr. Albert A. Bühlmann, the short duration dive to 140' would cause saturation of compartments 1-4 (out of 16), which are the short half time compartments ranging from 4 to 18.5 minutes. Compartment 5 would reach saturation during your ascent (27 minute half time).

A 200 minute dive at 40' would saturate compartments 6-8, which have longer half time periods of 38 to 77 minutes. Dive nitrox and you can stay at 40 feet until you reach your oxygen toxicity limit with no deco obligation.

 

[finnks]

Its a really good question.

My belief is that the long,shallow dive carries more risk for the following reasons:

On a short,deep dive only the fast compartments become significantly loaded.Upon surfacing the nitrogen may just be eliminated by the lungs. or,maybe due to exertion,some of it goes "sideways" into a longer half time compartment. No problem as that compartment has plenty of room in it.
Compare that with the long shallow dive. It would be quite possible to surface from that dive with many (all?) compartments heavily loaded. Any nitrogen going "sideways" can cause oversaturation and possibly DCS.

100 feet for 15-20 minutes is a profile done 1000's times a day at 100's of dive resorts every day around the world. Easily done on an 80 with a decent SAC rate.
The PADI NDL for a 40 foot dive is 140 minutes. I've never done that. It can't be done on an 80. It's an extremely unusual dive for which there is probably very little data.

Finally, if you do a good 5 minute safety stop after a 100 feet/20 minute dive that's going to reduce N2 loading significantly. A 5 minute stop after 40 for 140 will do next to nothing for the slow compartments.

Don't say it can't be done. My daughters excited-swimming SAC rate is roughly equal to my total relax - don't move rate at <~ 0.25. I'm not a small or fit guy, so can easily imagine many people have lower breathing rates. With that SAC at 40' for 140 minutes one would use 77.5 cu ft. This corresponds to a very slight over fill of an AL 80. Thus it is possible to do 140 minutes at 40' on an AL 80. Mind you the number of people who can is very small, and the number of them who would _ever_ bother is even smaller... Personally I've never hit 100 minutes on a dive, and don't really expect to ever hit 120 - much less 140.

 

[ianr33]

Don't say it can't be done. My daughters excited-swimming SAC rate is roughly equal to my total relax - don't move rate at <~ 0.25. I'm not a small or fit guy, so can easily imagine many people have lower breathing rates. With that SAC at 40' for 140 minutes one would use 77.5 cu ft. This corresponds to a very slight over fill of an AL 80. Thus it is possible to do 140 minutes at 40' on an AL 80. Mind you the number of people who can is very small, and the number of them who would _ever_ bother is even smaller... Personally I've never hit 100 minutes on a dive, and don't really expect to ever hit 120 - much less 140.

Have you considered:

1)Rock Bottom gas calculations.

2) A safety stop (Which would be a rather good idea after that dive )

 

[gcbryan]

On a short,deep dive only the fast compartments become significantly loaded.Upon surfacing the nitrogen may just be eliminated by the lungs. or,maybe due to exertion,some of it goes "sideways" into a longer half time compartment. No problem as that compartment has plenty of room in it.

"Sideways" into a compartment with more room in it?

 

[gcbryan]

Have you considered:

1)Rock Bottom gas calculations.

2) A safety stop (Which would be a rather good idea after that dive )

Finally, if you do a good 5 minute safety stop after a 100 feet/20 minute dive that's going to reduce N2 loading significantly. A 5 minute stop after 40 for 140 will do next to nothing for the slow compartments.

The real issue isn't whether it could be done anyway. Get a larger tank and it can be done. The point is both dives are within the NDL and should carry equal (theoretical) risk.

I'd prefer the deeper dive since I could do something about the loading while still underwater and if a second dive is planned there is less chance for screwing up with a too short surface interval.

 

[gcbryan]

According to Dr. Albert A. Bühlmann, the short duration dive to 140' would cause saturation of compartments 1-4 (out of 16), which are the short half time compartments ranging from 4 to 18.5 minutes. Compartment 5 would reach saturation during your ascent (27 minute half time).

A 200 minute dive at 40' would saturate compartments 6-8, which have longer half time periods of 38 to 77 minutes. Dive nitrox and you can stay at 40 feet until you reach your oxygen toxicity limit with no deco obligation.

How does a 10 minute dive to 140 fsw saturate any of those compartments? It takes 6 half-lives to saturate

A 200 minute dive would saturate half-lives shorter than 33 minutes.

The controlling compartments may be in the ranges that you've listed for the short and the longer dive but they wouldn't be saturated as you've described.

 

[ianr33]

"Sideways" into a compartment with more room in it?

Sideways as in into another compartment,whether it has room in it or not.

As I understand it,all the theoretical Buhlmann compartments are independent of each other.When nitrogen leaves that compartment it magically disappears rather than moving into another compartment. I suspect that may not be an accurate model of my body!

Consider 2 dives (These are not possible in reality,just a way of trying to get my point over)
After the first dive one compartment,lets say the 4 minute one,is at 95% of its allowed saturation level.The other compartments are empty.
After the second imaginary dive all 16 compartments are at 95% allowed saturation.
So far as software or computers are concerned these are equal risk as in each case the leading compartment is 95% saturated.
What happens though when you exert yourself climbing a ladder at the end of dive 2? Does all this nitrogen happily leave each compartment as the math is assuming it does? Or does some of it go from e.g. a thigh muscle to fat causing a bend?

Most compartments full = bad
Many compartments empty=good

There was an excellent discussion a long time back on TheDecoStop about O2 deco compared to backgas deco. What I got out of it was that O2 deco is safer (even though the 'theoretical"risk is the same) as using O2 the fast compartments will be essentially empty,leaving plenty of space for any wayward nitrogen.

The Deco Stop Posts 12 and 19 are especially relevant

 

[gcbryan]

I get the point about the model assuming no connectivity between compartments although I don't think exertion causes nitrogen to move into another compartment. I may be wrong.

I think exertion produces more micro nuclei around which nitrogen that has made it to the free state can cling and grow. I also can see how there actually is connectivity between varying half-life tissues in the body so the less that have reached critical supersaturation the better.

You mention saturation of tissues meaning that they are at equal risk. Saturation isn't what determines the risk but rather critical supersaturation. All of the tissues in your body are currently saturated (at sea level).

It's good to see more activity in this (interesting) subforum lately. I'd also like to learn more.

 

[ianr33]

Yep,should have said "supersaturation" or allowed M value. And you may well be correct about the exertion.

Good discussion.

 

[Rhone Man]

I was always taught more people get bent on long dives than deep dives, largely for the reasons ianr33 gave. I haven't had much exposure to DCS in my time, but such as I have had had largely endorsed that anecdotal wisdom.

P.S. 140 minute dive keeping exertion to a minimum? I'd freeze.