Dynamic or moving ceiling deco

willibo

I have read in many posts stuff about moving ceiling deco and dynamic ceiling deco. What is it? Is it different from, what, the ceilings in the tables?
Thanks in advanced

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Willi

Janos

If you plan a dive on tables, you might have stops at (say) 21m, 18m, 15m, 9m, and 6m.

My computer (Shearwater) also gives me stop depths in multiples of 3m. I will never show a stop at 20m or 19m etc.

However, on some computers (I think the Vision electronics, also the man-on-a-rope on the VR£) there is a constantly varying stop depth.

Ie, when you go into deco, it shows your first stop as being 21m. When you ascend and get to 21m, you are off-gassing, and after a bit [1] the stop depth will now be showing at 20m, or 20.5m, or 19.5m, depending on how much you have off-gassed. You can either ascend to this new stop depth, or you can wait until it shows 18m and then ascend to 18m.

If your ascent rate is slow enough, then you can conitinually ascend, without ever actually stopping until (normally) you reach your last few stops. This is sometimes called continous decompression.

When the stop depth is 0m (ie the surface) then it is safe for you to ascend.

Well that's how I understand it anyway.

Janos


[1] - A "bit" is a unit of time used in technical diving circles.

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willibo

Thanks a lot Janos. I find this very interesting. Now, I can imagine that this dynamic ceilings are more "real" and "actual" and less "square" than the ones from the tables. Of course the "static" ones would be more conservative than the dynamic ones, at least that is what I understand. Why would you prefer the "static" ones? just to be save? Which one do you personaly prefer?

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Andy Del

Deco with locked ceilings rather than a continual ascent will be more conservative, if only that it allows any tissues that have become overloaded to come back to a less 'gassed up' state. (nice new term, eh? Gassed up and ready to go... )

I can imagine if one's dive went outside of normal boundaries, such as cold, exertion, a sudden, unintended ascent, etc., that doing a continual ascent will add pressure to one's ability to offgas what has become a situation which the deco algorithm isn't taking into account.

Personally, after trial and error, I wait it out at the deco stop depth, and ignore the 'man on a rope'. Once cleared, I slowly ascend to the surface. I feel much better this way, but that's just me.

Hope that helps a bit more...

cheers

Andy

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ScubaDadMiami

Though it would all seem to be the same in the end, the last time I checked (and it has been a while), making stops seemed to lessen risk compared to making a continuous slow ascent. Logically (and who ever said logic applies to these things? ), it would seem that making a continous ascent at a slow enough rate should come out the same. However, as of the last time I read up on the subject, it did not in fact come out the same. This is why even chamber dives use dedicated stops (along with very slow ascents) rather than continous ascents once treatment depths have been completed.

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willibo

I think that to be conservative is a very good atittude in diving. As far as I know locked ceilings are set to key depths to release the overloaded tissues and that at the end of the time you are able to go to the next ceiling, and son one. Therefor , I guessed that dynamic ceilings would be the same but in shorter steps. If you tell me that in your expierence locked ceilings have made you feel better, then my guess was not right.
Scuba Dad, can you tell me where did you read the ceiling stuff??

Thanks.

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Willi

Janos

I too remember reading a study on this.

I'll try and look it up when I get to work. However I remember not being hugely impressed by the profile they were using. Although I could be getting confused with another experiment.

Janos

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ScubaDadMiami

Here is something I came across recently in Wienke's Reduced Gradient Bubble Model in Depth.

Wienke: "The impact of deep stops has been a revolution in diving circles. So have slower ascent rates across recreational and technical diving. In quatifiable terms, slower ascent rates are very much akin to deep stops, though not as pronounced as decompression stops. Deep stops plus slow ascent rates work together. And they work together safely and efficiently, particularly when coupled to helium decompression strategies." Page 56.

However, he cites the following: "Analysis of more than 16,000 actual dives by Divers' Alert Network (DAN) prompted Bennett to sugget that decompression injuries are likely due to ascending too quickly. He found that the introduction of deep stops, without changing the ascent rate, reduced high bubble grades to near zero, from 30.5% without deep stops. He concluded that a deep stop at half the dive depth should reduce the critical fast gas tensions and lower the DCS incidence rate. [Note that the ascent rate is not included here, and it might have been the old 60 feet per minute rate of the U.S. Navy Tables. It does not seem that this is in conflict with slower ascent rates or deep stops or using both.]

Marroni concluded concluded studies with DAN's European sample with much the same thought. Although he found that ascent speed itself did not reduce bubble formation, he suggested that a slowing down in the deeper phases of the dive (deep stops) should reduce bubble formation." Page 62.

I think that I was reading some of the early stuff (as I mentioned in my earlier post), which seemed to indicate in stronger language than this that the stops seemed to work better than just a slowed ascent rate. Reading this newer material, it almost sounds like you could choose one (slow continuous ascent) or the other (not a fast ascent but instead emphasis on stop times), and that, as many divers do, incorporating both is the best way to go.

Hope this helps as a start.

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willibo

Thank you very much for clearing this up for me guys. I stay with slow ascents and "locked stops", but not to slow ascents. Anyway, I always try to make my last ascent, from 15ft to surface, the slowest. I normaly take 2 min.
If you find something more about this issue I would be glad to read it.
I guess I understand the principle behind the "dynamic Ceilings" or "Man on the rope"; to me it sounds "logical" in the theory. But if the reports say that experiments produce different results, I guess I stay with the practical proves and keep on doing my slow ascents and my locked deco stops.
Thanks a lot.

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Willi