• Welcome to ScubaBoard


  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

Bent... advice sought on continuing to tech dive

Discussion in 'Advanced Scuba' started by Tortuga68, Nov 3, 2018.

  1. Tortuga68

    Tortuga68 Divemaster

    # of Dives: 1,000 - 2,499
    Location: Puerto Galera, Philippines
    4,104
    842
    Back in 2011 I posted this thread Increasing conservatism with Decoplanner/VPM seeking advice on increasing conservatism for technical dives after experiencing some 'minor' DCS symptoms. Rather than bump it I decided to make a new thread.

    I changed my profiles based around the feedback received (particularly about extending the 6m stop and doing a slower ascent to the surface on O2) and over the last 8 years and 1500 dives largely didn't have any problems until 3 weeks ago when I got properly bent (numbness and pain in upper right arm from shoulder to elbow after the dive) and did 2 treatments in the chamber (Table 6 and 5). I believe the problem was a shortened surface interval between 2 deco dives, and not really being in fit physical condition on the day; otherwise I have done the same profiles many times without issue. My own stupid fault on both points and kicking myself ever since.

    Anyway, moving on, I still want to continue to dive both rec & tech so thinking again how to adapt to the new challenge of now being predisposed for repeat DCS so I thought I would open myself up to my peers here and seek your advice once again.

    What's changed since the last thread: for several years I have been using a Shearwater computer with ZHL16C GF30/70. Doing more DPV dives now. I got trimix certified a couple of years ago but still mostly diving Air or Nitrox backgas to a max of 65m in the warm, clear waters of the Philippines where I now live since retiring. Obviously older now (50) and a bit fatter (BMI 26). Doing less diving in general now, 1-2 a day, 2-5 days a week, sometimes less/more - apart from the odd liveaboard trip which are obviously busier.

    For the record, the hyperbaric doctor gave me 4 weeks off diving, DAN gave me 6 (although there were 2 treatments, symptoms resolved after the first and the second was precautionary). Apart from getting tired easily the first few days I've felt fine since the first treatment and no reoccurrence of the numbness/pain. Interestingly I never really had any other symptoms, neurological exams were ok. no headaches etc.

    My first plan is obviously stick to the minimum 3 hour SI which I had always done prior to this event, and making sure I'm fit and hydrated, both of which I should have done in the first place. Beyond that I'm open to hearing any and all feedback.I'm still pretty down on myself about the incident but do want to get back to diving safely again as soon as prudently possible and in a manner conducive to avoiding further incidents, however at the same time being retired now, one that hopefully won't break the bank.

    Having said that, if you think trimix or rebreather or stop tech diving is the answer, that's what I want to hear. Interested in opinions on everything from how long to wait, how to ease back in and what to change gear/profile/gas etc. Thanks.
     
  2. KenGordon

    KenGordon Rebreather Pilot

    3,845
    2,630
    Are you doing two 50m plus dives the same day? The old thread was taking about 20m at 55 I think. Two of those might be a bad idea. Several days of two of those is too much for me. Bühlmann compartments are pretty much completely clear after a day, so the model isn’t helping you over several days. It is generally accepted that multiple days of diving is riskier.

    That thread came over as paranoid regarding CNS. Do you absolutely have to do your last stop at 6m? If you do then maybe don’t use 100% if you are worried about O2 exposures, but to be honest that doesn’t sound like a lot of O2 exposure. Another option would be to use something weaker as a final deco mix and get into that deeper. Then the stops at 12 or 9 get to be useful. Or you could (as suggested in the other thread) use a second deco mix. (What is your lost deco gas plan like otherwise?).

    Remember that deco is really statistics. You are never really doing it right, just trying to make the chances of it turning out badly lower. Sometimes your number can up. Maybe you don’t need to change anything. Of course if you keep getting bent then you need to see what is causing the increase in probability, but one event can’t really tell you anything.
     
  3. Tortuga68

    Tortuga68 Divemaster

    # of Dives: 1,000 - 2,499
    Location: Puerto Galera, Philippines
    4,104
    842
    Thanks. In the past I have done 2x 50m+ dives in a day, with 3 hour surface interval and using 50% and 100% for accelerated deco. But I think from now on I will only do 1 deco dive per day. I know what you mean about sounding paranoid about CNS in the last thread but I'm not really, or at least not any more. So 15-20 minutes on O2 is ok at 6m. Since I'm nearly always using either 50, or 50+100 for accelerated deco, and therefore getting off backgas at 21m, I don't feel that using a sub-100 mix would be a big benefit, since my idea would be rather to extended the initial stop times when the oxygen-window is first opened at high PP. I get that deco isn't an exact science but I really want to give myself the best chance of avoiding another trip to the chamber, without being totally over the top about it.

    To answer your questions, lost gas plan if only using 1 deco mix (50) is backgas, buddy-breathing or O2 stage dropped down from the boat to 6m. Usually I surface from most dives with >100 bar of backgas.
     
  4. KenGordon

    KenGordon Rebreather Pilot

    3,845
    2,630
    I am not arguing, but asking, is that still a thing?

    I assume you are referring to the extra reduction in total disolved gas pressures caused by a high ppO2 loading to more of the disolved o2 being used rather than bound o2.

    That is only going to help prevent bubbling, not off gassing. I suspect bubbling at 21m when you go to your 50% isn’t your issue. Staying longer there is trading that for some slow compartment on gassing (although obviously not much at a ppN2 of 1.5 ish).
     
  5. Tortuga68

    Tortuga68 Divemaster

    # of Dives: 1,000 - 2,499
    Location: Puerto Galera, Philippines
    4,104
    842
    That's what I'm talking about yes, aka the S-curve in Ratio Deco. I thought it was still a thing but I'm not a GUE diver so maybe I missed the memo. I do know Deep Stops seem to have gone out the window now. I experimented with the S-curve a few times but as a rule I haven't done it historically. I understand what you're saying... basically the same thing as doing your last stop at 4.5m rather than 6, getting away from the max PP and closer to the surface gradient
     
  6. Tortuga68

    Tortuga68 Divemaster

    # of Dives: 1,000 - 2,499
    Location: Puerto Galera, Philippines
    4,104
    842
    Maybe I can also do a better job of explaining my history...

    1. Started tech diving
    2. Over several years had various niggles which I wrote off at the time and either resolved without treatment or with oxygen
    3. Had a more serious issue in 2011 (right shoulder) which prompted my previous thread
    4. Changed my profiles as a result of various inputs and was largely asymptomatic for 8 years and 1500 dives (rec & tech)
    5. Now in 2018, had a serious issue (right shoulder again) and had 2 chamber rides

    Although I believe that it was the pre- and between-dive behaviours which caused the latest problem, rather than the profiles themselves, my thinking is that this combined history now makes me predisposed to get bent again and if you like puts me in the "yellow zone". Another similar incident could put me in the red zone and lead to either a lasting impairment or not being able to dive at all. That's what I'm seeking to avoid.
     
  7. rongoodman

    rongoodman ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Albany, NY
    5,321
    2,024
    You mention using a Shearwater. Have you been watching the tissue loading graph to keep an eye on loading in the slower compartments?
     
  8. KenGordon

    KenGordon Rebreather Pilot

    3,845
    2,630
    My naive view is that you are diving in a style which tries to protect fast compartments, but getting hits in tissues modelled by the slower compartments. So what i’d do, and this is what I do anyway so feel free to ignore me as justifying my own behaviour, is get shallower slightly more quickly (at least not hang round deep). Also get onto a vaguely proper deco gas early enough to take advantage of it while doing the stops in the 12 to 6 range which can take a while.

    I was on a boat in Norway diving the collected works of the RAF 1939 to 1945. The skipper had a rule which was approximately if your first dive is past 50 your second can’t be past 40. I was on a rebreather while my buddy was mostly an OC diver. On some dives I got to do 15 minutes of extra deco at high ppO2, I didn’t mind at all. The previous year I was on a boat with a bloke who wrote a book on the subject. He was only doing one deco dive a day.

    Once you get into multiple dives over multiple days it seems rather more hit and miss to me so being quite conservative with the last stop would be prudent.
     
  9. Tortuga68

    Tortuga68 Divemaster

    # of Dives: 1,000 - 2,499
    Location: Puerto Galera, Philippines
    4,104
    842
    Well... isn't that the job of the decompression algorithm? Are you saying I should use a different GF?
     
  10. rongoodman

    rongoodman ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Albany, NY
    5,321
    2,024
    I'm not commenting on your choice of algorithm or GFs, which seem pretty conservative. I'm pointing out that that there is a very useful tool on your computer which might provide some insight as to why you're having problems.
     

Share This Page