Fast descent or slow descent ????

[stefusa]

Doing some research on the different model of decompression and particularly on RGBM, I have read that it could be saffer to go down fast instead of slow.
They explain that by increasing the pressure faster reduces the chance that bubbles come together and therefore increase the number of small bubbles and reduces the numbers of relatively big bubbles that will be able to increase in size during ascent and cause DCS.

Doc????

Is it garbage what we learn in PADI formation, slow descent????? :huh:

thanks for the answers...

 

[H2Andy]

not sure on the DCS issue, but remember that fast descents are associated with greater narcosis effects

 

[dsteding]

Doing some research on the different model of decompression and particularly on RGBM, I have read that it could be saffer to go down fast instead of slow.
They explain that by increasing the pressure faster reduces the chance that bubbles come together and therefore increase the number of small bubbles and reduces the numbers of relatively big bubbles that will be able to increase in size during ascent and cause DCS.

Doc????

Is it garbage what we learn in PADI formation, slow descent????? :huh:

thanks for the answers...

Uh, you shouldn't have bubbles in your blood on descent. If you do, you have big issues. Bubble formation is a result of diffusion of gas from tissues as pressure is reduced, you need two things to achieve bubbles (1) gas in tissues; (2) a decrease in ambient pressure sufficient enough to have that gas come out of the tissues into the blood stream. If the decrease is fast enough, you'll get bubbles, if the bubbles are big enough, you'll get bent.

So, I guess I don't get the last part of your post regarding increasing pressure fast enough to keep bubble size small-there shouldn't, barring some sort of immediately preceding decompression incident, be bubbles in your blood at that point. Is this a reference to in-water recompression? That is the only time I could see the above scenario being relevant or applicable.

I'm not taking the bait on the PADI comment.

 

[stefusa]

VPM and RGBM are positive, there is always bubbles, they are too small to pose issues for diving but bubbles are inherents....

 

[stefusa]

Quote from VPM - THE INNER WORKINGS
Marcin Kaluza (www.hlplanner.com)
November 2005

In other words faster descents mean (potentially) shorter deco as illustrated in Fig. 4, and
conversely, slower descent rates may cause longer decompression times. This may be a major
inconvenience in cases where real descent rate differs from the plan. A slower descent
however usually means less time on the bottom (e.g. given a fixed dive runtime before
ascending) and therefore lower tissue saturation. The net effect is that although the allowed
supersaturation gradient is lower, the tissue contains less gas and differences in total deco
times are usually minor. Apart from this, slower compartments that control shallow stops are
slow enough not to be dependent on variations of descent rate.

 

[Luc Dupas]

During descent, meaning at the start of the dive, our body is still absorbing N2 and the amount and partial pressure of N2 in our body tissues at that time is still far below saturation and below the surrounding water pressure... so at that time there is no driving force (and hence no reason) for bubble formation...
Of course, if one would descend at an extremely (!) slow pace the picture might change but for a regular dive it doesn't matter... in terms of "bubble formation" during the descent. It does of course impact the final effective bottom time within the NDL limits, because the slower to get to the bottom, the more you already have absorbed N2 during descent and the less you can stay down within the NDL limits...

 

[Dr Deco]

Hello Stefusa:

Fast descents will almost assuredly not prevent tissue microbubble [nuclei] coalescence. I seriously doubt that this intermingling occurs, anyway.

Fast descents will probably cause the nuclei to shrink faster than incoming dissolved inert gas can arrive from the circulatory system. This would help surface tension reduce them in size. In this regard, it could be of some advantage, but I doubt it would be much for a recreational scuba diver.

Dr Deco :doctor: