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Is the CNS loading 'half life' concept the result of a misunderstanding?

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  • Turbanator
    Established TDF Member
    • Dec 2012
    • 870

    #16
    I'd read somewhere that our recommended limits were got by NOAA deciding the safe limits, then halving it.

    WRT to PO2, again when I learnt you were told 1.6 was the max , but to reduce by 0.05 for each of the following, cold, dark/stress and exertion, which brings you pretty close to BS-AC's 1.44. But then that's not really surprising when you think where the BS-ACs recommendations came from.

    As with a lot of things, knowing why we do something is more important then know what to do by rote.

    As Iain said, the original work is Kenneth Donald's, I'm not sure if there been any more experimentation since, or whether it's all extrapolation and fudge factors based on his work.
    Finally, a lot of what we get as recreational divers is old and 2nd hand because to the people paying for the research, (Comex et al.) this stuff has a commercial value.
    'I saw some purple slug things on the Scylla, so I squished them' - #MiniBodger

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    • gobfish1
      Last of the Mohicans
      • Jan 2013
      • 4303

      #17
      Originally posted by Garf
      Me personally? I don't pay any attention to the CNS clock whatsoever. I just choose standard gases and go diving. I've only ever thought about CNS once in the last 8 years, and that was when planning a 20 minute 100 metre dive. that dive didn't happen, and frankly was such a bollock ache to execute on OC that unless I go CCR it won't ever happen, so CNS is something I just don't waste any time thinking about.

      GUE divers have shortcuts for estimating CNS etc for those people that DO worry about such things. When you see what a blunt tool those shortcuts are you realise what an unknown it all is.
      id have to say im all most with you on the above , 20.000% dives may as will be out of space dives , for 99% of oc diving
      nitrox and mixgas lack of bottom gas / not wanting or able to do 4hr or more deco keeps you well out of harms way .

      trouble with the big out of space dives is , when it gets posted , we end up with one tank nitrox dives on a 30min bottom time with a low ppo2 wanting to take air brake.

      even a ccr diver doing 80m dives would be hard pushed to run up a big cns number , with out hitting 100% o2 ,

      it all maybe bollox , but over the last 20 odd year 1000,s and 1000,s of dives have been done , and only a v few have had a problem ,

      i think as a teaching tool noaa numbers are just fine , and if BASC or others want to move the numbers some what thats fine to ,
      None diver as of 2018.

      Comment

      • Gareth J
        TDF Member
        • Sep 2013
        • 265

        #18
        Originally posted by Garf
        I don't believe BSAC are "claiming" anything. They are merely making a recommendation to their divers. If they feel the need to build in conservatism above and beyond everyone else then that seems fair enough. Perhaps they are considering typical UK diving and factoring in the O2 toxicity factors of increased cold, stress, etc.

        Garf.
        I think you've hit the nail on the head.

        When I did my first course with IANTD, we had a PO2 max of 1.6, then subtract 0.1PO2 for environmental factors, like workload, cold, darkness ,etc.
        So if you where planning a dive in cold water, that was likely to be dark / poor visibility with a risk of high work load (tide) you had a maximum planned PO2 for the dive = 1.3 (1.6-0.1-0.1-0.1 = max PO2 1.3).

        Someone else has posted with the negative factor being 0.5, they may well be correct. If I get time I'll have a look at my old manuals.

        When I did my Advanced Nitrox Instructor and ERD certification with Jack Ingle he stated the reason that the BSAC had chosen 1.4PO2 as the maximum was based on the fact that in the UK you where likely to get at least two negative factors, so the BSAC choose 1.4PO2 as the safe limit.

        I also seem to remember the reason the BSAC settled on the 2 hour half life was that it just made the maths simpler.
        Again my original IANTD course had a 90 minute clock.

        I think Jack was always quite cautious with high PO2, I believe he had seen and experienced a number of O2 fits in the early days of using rich O2 mixes. That added to the fact that there was a lot of concern about the use of Nitrox within the BSAC at the time resulted in a fairly conservative attitude to the limits.

        Comment

        • gobfish1
          Last of the Mohicans
          • Jan 2013
          • 4303

          #19
          Originally posted by Gareth J
          I think you've hit the nail on the head.

          When I did my first course with IANTD, we had a PO2 max of 1.6, then subtract 0.1PO2 for environmental factors, like workload, cold, darkness ,etc.
          So if you where planning a dive in cold water, that was likely to be dark / poor visibility with a risk of high work load (tide) you had a maximum planned PO2 for the dive = 1.3 (1.6-0.1-0.1-0.1 = max PO2 1.3).

          Someone else has posted with the negative factor being 0.5, they may well be correct. If I get time I'll have a look at my old manuals.

          When I did my Advanced Nitrox Instructor and ERD certification with Jack Ingle he stated the reason that the BSAC had chosen 1.4PO2 as the maximum was based on the fact that in the UK you where likely to get at least two negative factors, so the BSAC choose 1.4PO2 as the safe limit.

          I also seem to remember the reason the BSAC settled on the 2 hour half life was that it just made the maths simpler.
          Again my original IANTD course had a 90 minute clock.

          I think Jack was always quite cautious with high PO2, I believe he had seen and experienced a number of O2 fits in the early days of using rich O2 mixes. That added to the fact that there was a lot of concern about the use of Nitrox within the BSAC at the time resulted in a fairly conservative attitude to the limits.
          yes it used to be .5 per cold / hard work ,
          and jack was a bit late to the game to have seen the eary days

          dark and bad vis lol

          I also seem to remember the reason the BSAC settled on the 2 hour half life was that it just made the maths simpler.



          what maths is that , dont BSAC dives need to look in a book to work out cns numbers ,?
          Last edited by gobfish1; 13-01-2014, 05:25 PM.
          None diver as of 2018.

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          • Dan Sumners
            New TDF Member
            • Aug 2013
            • 11

            #20
            Thanks to all who have replied so far, the historical info is much appreciated.
            I have a whole, and I'm fathoming it. Learn more at dansumners.co.uk.

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            • PeterL
              The artist formerly known as petlowe....
              • Dec 2012
              • 897

              #21
              Timely, I was talking to Dave Crockford about some DDRC diving he did back in the old and bold days...
              They ran the profile a few times in a pot then went out and did it for real........

              90M, 30 minutes bottom time, 90 mins back to surface, 4% N2 Heliox and PPO2's of 3.0 on deco using CCR+FFM... It worked.

              Comment

              • nigel hewitt
                Established TDF Member
                • Sep 2013
                • 3199

                #22
                Originally posted by PeterL
                It worked.
                My Russian O2 Rebreather is rated to 20 meters.
                Helium, because I'm worth it.
                Waterboarding at Guantanamo Bay sounded like a radical holiday opportunity until I looked it up.

                Comment

                • PeterL
                  The artist formerly known as petlowe....
                  • Dec 2012
                  • 897

                  #23
                  Originally posted by nigel hewitt
                  My Russian O2 Rebreather is rated to 20 meters.
                  You mean Russians are rated to 20M on O2.
                  Italians are rated to 12M.

                  Some nationalities just don't have good care facilities for the elderly......

                  Comment

                  • Hot Totty
                    Red Hot Totty ;)
                    • Dec 2012
                    • 3912

                    #24
                    Originally posted by nigel hewitt
                    My Russian O2 Rebreather is rated to 20 meters.
                    russian's are expendable
                    www.wapsac.co.uk
                    Apparently becoming a grandad

                    Comment

                    • PBrown
                      TDF Member
                      • Jan 2013
                      • 224

                      #25
                      Originally posted by Turbanator
                      I'd read somewhere that our recommended limits were got by NOAA deciding the safe limits, then halving it.
                      Not quite

                      As Iain said, the original work is Kenneth Donald's, I'm not sure if there been any more experimentation since, or whether it's all extrapolation and fudge factors based on his work.
                      Further work has been done, a nice summary is in a DAN article here: http://www.diversalertnetwork.org/me...ow_About_OXTOX

                      And more reading can be done here: http://rubicon-foundation.org/oxygen-toxicity/

                      I think those two must be about my most often posted links!

                      cheers,
                      Paul

                      Comment

                      • GLOC
                        Moderator
                        • Dec 2012
                        • 2844

                        #26
                        Originally posted by PeterL
                        90M, 30 minutes bottom time, 90 mins back to surface, 4% N2 Heliox and PPO2's of 3.0 on deco using CCR+FFM... It worked.
                        Bugger the Oxtox risk, what about the DCS! What is the GF for that!

                        Regards
                        Gareth

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                        • PeterL
                          The artist formerly known as petlowe....
                          • Dec 2012
                          • 897

                          #27
                          Originally posted by GLOC
                          Bugger the Oxtox risk, what about the DCS! What is the GF for that!

                          Regards
                          This is second hand only sorry but as the tale goes, the thing ran clean on their deco models, then doppler using 2 chamber rides so for the sake of science they did it for real......
                          As I said, older and bolder but you can't say it wouldn't have been the more entertaining way to see a wreck rather than the 5 odd hours hanging round like a mollusc.

                          Comment

                          • Dan Sumners
                            New TDF Member
                            • Aug 2013
                            • 11

                            #28
                            Originally posted by PBrown
                            Not quite

                            Further work has been done, a nice summary is in a DAN article here: http://www.diversalertnetwork.org/me...ow_About_OXTOX

                            And more reading can be done here: http://rubicon-foundation.org/oxygen-toxicity/

                            I think those two must be about my most often posted links!

                            cheers,
                            Paul
                            Great stuff, thanks!
                            I have a whole, and I'm fathoming it. Learn more at dansumners.co.uk.

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