Another Deco profile post.

[Remy B.]

As we observe all current Deco algorithms/profiles and so called strategies, the focus is on long times @ 6m, but since there is NO On-gassing from 12m and up and only Off-gassing is present, why is it that these Algorithms are not re-distributing the time @6m between 12m, 9m and 6m, not necessarily means divided by 3 but the time can be re-distributed.

The way I understand it, bubble size is kept smaller at 12m than at 6m and the surface tension as well is lower, and there only Off-gassing, it appear now days that the trend is as well shorter deco times @ 6m if we compare GF-50/70 to GF 50/80. or VPM+5 to VPM+2 for example.

Is there a particular reason why it is not done this way ? am I missing something or misunderstanding it.?

 

[fsardone]

Hi Remy,
in my understanding of the issue, it is the gradient between the dissolved phase of a given gas and its partial pressure in the respired gas that control the speed in which a gas dissolves or is eliminated into tissues (starting with blood).

Therefore all decompression algorithms want you to go shallow as soon as possible in order to achieve quick off gassing. Now when you breath oxygen, the partial pressure of all other inert gasses is zero and therefore you achieve best efficiency. If we could use 100% oxy at 12 meters we would do our stops there but as yuo probably know the risk of CNS toxicity and the speed of the CNS clock at ppO2>=1.6 precludes the use of pure O2 at pressure above 1.6 bar.

Theoretically we could lengthen the stops below 6 meters but decompression time will be longer.
I am justa practitioner but I am sure some other member more knowledgeable will chip in ...

Cheers

 

[victorzamora]

Fabio, my understanding is roughly the same.

Remy: If you stay at 12m instead of 6m you can't get on O2. Not breathing O2 means you're breathing in some inert gas. Breathing in inert gas means you're not off-gassing as quickly as possible as your gradient isn't maximized.

However, where'd you come up with "no possible ongassing at 12m and up"?

 

[Remy B.]

there is 50%O2 for that and you still can use 80 or 100% for the 9 and 6m stops, the time is not incremented it is re-distributed, there is no On-gassing at 12m ALL tissues/compartments are Off-gassing at 12m regardless of the diver breathing air or higher concentration of O2, of course with different rates depending of what he is breathing.

If the bubbles are kept small and the surface tension lower will not those inert gases defuse/dilute ( don't know if this is the right term ) faster from the tissues in the blood and be expelled/exchanged faster in the lungs ?

As will not everybody uses all the time 100% O2, most of my Deco dives I do with 50%

 

[fsardone]

@Remy B.
The inert gas trying to diffuse out of the tissues will have to do it against the pressure of the same gas in the lungs.
So, said equal the amount of gas dissolved in the blood (which is a tissue and it is the only one exchanging in the lungs at the alveoli) the amount of gas that moves out of the blood and into the lungs will be greater (faster off-gassing) the lower the partial pressure of said gas is in the lungs.

So with 100% oxygen Nitrogen PP is zero no matter the dept
with EAN50 at 6m N2PP is 0.8Bar at 9m is 0.9 and at 12m is 1.1Bar

That partial pressure is opposing the Nitrogen in the blood to move into the lungs and be eliminated with breathing.
So you will want to get a lower N2 PP as soon as possible by raising in the water column.
On the other end you have to limit your ascent to the maximum tolerable supersaturation to avoid DCS symptoms.

I would recommend a good book:
Deco for divers - Mark Powell, which will give you a good background to deal with decompression theories.

Cheers

 

[tbone1004]

there is 50%O2 for that and you still can use 80 or 100% for the 9 and 6m stops, the time is not incremented it is re-distributed, there is no On-gassing at 12m ALL tissues/compartments are Off-gassing at 12m regardless of the diver breathing air or higher concentration of O2, of course with different rates depending of what he is breathing.

If the bubbles are kept small and the surface tension lower will not those inert gases defuse/dilute ( don't know if this is the right term ) faster from the tissues in the blood and be expelled/exchanged faster in the lungs ?

As will not everybody uses all the time 100% O2, most of my Deco dives I do with 50%

How did you determine that you aren't on-gassing in any tissue at 12m regardless of inspired gas mix?

I think you answered your question though when you said of course with different rates. The lower the pN2 in the mix, the bigger the gradient is across your lungs. The bigger you make that gradient, the faster your decompression is. We want to minimize our decompression obligation, so we aim for the gas mixes with the biggest gradients. That means getting to 6m to get on O2. Once on O2, there is no benefit of staying deep in terms of deco efficiency, but you obviously have to stay deep long enough to not get bent. That's why most divers I know have their last stops set at 3m/10ft, and will go up there as soon as possible and finish deco up there so the pO2 is only 1.3 instead of 1.6

 

[boulderjohn]

That's why most divers I know have their last stops set at 3m/10ft, and will go up there as soon as possible and finish deco up there so the pO2 is only 1.3 instead of 1.6

The other nice thing about that is you don't have to be as precise in your depth on that stop as you do on the 6m/20 foot stop.

 

[tbone1004]

The other nice thing about that is you don't have to be as precise in your depth on that stop as you do on the 6m/20 foot stop.

yup. we may not actually go up to 10ft, in many caves we can't, and in the ocean we almost certainly won't, but it's nice to have that option and also be able to go up to say 14 or 15ft in the cave or up to a 15ft hang line in the ocean

 

[victorzamora]

Remy: You've big time misunderstood things. You absolutely CAN on-gas at 12m. You can on-gas at ANY depth with a PN2 or PHe greater than ambient.

PN2 at 12m is less than PN2 ambient when breathing EAN50. However, that definitely doesn't mean it's the best offgassing depth from a pure-efficiency perspective.

 

[KevinNM]

I think maybe he's confusing the depth where you can stay infinitely and ascend directly to the surface with not on-gassing.