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Thread: Deep Stops (rebreather dive charts)

  1. #1

    Deep Stops (rebreather dive charts)

    The charts below were requested by a few of you who wanted to see what a Rebreather dive might look like on the same basis as some of the NEDU deep stops discussion. First a few things to remember:

    1. The NEDU study provided hard data on a specific profile. The charts below are not hard data -- we have no real dives performed to validate whether one pattern or the other is better.

    2. The dives all match the run time of a VPM-B+4 profile starting with different low gradient factors. The profile was 270ft for 20min.

    3. Remember that the heat map is comparative only. The colors scale each compartment's supersaturations between the four profiles. "Red" does not mean danger, it means highest observed supersaturation between any profile for that compartment.

    4. Integral supersaturation is just supersaturation multiplied by time and summed. So a supersaturation of 1000mb for 10 minutes adds up to the same value as a supersaturation of 200 mb for 50 minutes. It is simply an index of the duration of supersaturation exposure. So, for example, we can say that VPM-B+4 exposes the diver to 31% more supersaturation at the surface than the GF 40/74 diver (i.e. 555.1 / 423.8). What we can't say is how that translates to risk (or at least I can't).

    Anyway, have at it if you find anything interesting. I'll post a few more charts above as I can't seem to get them all in this one. (Update: I can't seem to post charts that don't show as icons, so the charts below will have to do).
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    Last edited by UWSojourner; 27th January 2014 at 18:32.

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    Re: Deep Stops (rebreather dive charts)

    Fascinating!

    Thanks very much for these. Bound to start a debate I would guess. The integral supersaturation trends are particularly interesting. I'm sure Dr Doolette would agree that these charts require real world data to calibrate them, but the trends are interesting even so.

    Rich
    Last edited by RichardC; 27th January 2014 at 18:55.

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    Re: Deep Stops (rebreather dive charts)

    In your example the integral supersaturation goes down steadily as you pull in the GFs, and you can see the highest peak supersaturation move towards the fast compartments.

    One question that does come to mind is, if you did the same analysis for more aggressive profiles (VPM+3, 2, 1 etc,), do you ever encounter a turning point where the integral supersaturation starts to increase as you push the peak supersaturation higher and higher in the fast compartments? Does this happen before you get to GFs around 100?

    Rich

  4. #4

    Re: Deep Stops (rebreather dive charts)

    Quote Originally Posted by RichardC  View Original Post
    In your example the integral supersaturation goes down steadily as you pull in the GFs, and you can see the highest peak supersaturation move towards the fast compartments.

    One question that does come to mind is, if you did the same analysis for more aggressive profiles (VPM+3, 2, 1 etc,), do you ever encounter a turning point where the integral supersaturation starts to increase as you push the peak supersaturation higher and higher in the fast compartments? Does this happen before you get to GFs around 100?

    Rich
    I don't think so. At VPM-B+0 the pattern you observe for the integral SSs is the same. I think the flat GF would need to be around GF 90/90ish.

    The issue (that the NEDU study highlighted) is that the deep stops come at the price of continued loading of the slow compartments. That gas has to come out at some point. That point is normally the surface where you are breathing air.

    You decompress so much slower on air that the gas you loaded due to the deep stops comes at a high price in terms of SS-time exposure. Where the balance of additional risk posed by higher peak supersaturations in the fast compartments balances with the additional decompression time you have once you surface I can't say.

    I do wonder whether the surface integral SS measure shown in that chart correlates at all to the higher bubble counts some of you observed when you participated in some of the bubble count studies (I think in the Caymans?). Some divers decided to go with lower GF values which clearly show lower integral SS. If there is a correlation, then it would seem that integral SS could be tied to risk through the bubble counts studies. Perhaps those who keep up on the literature might already know whether there is a correlation.

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    Re: Deep Stops (rebreather dive charts)

    Quote Originally Posted by UWSojourner  View Original Post
    Where the balance of additional risk posed by higher peak supersaturations in the fast compartments balances with the additional decompression time you have once you surface I can't say.
    That, as they say, is the question. You clearly saw where I was going with this. Thanks.

    Rich

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    Re: Deep Stops (rebreather dive charts)

    Quote Originally Posted by RichardC  View Original Post
    That, as they say, is the question. You clearly saw where I was going with this. Thanks.
    Rich
    Hello Richard,

    Kevin has been appropriately cautious with his words, but his latest analysis of profiles that are closer to what happens in our everyday diving is important.

    Others have expended considerable energy trying to dismiss the NEDU study as having no relevance to typical technical diving, but its relevance
    (as we have repeated many times) is that it compared hard outcomes for profiles that generated representative patterns of tissue supersaturation. Those patterns are replicated in different real world technical diving profiles as can be seen in Kevin's latest analysis. The NEDU profile that emphasized deep stops and protected the faster tissues early in the ascent at the expense of greater supersaturation in slower tissues later had worse outcomes. That is the only hard data point we have. You can see from Kevin's diagram that the profiles that "de-emphasize" deep stops are the ones that best avoid the pattern that seemed disadvantageous in the NEDU study.

    I repeat that Kevin was correct to be cautious in his wording, and in respect of the conclusions that can be drawn from this analysis. However, I agree with you that this is fascinating.

    On the issue of bubble counts... this was thrashed to death in the other thread, but there is no doubt that a profile that consistently generates higher VGE counts than another is associated with higher risk of DCS, particularly if those higher counts are in the grade III to IV range.

    Simon M
    Last edited by Simon Mitchell; 28th January 2014 at 17:40.

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    Re: Deep Stops (rebreather dive charts)

    So, the thing we all want to know is:

    When is Kevin going to write some deco software that generates its own 'supersaturation heat maps' and lets us tweak GFs to produce one that we like for the dives that we do?

    Please? It would be a killer feature!

    Andy

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    Re: Deep Stops (rebreather dive charts)

    Quote Originally Posted by apitkin  View Original Post
    So, the thing we all want to know is:

    When is Kevin going to write some deco software that generates its own 'supersaturation heat maps' and lets us tweak GFs to produce one that we like for the dives that we do?

    Please? It would be a killer feature!

    Andy
    Yeah, I asked the same thing. How cool would that be?
    Andrew Ainslie

    Buhlmann = Bend and Mend
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    GF 35-50/70-85 = Mend and Mend

  9. #9

    Re: Deep Stops (rebreather dive charts)

    Quote Originally Posted by Simon Mitchell  View Original Post
    ... the NEDU study ... relevance
    (as we have repeated many times) is that it compared hard outcomes for profiles that generated representative patterns of tissue supersaturation. Those patterns are replicated in different real world technical diving profiles as can be seen in Kevin's latest analysis. The NEDU profile that emphasized deep stops and protected the faster tissues early in the ascent at the expense of greater supersaturation in slower tissues later had worse outcomes.
    One pattern that caught my eye in the Rebreather chart that wasn't in the NEDU study can be seen in the fast compartments upon surfacing (top right of the heat maps). The rebreather washes out the fast compartments prior to surfacing to the point that C1-C5 (for VPM) and C1-C6 for the GF profiles all start on gassing when the diver comes off the loop.

    The NEDU study showed that emphasizing the fast compartments was not as beneficial as curtailing continued on gassing of the slow compartments. That result was seen in an air only dive. When you add a Rebreather, those faster compartments also get quite the treatment prior to surfacing. For example, in the NEDU study the average inert gas level of C1-C5 upon surfacing was about 1020mb for both A1 and A2 (and all were supersaturated). In the Rebreather profile we're discussing, the average inert gas load at the surface for C1-C5 is about 160mb (all on gassing when you come off the loop).

    If the NEDU study showed an incremental level of supersaturation in fast compartments was beneficial, it seems like a reasonable guess that the same additional supersaturation would be even less concerning on a Rebreather just because those faster compartments get so thoroughly washed out prior to surfacing. Perhaps especially in a Rebreather environment favoring the fastest compartments is likely counterproductive.

    Standard conjecture alert ... .

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    Re: Deep Stops (rebreather dive charts)

    Quote Originally Posted by UWSojourner  View Original Post
    If the NEDU study showed an incremental level of supersaturation in fast compartments was beneficial, it seems like a reasonable guess that the same additional supersaturation would be even less concerning on a Rebreather just because those faster compartments get so thoroughly washed out prior to surfacing. Perhaps especially in a Rebreather environment favoring the fastest compartments is likely counterproductive.

    Standard conjecture alert ... .
    Hi Kevin,

    My take on what the NEDU study showed was that incremental supersaturation in fast compartments didn't matter (as opposed to being beneficial per se). It was probably the reduced supersaturation in slow compartments that was "beneficial". But other than that largely semantic observation, I competely agree with you.

    Simon M

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