Three Deco Questions

[mike_mike]

1. How does oxygen window relate to the use of high O2 mixes for deco, especially if we already know that diffusion of one gas in solution is not affected by the presence of other gases (and therefore enlarging the oxygen window will unlikely promote tissue gas removal)?

2. If pressure gradient is the single parameter dictating the deco efficiency (and DCI risk), why do we use high O2 mixes at depth instead of just ascending to a shallower depth with our backgas to get to the same pressure gradient of non-metabolic gases?

3. Is it possible to have oxygen bubbles formed when one switches from pure O2 to backgas during air breaks?

Thank you for any insights.

 

[Agility]

1. by removing N2 from the inspired gas the gradient for N2 is higher, so more N2 can diffuse from the capillaries into the lung to be exhaled.
2. because the gases we want to offgas are in the backgas (see 1.) and the level you can ascend to is limited by the inert gasses in solution in the body tissues, if ambient pressure is reduced too much bubbles will expand beyond the size (allowed by the decompression model)
3. why should oxygen bubbles be formed when the ambient pressure is not changed?

If you are interested in an in depth answer, you might want to read http://www.dive-tech.co.uk/deco for divers.htm

 

[TSandM]

Another recommended read: Mark Powell's Deco for Divers.

 

[Michael Guerrero]

Bubbles respond to ambient pressure change. At a deco stop, O2 improves off-gassing without the risk of increasing bubble formation because ambient remains the same, and the pressure differential of N2 is dramatically decreased in the inspired air vs. the tissues, allowing more rapid diffusion than would normally be allowed by M-values without risking bubbles.

O2 bubbles won't be formed because, as was said, ambient pressure hasn't changed. But by switching to back gas you have significantly reduced the pressure differential between inspired O2 and that in the tissue, which would mean O2 should diffuse out of the tissue and into the blood. Since O2 is a metabolic gas, it behaves differently than N2 or He. I'd have to go back to the book to provide a better answer though.

 

[Michael Guerrero]

Thought I would finish answering the question about O2 bubbles. Looked it up and basically the metabolic process reduces the tissue tension by absorbing O2 to such a degree that bubbling is not a concern.

N2 and He don't behave this way because they are inert (not metabolically active).