Nitrogen loading and DCS risk based of depth

[rubixcube]

Thanks for even more responses everyone! In essence it seems that many are saying that although both dives are theoretically of the same risk level, the shallower dive is also hypothetically riskier :blinking:. Well looks like you've done it again scubaboard, I've gotta read up on deco theory now .

 

[gcbryan]

Thanks for even more responses everyone! In essence it seems that many are saying that although both dives are theoretically of the same risk level, the shallower dive is also hypothetically riskier :blinking:. Well looks like you've done it again scubaboard, I've gotta read up on deco theory now .

Always a good thing to do

I think I've learned over the years that everything bad in scuba happens on the way up and slower is usually better!

Do the deepest part of the dive first and spend as much time as practical at the end of the dive shallower than 30 fsw.

I've never been a fan of the long 60 fsw dives.

Now if someone will just volunteer for the dive experiment that TS&M suggests where you put a nitrogen probe in your tissues for a direct reading

 

[gcbryan]

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!

The other point that I forgot to bring up with your comments above is that traditional diffused gas theory is considered to be perfusion limited. Slower compartments are slower in part because they are not as well supplied by blood vessels (fat, bone, etc) and this is how on gassing and off gassing takes place (through diffusion but requiring perfusion). So even if you have faster tissues nearby that have cleared out it still doesn't mean that fat tissues nearby will benefit if they are not as well supplied by blood vessels (capillaries)

I know there are some more modern theories that are considered to be diffusion limited or mixed (diffusion/perfusion). I'm not sure that this includes bubble theory however.

Can anyone clear this up for me?

 

[Mr Carcharodon]

I know there are some more modern theories that are considered to be diffusion limited or mixed (diffusion/perfusion). I'm not sure that this includes bubble theory however.

Can anyone clear this up for me?

VPM (Yount's bubble model) uses the same 16 parallel compartments that Buhlman did (you can see the code for yourself at the decompression.org FTP site.

Something that is getting lost in the discussion is that the saturation that is important is the one that occurs on the way back up. So you can be at 50% saturation at 160 fsw and get back to the surface more supersaturated than a 100% saturated dive to 40 fsw due to the ambient pressure being 2.5x greater at 160 feet. In the dissolved gas model the limits are pressures so it is more useful to track pressure than %saturation.

 

[gcbryan]

I think people just are using the terms saturation, supersaturation, and critical supersaturation incorrectly in this context. As you point out in effect, you can be saturated at 20 fsw and go directly to the surface . Your fastest compartment can be saturated if you at a 100 fsw and you can still go directly to the surface. It's only when you reach critical supersaturation that you can't go directly to the surface.

 

[beertiki]

Every table will show a higher RNT on the long shallow dive than the faster deep dive. At 40 feet you are loading nitrogen over 2 times faster than breathing at the surface for over 2 hours. At 130 feet you are loading 5 times faster, but for only 10 minutes. Look at the difference in surface intervals necessary to make another dive. The shallow dive loads much more nitrogen. DCS is the incomplete removal of that nitrogen.

 

[beertiki]

Here is what some tables say -

Dive 1 - 40 feet for 140 minutes (shallow) or 130 feet for 10 minutes (deep)
a ten minute surface interval
Dive 2 - 40 feet for as long as the tables will allow

Padi - Shallow dive 104 RNT with 36 minutes at 40 feet. Deep dive 31 RNT with 109 minutes at 40 feet.

US Navy - Shallow dive 152 RNT with 11 minutes at 40 feet. Deep dive 45 RNT with 118 minutes at 40 feet.

IANTD (Buhlmann 16) - Shallow dive could only be for 125 minutes RNT of 137, longer surface interval needed for next dive. Deep dive 37 RNT with 88 minutes at 40 feet.

Naui - Shallow dive 161 RNT, longer SI need for next dive. Deep dive 49 RNT with 81 minutes at 40 feet.

I did break the rules on the padi table, being a "Z" diver means a 3 hour SI on the shallow dive. All the tables show the RNT 3 times higher on the shallow dive. The long shallow dive carries much more risk of DCS.

 

[MarineResearch]

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.

Correct and a complete brain fart. I meant supersaturation.