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View Full Version : Is the CNS loading 'half life' concept the result of a misunderstanding?



Dan Sumners
13-01-2014, 02:22 PM
My BSAC Sports Diver manual states, with regard to CNS oxygen uptake, "For every two hours on the surface breathing air, the CNS % is reduced by half".

However, the recently published 'Staying alive: risk management techniques for advanced scuba diving' by Steve Lewis, states "...some divers believe that if a diver surfaces with a CNS loading of X per cent, after 90 minutes on the surface that loading will have dropped to 1/2 X per cent.

"This is not a technique suggested or recommended in any NOAA manual that I know of and was characterised as 'poor science' by Dr Bill Hamilton [who helped prepare the NOAA oxygen exposure limits].

"The only CNS loading tied to a 90 minute half time in NOAA publications are exposures delivering an oxygen partial pressure of 1.6 bar/ata. These are the ONLY [his emphasis] exposures given a half time of any sort.

"If we check out the single dive limit for 1.6 bar (45 minutes) and compare it to the daily limit (150 minutes), it becomes apparent something different goes on compared to what happens with lesser ppO2s [eg single exposure for 1.5 bar is 120 mins and 24 hour is 180 mins]."

I know BSAC isn't the only agency that teaches the half life, and a quick search online shows many believe it to be the case, as one would expect if it's being taught. However, BSAC does seem to be the only agency that says the half life is 2 hours, rather than 90 minutes.

Can anyone shed any further light? Does anyone have any information to suggest Steve Lewis is wrong? On what evidence does BSAC base its claim?

gobfish1
13-01-2014, 03:01 PM
90 minute half life is right ,

bsac just rehash the numbers, to teach and sell books , and edge on the safe side , well they will say that , but in truth ist hard to sell someones work , as your own , if you dont change it some what ,

2hr prob works in with what ever else they are teaching ,

NOAA publications are what id go off ,

and i know mike r , was reading the same noaa books as my self back in 1990, so he know the numbers to,
bsac came late to the party , and need new numbers , to sell books ,

all scuba certification agencies try to reinvent the wheel , you cant sell the same old shite twice ,

lol but it dont stop them trying :rofl:

Mark Powell
13-01-2014, 03:28 PM
I saw the comment in Steve's book when I was reviewing it and was wondering how long it would take before this discussion came up.

The following is just my view.......

I agree with Bill Hamilton that the 90 minute half life is "poor science" but then you could say that the vast majority of diving theory is based on "poor science" although that doesn't mean that it's wrong.

There is very little scientific basis for the 90 minute half life but that doesn't mean there is no evidence for it. Just look how many divers are using it.

Dan Sumners
13-01-2014, 03:31 PM
90 minute half life is right ,

bsac just rehash the numbers, to teach and sell books , and edge on the safe side , well they will say that , but in truth ist hard to sell someones work , as your own , if you dont change it some what ,

2hr prob works in with what ever else they are teaching ,

NOAA publications are what id go off ,

and i know mike r , was reading the same noaa books as my self back in 1990, so he know the numbers to,
bsac came late to the party , and need new numbers , to sell books ,

all scuba certification agencies try to reinvent the wheel , you cant sell the same old shite twice ,

lol but it dont stop them trying :rofl:

Thanks for the quick response.

However, Steve Lewis is saying that the NOAA publications say the 90 minute half life ONLY applies to "exposures delivering an oxygen partial pressure of 1.6 bar/ata". So for any other exposure, the full % should be counted until 24 hours after that dive.

Is that not your understanding?

Dan Sumners
13-01-2014, 03:34 PM
I saw the comment in Steve's book when I was reviewing it and was wondering how long it would take before this discussion came up.

The following is just my view.......

I agree with Bill Hamilton that the 90 minute half life is "poor science" but then you could say that the vast majority of diving theory is based on "poor science" although that doesn't mean that it's wrong.

There is very little scientific basis for the 90 minute half life but that doesn't mean there is no evidence for it. Just look how many divers are using it.

Thanks Mark - and, while I have the opportunity, for 'Deco for divers'.

So do you feel there is enough anecdotal - for want of a better term - evidence to suggest the half life is a valid concept?

gobfish1
13-01-2014, 03:41 PM
Thanks for the quick response.

However, Steve Lewis is saying that the NOAA publications say the 90 minute half life ONLY applies to "exposures delivering an oxygen partial pressure of 1.6 bar/ata". So for any other exposure, the full % should be counted until 24 hours after that dive.

Is that not your understanding?

well i use 90min half life for all my diving for ppo2 of 1.6 or less , ppo2 above 1.6 i dont know as i dont use them so never botherd to remember what the noaa book says , tho i do still have all my old noaa and iantd work books

been using said numbers for more than 22 years , so im happy to stick with it ,

Garf
13-01-2014, 03:51 PM
Can anyone shed any further light? Does anyone have any information to suggest Steve Lewis is wrong? On what evidence does BSAC base its claim?

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.

gobfish1
13-01-2014, 03:58 PM
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.

yes then the next lot of tec dives come along and think 2hr is the bottom line and ad in some more conservatism
if 2hr was true some of my 2nd dives over the last 22 years would make a joke out of the hole cns /ppo2 numbers ,

im sure loads of dives would say cns/noaa numbers are a joke ,
i all ways liked them my self , give s you something to work with , weather or not you think 100% on the cns clock is the right number .

Dan Sumners
13-01-2014, 04:00 PM
well i use 90min half life for all my diving for ppo2 of 1.6 or less , ppo2 above 1.6 i dont know as i dont use them so never botherd to remember what the noaa book says , tho i do still have all my old noaa and iantd work books

been using said numbers for more than 22 years , so im happy to stick with it ,

Thanks again!

Garf
13-01-2014, 04:01 PM
yes then the next lot of tec dives come along and think 2hr is the bottom line and ad in some more conservatism
if 2hr was true some of my 2nd dives over the last 22 years would make a joke out of the hole cns /ppo2 numbers ,

Couldn't agree more. Never said I agreed with it, just that I can see reasons for it.

Iain Smith
13-01-2014, 04:03 PM
So do you feel there is enough anecdotal - for want of a better term - evidence to suggest the half life is a valid concept?

One deep cave exploration group report dives involving CNS oxygen exposures in excess of 20,000%.

If you get a chance to read Kenneth Donald's "Oxygen and the Diver" you'll see the massive day-to-day variation in tolerance to elevated oxygen pressures within the same individual.

The fact that people don't appear to be getting hurt within a range of lower oxygen pressures doesn't mean that a half time of 90minutes has any validity at all. It may simply be that the oxygen exposure limits are sufficiently conservative that divers are kept far enough from the seizure threshold that they don't run into difficulties.

The fact that application of back-gas breaks appears to extend oxygen tolerance so far beyond the traditional limits suggests that there is much we do not understand about the response to elevated pO2. (I should note that I am not aware of any peer reviewed studies validating the claims made by WKPP divers, however I have no reason to doubt them. Equally the plural of "anecdote" is not "data"!)

Iain

Dan Sumners
13-01-2014, 04:04 PM
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.

Thanks Garf. Yes; I meant the term 'claim' in a very general sense, ie something that is said, but I see it seems stronger. Probably a result of my work in the political world!

So do you think that, even though NOAA only talks about a half life for exposures of 1.6, enough divers have used it 'successfully' to make it a valid concept?

Hot Totty
13-01-2014, 04:08 PM
Then we'll get into the instant death zone of more than 100%cns - it's all a very grey area with a large amount of variables. If deco is basically voodoo then the whole Cns loading issues and tactics invented to mitigate I call voodoo + . ;)

Garf
13-01-2014, 04:10 PM
Thanks Garf. Yes; I meant the term 'claim' in a very general sense, ie something that is said, but I see it seems stronger. Probably a result of my work in the political world!

So do you think that, even though NOAA only talks about a half life for exposures of 1.6, enough divers have used it 'successfully' to make it a valid concept?

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.

Hot Totty
13-01-2014, 04:11 PM
Me personally? I don't pay any attention to the CNS clock whatsoever. I just go diving.

Dive ccr, then it can be another issue to take on board ;)

Turbanator
13-01-2014, 04:45 PM
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.

gobfish1
13-01-2014, 05:04 PM
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 ,

Gareth J
13-01-2014, 05:06 PM
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.

gobfish1
13-01-2014, 05:12 PM
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 :x:

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 ,?

Dan Sumners
13-01-2014, 05:14 PM
Thanks to all who have replied so far, the historical info is much appreciated.

PeterL
13-01-2014, 05:35 PM
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.

nigel hewitt
13-01-2014, 07:01 PM
It worked.
My Russian O2 Rebreather is rated to 20 meters.

PeterL
13-01-2014, 07:04 PM
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...... :)

Hot Totty
13-01-2014, 07:04 PM
My Russian O2 Rebreather is rated to 20 meters.
russian's are expendable ;)

PBrown
14-01-2014, 12:44 PM
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/medical/articles/OXTOX_If_You_Dive_Nitrox_You_Should_Know_About_OXT OX

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

GLOC
14-01-2014, 04:15 PM
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

PeterL
14-01-2014, 05:15 PM
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.

Dan Sumners
15-01-2014, 11:37 AM
Not quite

Further work has been done, a nice summary is in a DAN article here: http://www.diversalertnetwork.org/medical/articles/OXTOX_If_You_Dive_Nitrox_You_Should_Know_About_OXT OX

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!