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What's the current thinking on a reverse profile series for technical dives?

Discussion in 'Decompression Theory' started by Tortuga68, May 19, 2011.

  1. Tortuga68

    Tortuga68 Divemaster

    # of Dives: 1,000 - 2,499
    Location: Puerto Galera, Philippines
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    Like the title says, what is the current theory, or your theory, on doing a series of technical dives in reverse profile ie doing a shallow(er) decompression dive as the first dive of a series (SI <12 hours but more typically 2-4 hours) followed by a deeper dive? Say between depths of 40-75m

    In 2000 DAN's "Reverse Dive Profiles" workshop on NDL dives concluded that:

    &#8220;&#8230;no convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of decompression sickness&#8221; and &#8220;We find no reason for the diving communities to prohibit reverse dive profiles for no-decompression dives less than 130 feet / 40.6 meters and depth differentials less than 40 feet / 12.5 meters&#8221;

    http://rubicon-foundation.org/dspace/bitstream/123456789/7677/1/SPUMS_V32N2_9.pdf

    What about technical/decmpression dives beyond these depths, or with a greater differential? What about an NDL dive <41m followed by a technical deco dive >41m?

    I have done reverse profile quite a few times, typically the biggest differential would be something like 45m deco -> 55m deco, or 20m NDL -> 50m deco

    John Lippmann from DAN AP has said that they "...see problems with reverse profile not uncommonly [sic]" although I don't know any details on depth, NDL/deco etc

    Please share your thoughts - what is acceptable?

    Thanks
     
    Last edited: May 19, 2011
  2. Tortuga68

    Tortuga68 Divemaster

    # of Dives: 1,000 - 2,499
    Location: Puerto Galera, Philippines
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    Don't all answer at once now...
     
  3. DevonDiver

    DevonDiver N/A

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    I'd be interested to hear some qualified opinions on this also. I've never been in the situation where I have needed to drastically reverse profile on a tech day-trip, so I've not thought about having a clear personal policy on the matter. I suspect many others would feel simularly. LOL
     
  4. r4e

    r4e Dive Shop

    # of Dives: 500 - 999
    Location: Finland, Europe
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    Disclaimer: my theory

    Three possible considerations come to mind:
    1) How well do the tables and/or software take the reverse profile series into account? What is the right amount of additional conservatism in such a case? Additionally, the normal series is far more tested by the navy/science/industry. Consequently there might not be enough data for testing/adjusting the calculations for a reverse series, and, it could be that some algorithms actually punish you more than needed.

    2) The remaining microbubbles in your blood stream might somewhat increase your susceptibility to DCS if the latter dives are more demanding.

    3) Exhaustion and cold. I prefer to have maximum energy levels for the more demanding dive. In a reverse series, your energy levels will be lower during the remaining dives. This applies both to exhaustion and coldness. I fear this effect might be increased by feeling more chilled during the more demanding and thus longer second dive. This will slow down blood circulation during the dive thus making on/off gasing less symmetrical and messing up calculations.

    As a summary, for a pair of technical and none-technical dives, I would normally place the technical dive first thus allowing me to maximize dive time etc. I do not mind doing the dives in reverse order, but then I'll be more conservative for the technical dive.
     
  5. fdog

    fdog ScubaBoard Supporter ScubaBoard Supporter

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    At first blush, since Reverse Profile as it's used for recreational diving is generally for a pair of dives of ~50' and ~100', it appears that for deco dives the % change would be much less.

    For example, the pressure delta for recreational would be ~38%, and for a pair of deco dives at 150' and 200' would be ~20%. Translated to recreational terms that would be like worrying over reverse profiling a pair of dives to 75' and 100'.

    <shrugs> I'd play with Vplanner a bit. Can't say I've ever heard of it being brought up, not surprising with the way we usually dive - either just one dive a day, or two dives to the same site.


    All the best, James
     
  6. Tortuga68

    Tortuga68 Divemaster

    # of Dives: 1,000 - 2,499
    Location: Puerto Galera, Philippines
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    Thanks for the input

    I've been thinking about this point with reference to the DAN study I quoted earlier... it occured to me that since most rec divers have "don't do reverse profiles" drilled into them, the available data could reflect a pretty low sample size, which may render the results questionable


    I can't say I've ever *needed* to do it, but sometimes it's more convenient. For eg, if the shop is busy we do guided rec dives in the morning and get a tech dive in later after the customers have lost interest for the day


    I've always used Decoplanner (v3.1.4, VPM) and put the earlier dive + SI in as part of the series - not had a problem doing that, but just wondering

    I just plugged in 45m for 20 & 55m for 15 with a 2 hour SI, and vice versa... results:

    Reverse profile: Total deco 17 & 20 (37)

    Non-reverse profile: Total deco 19 & 21 (40)

    Hmm...


    NB I have asked for this thread to be moved to Diving Medicine so we can get some more input - a link will be placed in T2T
     
  7. ScubaFeenD

    ScubaFeenD Marine Scientist

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    The DAN workshop indicating no significant risk of DCS with a reverse profile is interesting, because I distincly remember reading research that indicated that reverse profiles significantly INCREASED the risk of DCS. Ill try to find that.
     
  8. Cave Diver

    Cave Diver Divemaster

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    I seem to recall studies indicating reverse profiles weren't significantly more risky during a discussion on various sawtooth and reverse profiles found while cave diving.

    A quick search of Rubicon turned up several articles. I haven't had time to read through yet, but will post the links to a couple of them here for further discussion:

    Rubicon Research Repository: Item 123456789/1799

    http://rubicon-foundation.org/dspace/bitstream/123456789/7677/1/SPUMS_V32N2_9.pdf

    http://rubicon-foundation.org/dspace/bitstream/123456789/4059/1/16509284.pdf

    Rubicon Research Repository: Item 123456789/7677
     
  9. Tortuga68

    Tortuga68 Divemaster

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    Location: Puerto Galera, Philippines
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    Thanks for the links - I had a quick read...

    lol when they said guinea pigs I thought they meant test divers but it seems they actually meany guinea pigs!?

    This is the link I posted in the OP

    Guinea pigs again... they mention GPs being analogous, this is the first I've read of them being used (and here I thought exploding goats were more common?)

    Appears to be a summary of the 2000 DAN workshop
     
  10. ScubaFeenD

    ScubaFeenD Marine Scientist

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    Location: Baltimore, MD
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    Thanks. The first one is the one that was in my mind. Of course, it isnt retrospective and it is in guinea pigs, but I think the results are valid, because they are still mammals and the physiology isnt actually that much different from human physiology (alot of what i was taught in physiology class was actually worked out in non-human mammals). That being said, I would likely just run deco software to determine if a profile is OK, because from what i understand the models I use are based on calculated nitrogen loading and mvalues based on that. That said, i try to avoid reverse profiles, because psychologically it is less "good" in my mind and I havent had trouble avoiding them so far.
     

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