Re: Stepped off-gassing

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Posted by MHK on December 04, 2000 at 18:14:13:

In Reply to: Stepped off-gassing posted by Eins on December 04, 2000 at 17:30:31:

I'm confused about a few parts of your question.. But let me start by saying that decompression algorithms are a very inexact science and in terms of added 5' to your dive I believe may be taking this to an extreme that we will simply not be able to resolve, so let's keep that in mind as this thread unfolds. Moreover there are other contributory factors such as dehydration, obesity, physical fitness, PFO's and the like but I'm happy to address the the issue but understand there are other factors.

The basic Haldane equation solves for CONSTANT DEPTHS ( NOT [ emphasis added ] on NOT for ascents or descents so in order to solve for the surfacing M-value you'll need to re-arrange the equation to solve for time and then look to the natural logarithm of both sides. By doing this you will solve for t, which is the time that it will take for the compartment to load gas ( ON or OFF) from the initial compartment pressure, Po, to the final pressure, P.

Please note that the above is the Haldane model which differ's from the current Weinke RGBM and that the above paragraph lies the difference between the two models.. I can remind you of the distinction or you can review my earlier posts on this subject.

However to the extent that you are simply seeking to solve for the surfacing M-value, as opposed to the final pressure you can use the following:

t= (-1/k)*In[(P1-Mo)/Pi-Po)].

In this equation t, is the NDL..

Furthermore, in terms of your comments about the delta saturation and the tissue gradient you may want to look to this equation which really is the driving force for *on-gasing* and *off-gasing*.

P = Po + (Pi - Po)(1 - e^-kt)

This is the entire equation for the constant depth that I spoke about at the beginning but the exponential gas loading is given in the second part, (Pi - Po)(1-e^-kt) noting that Pi-Po is the pressure gradient between the inspired gas pressure and the initial compartment pressure..

Hope this helps but I'm leaving the office in a few minutes so any follow up's I'll answer tomorrow..


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