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John wrote: >Is super saturation a bad thing? If it is I think we would be >commited to depth, never to return to the surface. My Mom(Mum for the >limies)would be pissed. Now I know to much super saturation can be a bad >thing. I belive this would be at an unmanageable point of bubbledge. >Some refer to this as critical super saturation. But its all in how you >interput it. >This is were they developed that hypothetical M-value thing. The >M-Value was designed to give a name to an allowable supersaturatio ratio >to allowable absolute inert gas pressure. Supersaturation is a bad thing. The M-values try to limit it so it won't get excessive and create (too much) bubbles. I think you're confusion supersaturation with offgassing gradient (driving pressure). A high gradient, accomplished by a productive gas switch does not cause supersaturation since the ambient pressure keeps the dissolved gas in solution. However, it causes a gradient or "steeper downhill" for washing out inert gas. This is what nitrox/ oxygen deco is all about. Further, you'd want to switch from faster to slower inert gases. Switching to more helium during ascent is hardly productive. This means that you ongas with helium faster than you offgas what's already dissolved in your tissues. This may create a surge of more gas in your body, before it drops again. The extra helium rushes in and adds to the nitrogen, which takes longer to diffuse out of your body. This is what may cause counter-diffusion problems: You carry a high load of a slow gas (as nitrogen) and suddenly dump a lot of fast gas (helium) on top of it. True, the gradient for eliminating nitrogen would be excellent but the sluggish gas doesn't leave as fast as the fast gas ongasses -- thus, total gas peaks shortly after that switch. That peak may give you a sudden, transient bubble trouble before things level off. Conversely, when you go from helium on the bottom to less helium/ nitrox you lose the helium faster than you ongas with nitrogen. Thus, total gas drops faster. The fast offgassing of helium turns into a blessing the moment you discontinue breathing helium -- from there on helium rushes out of your tissues fast. regards, Hans -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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