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Could you tell me why it would be safer to do (2) 40 meter deco dives in one day then then it would be to do a 12 meter dive first then a 40 meter deco dive second in the same day. I have always followed the training agencies recommendations for deep dive first but no one ever gave me a reason why.
If you are making your deepest dive first, you will be able to have more bottom time, but it is no safer than making the shallowest dive first. 40M (131 ft) is a tad deep for most divers.
I experimented with a few reverse profile plans on my decoplanner. It chewed up your NDL on the second dive. In addition, it appears to screw up the M-Values on those compartments that haven't finished outgassing.
Note that the foregoing was theoretical. I'm not planning on trying it and don't recommend it to anyone else.
for one, in the decompression models used there is nothing in those models implying that reverse profiling is bad or not allowed. The models/tables are blindly followed by most divers and in training they are told reverse profiling is bad, although recently the agencies have begun to realize that there really never was any reason to NOT do a reverse profile. One problem is that students are told that a direct ascent to the surface is fine as long as NDL's are not exceeded which is nonsense and causes DCS symptoms a lot of times.
what i see a lot as well is divers making a nice long safety stop at 20ft after which they rocket to the surface from 20 ft thinking they did it right, and they just undid the benefit of the stop by screwing up the rest of the ascent, then they might as well not have made the stop at all. The biggest bubblerush occurs in those last 20ft and after the stop at 20ft the remaining 20 ft to the surface should be very very slow.
as a matter of fact the best way to do multiple dives is to do the shallower dive(s) FIRST. Of course this is assuming one does not screw up the ascent from the deeper dive.
Just a comment or two on models and RPs (reverse profiles).
Risk on DCS for RPs goes up fast with increments greater
than 40 fsw, and at depths beyond 130 fsw on any mix --
see Smithsonian Institute Porceeding On RPs. So it's NOT
true that RPs are OK. And additional time for repets on
forward profiles is a hangover from haldane dissolved gas
models and tables ONLY. And RPs for deco diving are an
absolute no-no -- insanity with plenty of hits (see same
Smithsonian publication).
Modern phase models suggest more bubbles are excited into
growth by RPs (the greater the increment and deeper, the more
seeds excited into growth. Those of you with Suunto, Plexus,
and Hydrospace computers will see at reduction in M-values
to accomodate this additional phase on RPs (hence shortened
NDLs) with persistence time scales of 1 -3 hrs after the first dive.
Such, however, will not come into play until depths are beyond
50 - 60 fsw.
sheck33 once bubbled... agencies have begun to realize that there really never was any reason to NOT do a reverse profile.
Let's take a specific example, using the Navy tables to plan a couple of recreational dives, remaining within NDL. We'd like to make a dive to 60 feet for 40 minutes and a dive to 40 feet for 60 minutes. If we do the 60 foot dive first, a surface interval of 41 minutes between dives is required. If we do the 40 foot dive first, however, our required surface interval is 7 hours, 36 minutes! So while it is true to say that doing the shallow dive first is ok, the penalty in surface interval certainly provides me with a reason to avoid the reverse profile for these two dives.
One problem is that students are told that a direct ascent to the surface is fine as long as NDL's are not exceeded which is nonsense and causes DCS symptoms a lot of times.
Sorry, this statement is just false. The NDL tables are built around a direct ascent, and DCS symptoms following a direct ascent using the ascent rate specified by the table are so extremely rare as to be practically non-existent.
what i see a lot as well is divers making a nice long safety stop at 20ft after which they rocket to the surface from 20 ft thinking they did it right, and they just undid the benefit of the stop by screwing up the rest of the ascent, then they might as well not have made the stop at all. The biggest bubblerush occurs in those last 20ft and after the stop at 20ft the remaining 20 ft to the surface should be very very slow.
Good point.
as a matter of fact the best way to do multiple dives is to do the shallower dive(s) FIRST.
If you're willing to accept the entire profile, including the surface interval penalty, this is likely true - though often not very practical.
Rick
"You can have peace, or you can have Freedom. Don't ever count on having both at once." (Heinlein)
"... they saw the deeds of the LORD, his wondrous works in the deep." (Ps107:24)
Let's take a specific example, using the Navy tables........
I learned how to dive with the Navy tables. I cant believe they still use them. These tables suck. They get you out of the water and into a chamber the fastest by getting you bent on the way up and then 'fixing' it by doing long stops. So frankly i dont care what results a Navy table gives. According to those tables i'd be dead a long time.
yes, the NDL tables are based on a direct ascent to the surface at 60 ft / min which is too fast and especially far too fast for ascending the last 20 ft. Doing a 60 ft / min ascent from say 70 ft all the way to the surface does not give the body enough time to properly offgas. It takes time for the blood to circulate in the body and that fact is not taken into account.
why would doing a deeper dive after a shallow one not be practical?
people do deep dives first, have some surface interval and then do a repetitive shallow dive where usually not much attention is paid to ascent rates since 'it's only a shallow dive' My sister got bent not too long ago because of two reasons, one: she should have been hydrated better, she was doing multiple dives over multiple days, two: the stupidity taught with regards to repetitive diving, this whole issue is the very reason why WKPP does not allow their divers to do short, shallow bounce dives to 30ft to pick up deco bottles, after a deep dive because THATS where they got bent!
since i have switched to ding my deeper dives first and ascending extremely slowly, especially from the 20 ft stop i do feel better after these dives.
I just noticed an old thread under General Tec Discussions reference by Spectre. I did not originally see that as I was away at that time without my laptop. The subject of arterialization came up on the thread and the need for the PFO. In actuality, clinicians who measure arterialization (to determine the cause of a stroke in an individual) do find instances where the arterialization of the saline contrast gas bubbles occurs and a PFO is absent. In this case, it is postulated that the route of the gas bubbles is through pulmonary shunts), that is, though blood vessels larger than lung capillaries. This is based on the observation that PFO bubbles are in the left ventricle in one or two heartbeats, while with pulmonary shunts, it requires five or more heat beats.
WKPP and Arterialization
The observation that individuals who re-dive are at risk for DCS is an important one. What I was debating was whether the mechanism was one of passage through the lungs with repressurization. I did not doubt the observation that DCS occurred. Laboratory trials indicate that passage does not occur. HOWEVER, evidence collected since 1995 by clinicians (regarding so-called “paradoxical stroke” = stroke from clots originating on the venous side of the circulatory system) implicate Valsalva-like maneuvers to a large degree. This is somewhat different than the laboratory studies of the 1980s. In these studies, Valsalva-like maneuvers were not performed. Neither were straining maneuvers performed when reaching surface after a very short dive. This might color the outcome and is what makes this forum interesting for me; I receive information that I would not otherwise find.
Microbubbles and Repetitive Dives
I am more a proponent of the microbubbles-through-hydrodynamic-cavitation hypothesis. The concepts of Dr Wienke stress these changes less, most likely because they are less deterministic (= more random) and not as easily modeled. The concepts that I present were developed more to explain decompression risk in a gravity-free environment with its attendent reduced level of musculoskeletal activity. In addition, prior compression is not involved in hypobaric activity as it is in diving.
During the initial descent, I have little reason to doubt that the micronuclei are compressed. It is during the surface interval that musculoskeletal activity will make its being appearance with respect to microbubbles. Here we see not only microemboli formation, but also growth while tissues are in a state of supersaturation. Repetitive decompression while under these conditions is apparently very harmful.
Laboratory Experiments
It is obvious that experiments with humans would be very difficult to perform for many of these questions to be answered. It would involve exposing people to situations where arterialization would be provoked and made to occur. While the proper setting could be obtained to virtually assure subject safety, it is still not totally risk free. There are questions involved regarding the necessity of doing this study as well as questions of subject compensation if something horrible comes about that can not be reversed with pressure therapy.:boom:
In the absence of direct trials, recourse can sometimes be made to research work in the field of vein-to-artery (paradoxical) stroke. Here we have studies where the individuals are at a greater risk if nothing is done. I have taken some of these studies and “imposed” the results on the diving situation. At the present, that is the best information that we will obtain.
Michael R. Powell, M.S., Ph.D. "If a man empties his purse into his head, no man can take it away from him. An investment in Knowledge always pays the best Interest." - Benjamin Franklin
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