A breakdown

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Posted by JRM on December 05, 2000 at 11:10:36:

In Reply to: Re: on-gassing while ascending posted by Gerry on December 04, 2000 at 15:44:52:

OK, maybe I didn't grasp the concepts the first couple of reads. So I'm going to parrot back what I think I got, and you guys can correct my mistakes.

The partial pressure of Nitrogen in tissues increases as the PPN increases in the lungs as one descends. However, tissues "on-gas" at different rates, and are thus broken into "compartments" of similar half-times. Although the compartments are not broken down anatomically, they are differentiated based on time. Different theories assign different half-times, and differing numbers of compartments.

An "M" value is the maximum PPN of a tissue that would allow direct ascent to the surface without a significant risk of a DCS hit. "M" values are estimated for different compartments.

Saturation is the point when the PPN of a tissue is near PPN of breathing gas in the lungs. One author cited 96% PPN in tissue as saturation.

Nitrogen obeys the law of diffusion. N flow will always be from higher PPN to lower PPN. So if the PPN in the gas in the lungs exceeds the PPN in any tissue compartment, that tissue will continue to "on-gas" reguardless of whether the person is ascending or descending. (This is what hosed me before. I think I got it now).

That ought to be enough to prove my ignorance beyond a shadow of a doubt :-)


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