Michael Guerrero
Contributor
Recently I was party to a conversation regarding what depth to use for last stops and why. I want to outline my understanding of the relevant factors with respect to deco efficacy only, not environmental conditions (surge, temp, etc.) and solicit your comments on the accuracy of my statements.
1. Decompression occurs when the gas tissue tension exceeds the inspired partial pressure (pp) of the same gas.
2. Gases on- and off-gas independent of other inert or metabolic gases and rates are based on gas gradients, perfusion, diffusion, and solubility.
3. The principle way to create an inert gas gradient is to change ambient pressure (descending and ascending), and an alternative way is to change the fraction of inspired inert gas.
4. During ascent, ambient pressure decreases, lowering the pp of inspired gas relative to tissue tension of the same gas.
5. When the inspired pp of an inert gas is lower than the tissue tension of the same inert gas, the inert gas diffuses out of the tissue, into the venous bloodstream, to the lungs, and out of the body.
6. Off-gassing is dependent in pp, which is normally linked to ambient pressure.
7. When we use oxygen to accelerate decompression, we are decoupling pp from ambient pressure by creating a greater reduction in pp of a specific inert gas (increasing the gradient).
8. This is known to some people as the oxygen window (there are other definitions for the oxygen window), and allows accelerated inert gas off-gassing, potentially exceeding compartment m-values.
9. This does not increase our chances of creating bubbles because bubbles are affected by the relationship between tissue tension and ambient pressure.
10. The presence of oxygen in inspired gas does not in and of itself accelerate off-gassing.
11. It is only when oxygen (or another gas) displaces a portion of an inspired inert gas during decompression that accelerated off-gassing occurs.
12. Pp is affected by ambient pressure (depth), and reducing depth during decompression increases the inert gas gradient between the tissues and inspired gas (assuming no change in inspired gas).
13. A shallower depth leads to an increase in gradient, and greater off-gassing due to decreased inert gas pp.
14. Oxygen is considered dangerous at a pp greater than 1.6 during deco, and reducing depth reduces the pp of oxygen (i.e., the pp of oxygen is lower at 10ft than at 20ft if breathing pure oxygen).
15. Greater off-gassing does not occur between 20ft and 10ft on pure oxygen because the pp of inspired inert gas is 0, and thus the gradient cannot be increased once on pure oxygen.
16. Your risk of oxygen toxicity is less at 10ft than 20ft due to the reduced pp of oxygen (1.3 vs. 1.6).
17. A pp of 1.6 is used strictly for prevention of oxygen toxicity and not off-gassing efficacy (i.e., inspired inert gas displacement is what really accelerates decompression).
Given the above, and precluding any environmental factors increasing the risk of holding a 10ft stop vs. a 20ft stop, it is safer to do your last deco stop at 10ft rather than 20ft by virtue of reduced exposure to a high oxygen pp.
My sources for all this are Mark Powell's book Deco for Divers, Simon Mitchell's presentations at the 2008 DAN Tech Diving Conference and DAN's Rebreather Forum 3.0 conference, David Doolette's presentations at both conferences, as well as some others from the 2008 DAN conference.
1. Decompression occurs when the gas tissue tension exceeds the inspired partial pressure (pp) of the same gas.
2. Gases on- and off-gas independent of other inert or metabolic gases and rates are based on gas gradients, perfusion, diffusion, and solubility.
3. The principle way to create an inert gas gradient is to change ambient pressure (descending and ascending), and an alternative way is to change the fraction of inspired inert gas.
4. During ascent, ambient pressure decreases, lowering the pp of inspired gas relative to tissue tension of the same gas.
5. When the inspired pp of an inert gas is lower than the tissue tension of the same inert gas, the inert gas diffuses out of the tissue, into the venous bloodstream, to the lungs, and out of the body.
6. Off-gassing is dependent in pp, which is normally linked to ambient pressure.
7. When we use oxygen to accelerate decompression, we are decoupling pp from ambient pressure by creating a greater reduction in pp of a specific inert gas (increasing the gradient).
8. This is known to some people as the oxygen window (there are other definitions for the oxygen window), and allows accelerated inert gas off-gassing, potentially exceeding compartment m-values.
9. This does not increase our chances of creating bubbles because bubbles are affected by the relationship between tissue tension and ambient pressure.
10. The presence of oxygen in inspired gas does not in and of itself accelerate off-gassing.
11. It is only when oxygen (or another gas) displaces a portion of an inspired inert gas during decompression that accelerated off-gassing occurs.
12. Pp is affected by ambient pressure (depth), and reducing depth during decompression increases the inert gas gradient between the tissues and inspired gas (assuming no change in inspired gas).
13. A shallower depth leads to an increase in gradient, and greater off-gassing due to decreased inert gas pp.
14. Oxygen is considered dangerous at a pp greater than 1.6 during deco, and reducing depth reduces the pp of oxygen (i.e., the pp of oxygen is lower at 10ft than at 20ft if breathing pure oxygen).
15. Greater off-gassing does not occur between 20ft and 10ft on pure oxygen because the pp of inspired inert gas is 0, and thus the gradient cannot be increased once on pure oxygen.
16. Your risk of oxygen toxicity is less at 10ft than 20ft due to the reduced pp of oxygen (1.3 vs. 1.6).
17. A pp of 1.6 is used strictly for prevention of oxygen toxicity and not off-gassing efficacy (i.e., inspired inert gas displacement is what really accelerates decompression).
Given the above, and precluding any environmental factors increasing the risk of holding a 10ft stop vs. a 20ft stop, it is safer to do your last deco stop at 10ft rather than 20ft by virtue of reduced exposure to a high oxygen pp.
My sources for all this are Mark Powell's book Deco for Divers, Simon Mitchell's presentations at the 2008 DAN Tech Diving Conference and DAN's Rebreather Forum 3.0 conference, David Doolette's presentations at both conferences, as well as some others from the 2008 DAN conference.