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  1. #1

    Deep stops for T2 dive in light of latest deco thinking

    So, I'm planning for a 78m dive with up to 25 mins bottom time. (I.e. actually planning to do it, not theoretically.)

    Standard procedures for this bottom time would call for 1 min stops from 57m (with gas switch to 21/35 at 57m), and 2 min stops from 39m.

    In light of the many discussions around the web on the latest thinking on the benefits/wisdom of deep stops, I'm wondering if those 1 minute stops - totalling 6 minutes - spent between 57 and 42m are the best approach. Using Buhlmann theory to analyse the inferred profile shows that the gradient factors actually experienced during those 1 min stops are ridiculously low; they also add to the total deco time (greatly in case of loss of the 21/35 deco gas) and add to minimum gas requirements, for little benefit as it appears.

    However, using 1 minute stops between 39 and 24m would mean arriving at 21m for the gas switch at around GFHi 85, which seems a little aggressive for that point in the ascent. (And even more aggressive in case of loss of the 57 bottle.)

    So I'm proposing 0.5 mins (or, 6m/min) and 2 mins (or, 1.5m/min) as deep stops, with depth ranges as usual. Has anyone else done something like this? And have any comments?

    (Incidentally and for clarity, since this is the internet; I'm perfectly capable and willing to decide on my own whether this is a reasonable thing to do , nor am I looking for reassurance - but I'm still interested in discussion and other opinions I've also done the 1 & 2 minute deep stop profile in the past with no issues so it's not that I think the existing procedure is broken, just a way of responding to latest thinking without going overboard, and with retaining the benefits of a ratio approach to deco.)

    The analysed plan then looks like:

    Phase Depth - Time RunTime Mix O2/He GFHi
    ----- ------------ ------- --------- --
    Descent To: 78m - 5.2 min 5:13 15/55 Ceil:3 / GF:-11
    Bottom Phase: 78m - 19.8 min 25:0 15/55 Ceil:28 / GF:-35
    Ascent To: 57m - 2.3 min 27:20 15/55 Ceil:29 / GF:-11
    Deep Stops 1: 57m - 0.5 min 27:50 21/35 Ceil:28 / GF:3
    Deep Stops 1: 54m - 0.5 min 28:20 21/35 Ceil:27 / GF:5
    Deep Stops 1: 51m - 0.5 min 28:50 21/35 Ceil:27 / GF:7
    Deep Stops 1: 48m - 0.5 min 29:20 21/35 Ceil:26 / GF:11
    Deep Stops 1: 45m - 0.5 min 29:50 21/35 Ceil:26 / GF:16
    Deep Stops 1: 42m - 0.5 min 30:20 21/35 Ceil:25 / GF:21
    Deep Stops 2: 39m - 2 min 32:20 21/35 Ceil:23 / GF:28
    Deep Stops 2: 36m - 2 min 34:20 21/35 Ceil:21 / GF:27
    Deep Stops 2: 33m - 2 min 36:20 21/35 Ceil:20 / GF:28
    Deep Stops 2: 30m - 2 min 38:20 21/35 Ceil:18 / GF:30
    Deep Stops 2: 27m - 2 min 40:20 21/35 Ceil:17 / GF:38
    Deep Stops 2: 24m - 2 min 42:20 21/35 Ceil:16 / GF:46
    Intermediate Stop: 21m - 6 min 48:20 50/0 Ceil:12 / GF:57
    Intermediate Stop: 18m - 6 min 54:20 50/0 Ceil:10 / GF:39
    Intermediate Stop: 15m - 6 min 60:20 50/0 Ceil:8 / GF:34
    Intermediate Stop: 12m - 6 min 66:20 50/0 Ceil:6 / GF:38
    Intermediate Stop: 9m - 6 min 72:20 50/0 Ceil:5 / GF:53
    6m Stop: 6m - 33 min 105:20 100/0 Ceil:0 / GF:75
    Final Ascent: 5m - 1 min 106:20 100/0 Ceil:0 / GF:0
    Final Ascent: 4m - 1 min 107:20 100/0 Ceil:0 / GF:15
    Final Ascent: 3m - 1 min 108:20 100/0 Ceil:0 / GF:30
    Final Ascent: 2m - 1 min 109:20 100/0 Ceil:0 / GF:47
    Final Ascent: 1m - 1 min 110:20 100/0 Ceil:0 / GF:65
    Surface: 110:20 100/0 Ceil:0 / GF:84

    Plan for 78m for 25 mins on 15/55 with 21/35 + 50% + O2
    Total Deco: 63 mins - as 6+6+6+6+6+33
    Deep Stops: 0.5 / 2
    Estimated Total Runtime (including switches and gas breaks): 127 mins
    Last edited by huwporter; August 13th, 2016 at 05:08 AM. Reason: formatting / clarity

  2. #2
    Hi Huw

    I have had great success with 20/85 I don't feel any reason to change for T2 dives (or T2+ - which your dive looks like ). Do you (and greater readership) have a reason to change?

    On bigger dives (5-6 deco) I don't follow 20/85 but I also don't skip deeper stops.

    Graham
    Last edited by graham_hk; August 14th, 2016 at 01:31 AM.
    ah herro

  3. #3
    Thanks Graham,

    I'm sure you can't have missed the epic threads sprawling across multiple dive forums for the past couple of years on the latest evidence around the practice of deep stops (with science and evidence on one side, and commercial interests on the other...) In a nutshell, for anyone that has managed to miss them, the best and latest available evidence seems to indicate that deep stops *may* have come to be over-emphasised to some extent in some deco theories, and that the optimum ascent strategy may involve (slightly?) shallower stops than some models dictate.

    You'll also know that GUE procedures call for modifying the ascent rate starting from around 75% max depth. Analysing this using Buhlmann theory, on arriving at 75% max depth you are usually at a way lower gradient factor than 20 - case in point, in the example profile above, 25 minutes at 78m, when you arrive at 57m you are at gradient factor *3*; not 20. So following this protocol you aren't really diving 20/85, in actuality more like 3/85. To actually follow 20/85, you would ascend directly to 48m.

    On the other hand, hopefully I don't need to persuade you of the benefits of a standardised ascent protocol in getting a team on the same page during an ascent. So, moving to 30 second stops in place of 1 minute during the first phase of deep stops seems like a way of incorporating the latest evidence in a measured way without throwing out all the benefits of the ascent standards?

    ...The dive went great by the way, pretty much exactly to the plan above, with the exceptions that the Shearwater on my buddies JJ asked for an additional 2 minutes at 24m, an additional 4 minutes at 9m and an additional 14 minutes at 6 - but then he was running a higher %He in his dil/backgas. And also provided entertainment from the look on his face as he realised that the only remaining option when the motor on his antique Gavin ran away at 78m (-not a stuck trigger-) was sticking his fist in the prop...
    Last edited by huwporter; August 14th, 2016 at 06:34 AM.

  4. #4
    Quote Originally Posted by huwporter View Post
    Thanks Graham,

    I'm sure you can't have missed the epic threads sprawling across multiple dive forums for the past couple of years on the latest evidence around the practice of deep stops (with science and evidence on one side, and commercial interests on the other...) In a nutshell, for anyone that has managed to miss them, the best and latest available evidence seems to indicate that deep stops *may* have come to be over-emphasised to some extent in some deco theories, and that the optimum ascent strategy may involve (slightly?) shallower stops than some models dictate.

    You'll also know that GUE procedures call for modifying the ascent rate starting from around 75% max depth. Analysing this using Buhlmann theory, on arriving at 75% max depth you are usually at a way lower gradient factor than 20 - case in point, in the example profile above, 25 minutes at 78m, when you arrive at 57m you are at gradient factor *3*; not 20. So following this protocol you aren't really diving 20/85, in actuality more like 3/85. To actually follow 20/85, you would ascend directly to 48m.

    On the other hand, hopefully I don't need to persuade you of the benefits of a standardised ascent protocol in getting a team on the same page during an ascent. So, moving to 30 second stops in place of 1 minute during the first phase of deep stops seems like a way of incorporating the latest evidence in a measured way without throwing out all the benefits of the ascent standards?

    ...The dive went great by the way, pretty much exactly to the plan above, with the exceptions that the Shearwater on my buddies JJ asked for an additional 2 minutes at 24m, an additional 4 minutes at 9m and an additional 14 minutes at 6 - but then he was running a higher %He in his dil/backgas. And also provided entertainment from the look on his face as he realised that the only remaining option when the motor on his antique Gavin ran away at 78m (-not a stuck trigger-) was sticking his fist in the prop...
    Hi Huw

    No I haven't missed the threads :D There is always an ebb and flow of information and I remember in late 90s when folks were racing out of the water on short deep dives and getting bent as they tried to replicate information from the internet or start using "new" bubble models that promised faster/better deco at that time. Should we all jump ship and move to something else now - even though what we have is working (as you found out on your dive)?

    GUE has not had a deep stop methodology for many years - we have a variable ascent rate that slows at 75% yes - but these are not a deep stops. Even with this variable rate we are moving much faster out of deep water than the general (believe DAN collected data and average is 4-5m per min) tech diving population upto our decompression stops.

    I can't replicate a 3% GF in decoplanner and in the real world with your example of ascend directly to 48 that essentially means that an extra 2min between 57 and 48 (40sec at each of 3 stops) - do you think this makes a difference? Especially since you have made a gas switch ...

    There is a lot of interesting research for us - like He and N2 half times actually being similar in real tissues (as opposed to estimations based on physical properties). DAN studies done in Italy on deep stops showing that there are two groups of people those who do well on shallow profiles and those who do well on deep profiles - but the shallow group don't suffer as much doing a deep profile as the deep lovers do going shallow. There are diving doctors who think what GUE does is a good way to manage ascents and Simon Mitchell has numerous times said somthing along the lines of: what GUE does works for GUE because they do what they plan.

    So I don't feel any need to experiment too much with 25@78.
    ah herro

  5. #5
    Fair enough - as you say, it is a minor point in the grand scheme of things. We wrestle with the big questions here on DIRx...

    I'll certainly agree that differences between theory and practice (e.g. your DAN collected ascent rates example) mean any anecdote has even more limited value than anecdote usually has. (Although I will say that the difference between a 3m/min variable ascent rate and 1 min deep stops seems an awful lot like semantics ).

    I'm happy to continue to experiment with 6m/min, but equally if my team on a dive wishes to stick to 3m/min that is cool too.

    Edit:
    Quote Originally Posted by graham_hk View Post
    DAN studies done in Italy on deep stops showing that there are two groups of people those who do well on shallow profiles and those who do well on deep profiles - but the shallow group don't suffer as much doing a deep profile as the deep lovers do going shallow.
    Do you have any details for this? This is something I have suspected (without any support beyond a hunch) for a while.
    Last edited by huwporter; August 14th, 2016 at 11:13 PM. Reason: added second part

  6. #6
    Quote Originally Posted by huwporter View Post
    Fair enough - as you say, it is a minor point in the grand scheme of things. We wrestle with the big questions here on DIRx...

    I'll certainly agree that differences between theory and practice (e.g. your DAN collected ascent rates example) mean any anecdote has even more limited value than anecdote usually has. (Although I will say that the difference between a 3m/min variable ascent rate and 1 min deep stops seems an awful lot like semantics ).

    I'm happy to continue to experiment with 6m/min, but equally if my team on a dive wishes to stick to 3m/min that is cool too.

    Edit:


    Do you have any details for this? This is something I have suspected (without any support beyond a hunch) for a while.
    There are a lot of semantics in diving For most the difference between stopping at 75% and GF 40/60 will be minutes and probably meaningless in grand scheme of things :D :D :D

    The divers group I have learned from my Italian dive buddies that were part of the DAN study - not sure if its been published yet. I'll find out and post a link if possible.
    ah herro

  7. #7
    Did you ever find a link to that study?

    In his decompression controversies talk, he uses the NEDU study to argue for a higher GF-low. I think his explanation is worth thinking about, because he argues that the VPM-B+4 is worse than GF40/75 by looking at supersaturation modelled with Buehlmann... That is obvious, isn't it?
    The whole argument for VPM style models is that you lose decompression efficiency due to bubble growth, which is not modelled in Buehlmann at all.

  8. #8
    Quote Originally Posted by data2 View Post
    Did you ever find a link to that study?

    In his decompression controversies talk, he uses the NEDU study to argue for a higher GF-low. I think his explanation is worth thinking about, because he argues that the VPM-B+4 is worse than GF40/75 by looking at supersaturation modelled with Buehlmann... That is obvious, isn't it?
    The whole argument for VPM style models is that you lose decompression efficiency due to bubble growth, which is not modelled in Buehlmann at all.
    The argument around the NEDU study is actually a bit more sophisticated/complex than that.

    The clear fact is that the "bubble model" tested in the NEDU study resulted in a higher rate of DCS than the "dissolved gas model", on profiles of the same total length. (Bottom time and total deco time were the same for both profiles, just the distribution of stops was different - with longer deeper stops and shorter shallower stops in the bubble model profile. The same total length of the profiles is important - the study was trying to evaluate the most efficient way of ascending in a given time.)

    The bubble model wasn't VPM, and the dissolved gas model wasn't Buhlmann (both were US navy models), however you can use the Buhlmann algorithm to analyse supersaturations during the profiles tested, and doing this you can see that the bubble model profile resulted in higher levels of supersaturation, particularly in slow tissues, towards the end of the ascent. At the least, this can be said to be correlated with the higher rate of DCS.

    You can similarly use Buhlmann to analyse the supersaturation levels experienced during a GF say 40/75 profile and a VPM profile of the same total length, and what you see is a similar pattern of raised supersaturation in slow tissues in the VPM profile to the pattern seen in the bubble model in the NEDU study that was correlated with raised DCS risk.

    This is NOT conclusive proof that the VPM bubble profile is inferior to the shallower GF profile, but it seems reasonable to suspect that VPM (/bubble models) may overemphasise deep stops at the expense of shallow stops in a way that is non-optimal for the most efficient deco profile within a given time in the real world, and that a shallower stop profile would actually be more efficient.

    ...And that is all the study says, and all that the supporters of the study have ever claimed. Unfortunately the waters then become muddied by a few highly vocal financially-interested supporters of VPM who have spread FUD to the tune of literally thousands of forum posts across multiple forums. (All I'll comment on that is that in my opinion, although I may have many questions on how well and how directly the NEDU study applies to technical diving a la DIR style, VPM itself has been entirely discredited as a dive planning tool by its own supporters in my eyes.)

    And I'd still like to see Graham's study.
    Last edited by huwporter; November 16th, 2016 at 10:19 AM. Reason: clarity

  9. #9
    Quote Originally Posted by huwporter View Post
    The argument around the NEDU study is actually a bit more sophisticated/complex than that.

    The clear fact is that the "bubble model" tested in the NEDU study resulted in a higher rate of DCS than the "dissolved gas model", on profiles of the same total length. (Bottom time and total deco time were the same for both profiles, just the distribution of stops was different - with longer deeper stops and shorter shallower stops in the bubble model profile. The same total length of the profiles is important - the study was trying to evaluate the most efficient way of ascending in a given time.)
    That is true, and I think relatively important.

    The bubble model wasn't VPM, and the dissolved gas model wasn't Buhlmann (both were US navy models), however you can use the Buhlmann algorithm to analyse supersaturations during the profiles tested, and doing this you can see that the bubble model profile resulted in higher levels of supersaturation, particularly in slow tissues, towards the end of the ascent. At the least, this can be said to be correlated with the higher rate of DCS.
    Well, that is going to be true for any profile with deeper stops; that is simply what the Buhlmann algorithm does. This was clear even before the NEDU study


    This is NOT conclusive proof that the VPM bubble profile is inferior to the shallower GF profile, but it seems reasonable to suspect that VPM (/bubble models) may overemphasise deep stops at the expense of shallow stops in a way that is non-optimal for the most efficient deco profile within a given time in the real world, and that a shallower stop profile would actually be more efficient.
    I guess that is a good summary. So: do we continue with out GF20/85 dives? Or do we put more emphasis on lower supersaturation and reduce the GF-high to 75? If we do the latter, than I think it becomes necessary to increase GF-low to not run into excessive decompression times, leading us to 40/75 or even 50/75 (which, btw. get's very close to straight buhlmann with more conservative M-values. Full circle anyone?)

  10. #10
    Quote Originally Posted by data2 View Post
    I guess that is a good summary. So: do we continue with out GF20/85 dives? Or do we put more emphasis on lower supersaturation and reduce the GF-high to 75? If we do the latter, than I think it becomes necessary to increase GF-low to not run into excessive decompression times, leading us to 40/75 or even 50/75 (which, btw. get's very close to straight buhlmann with more conservative M-values. Full circle anyone?)
    Well who knows. It's not as though we were actually diving 20/85 anyway, after all the modifications. Looking at the last profile I ran for 30 mins at 60m, 18/45 + 50% + O2, unmodified 20/85 gives a 92 minute run time. 50/75 also gives 92 mins. GUE first stop is at 45m, 20/85 at 36m and 50/75 at 27(!)m. If the conclusion from the study applies, 50/75 may be a better way of spending your 92 minutes. The revised GUE ascent procedures are due out shortly, de-emphasising deep stops but otherwise sticking with modified 20/85. Times they are-a-changing.

    If you listen carefully, on a quiet night you can hear George crying.
    Last edited by huwporter; November 17th, 2016 at 11:48 PM.

 

 

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