narcosis question.

wedivebc

That all depends on where you stand on this issue I have gone from a N2 rich loop to an O2 rich loop and experienced a pronounced change in perception and narcosis.

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tibby

Are you saying 215fsw on a .7 SP with air dil? That would make your END about 260fsw which is pretty bad. It would take a very fast descent to not notice this until too narced to care.
I'm almost afraid to ask but, is this what happened on the "O"? You don't have to answer this if I'm too pushy.
Thanks
Tibby

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Does the voice of reason change when you add helium to the mix?...hmm

Abbo

Let's start by considering air as if it were nitrox 20.9, with nothing else in there, and let's pretend that nitrogen is the only narcotic component. We'll also assume the idea gas law. That keeps things simple. As long as the FO2 in the loop is higher than the FO2 of air, 20.9%, then the PN2 is lower, and the level of narcosis would be lower. The absolute pressure where the narcosis level is the same for both OC and CC is
P=PO2/FO2=Set point/0.21.
Depth = {(SP/0.21)-1}*10
The narcosis level is less on CC than for OC for any depth up to 57m if the SP is 1.4, 52m for SP=1.3, 47m for 1.2 and so on. At 40m, the END is 40m with nitrox 21 but 35.5m with an SP of 1.4.

Before considering other effects, let's remind ourselves what narcosis is. Our understanding of narcosis comes from anaesthesia studies. Think of a nerve cell as a junction box filled with oil: the more that gets disolved in the oil, the less the junction box works. A couple of researchers, Meyer & Overton, noticed that the anaesthetic effect of a substance is proportional to its solubility on oil. If I remember right, they used olive oil.
Minimum alveolar concentration - Wikipedia, the free encyclopedia

CO2 is 13 times more soluble in oil than nitrogen, so it should be 13x as narcotic. The measured effect, however, is around 170x as large as nitrogen*. On average, the CO2 content of air is 385ppm. By definition, you have an END of 40m breathing Nitrox 20.9 at 40m, but you have an END of 44.1m taking into account the narcotic effect of CO2. With a rebreather on a set point of 1.4, with 385ppm of CO2, your effective END is 39.6m. Your CO2 would have to rise to 420ppm to get the same effect as with Nitrox 20.9, and 800ppm before you had the same effect as with air. Whether the FCO2 gets that high is a function of your fitness and the WOB of the rebreather. Whereas a good reg has a WOB of around 0.7Joules/litre at that depth, most rebreathers are around 2J/l. The effect of the CO2 can quickly dwarf the effect of the nitrogen: if the FCO2 hit 1%, your effective END would be 142m!!

The learning point is that though on the face of it your narcosis levels ought to be lower with a rebreather, that is only the case if you keep your FCO2 low. Use trimix shallower than with OC to reduce WOB, be fit, don't over-exert yourself and change the kitty litter.

* Source: Effects of CO2 and N2 partial pressures on cognitive and psychomotor performance, 1991, Fothergill, DM; Hedges, D; Morrison, JB - if I remember correctly

PhilSiswick

Ah, good point, thanks. Any idea what a sensible reading in the loop under normal conditions is?

Cheers,

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Mdemon

If we're talking about a working rebreather used in normal, un-stressed conditions then there should be negligible CO2 in the loop, right?

And you're breathing like you would on the surface because you have endless gas, right? And when you're on OC, we all tend to slow the breathing down, if not actually skip-breathe?

So the retained CO2 in the OC diver may be slightly higher?

Additionally, swapping some of the nitrogen for Oxygen is going to reduce the dose of Nitrogen mentioned above in the olive oil scenario. So my feeling is that narcosis should be less on CCR. I can't remember there being a difference myself.

That said, several of my friends have found that narcosis hits them shallower on the Box. I wonder if this is because using a Rebreather requires more concentration so the effects are more noticeable?

On the narcotic O2 question; I'm sure the scientists would say otherwise but I don't find it narcotic compared with nitrogen. My reason for saying this is that during air breaks in the Pot, switching to air made me feel noticeably more narced and this reversed equally quickly when switched back to O2. Whilst O2 may or may not have a narcotic effect, it seemed to me to be insignificant compared with the Nitrogen.

That's my experience. You may react differently!

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divingtoors

Hi

My recent experience is very relative to this subject, if I may explain.

I did a dive to 31m (102 Ft) about two weeks ago, and I am very sure that I was deffinately narked, to such an extent that I actually muddled up my PPO2 on arrival on the bottom. Luckily I had the sanity to get off the loop and go to the bailout until I could get the PPO2 spike sorted out.

Dive conditions were as follows:
Low PPO2 @ start of the dive: 0.4 Bar +/-
Viz: surface to 20m +/-3-5 m
Bottom: clear but dark and gloomy due to dirty topwater, and we needed torches to see detail
Temp: 10' Celsius
PPO2 on botom part of dive +/- 1.0 Bar

The whole dive I felt that familiar feeling of being disconected to the dive, with little memory recall and task fixation, (which in my case was monitoring the PPO2, I dive a KISS converted IDA-71)
I really had to concentrate on my PPO2 displays and VR3 to make any sense of what they were telling me !! Not good....

My comment to my OC buddy after the dive was that I was so busy "surviving" I didn't have time to dive..

Thinking back on the dive, I was quite concerned about my apparent narcosis, although I had less PPN2 than on OC. So I dismissed it (very stupidly) as one of those things..Maybe I just had a bad day diving.

Therefore I am very glad for this thread, as it is deffinately giving me insight into what happened on that dive.
I was taught that many outside factors also play a significant role in Narcosis such as rate of descent, visibility, temperature, etc.
Maybe the ambient conditions coupled with some CO2 retention was all the ingredients that was needed to bake the "narcosis cake"

I have done many OC dives in the same conditions and the levels of narcosis were deffinately less than on Rebreather.

Maybe it is time that I also start using mix maybe a 20/20 trimix or similar to counter the effects of narcosis. It will also reduce WOB and maybe CO2 retention??

Deffinately something to try this weekend

Thanx for a very enlightening thread

Regards

Johan

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PhilSiswick

Based on personal experience - definitely try it. I tend to run 21/35 or 21/20 (the result of air topping a part-used 21/35 :-). I've always known that I had sub-clinical narcosis (i.e I was narked, but it didn't directly affect my safety) on most sub 35m dives on OC, but not how much it was affecting my enjoyment and memory of the dive itself.

I was up in Scapa a couple of weeks ago. I haven't yet got over how much more I remember of dives that I have done several times in the past. The viz and the light seemed much better than normal too.

Cheers,

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Phil (WSKD 0001)

I have always felt that the dive I am on is not nearly important as the dives I plan to be on the rest of my life. Tom Rose, 2007

The person who gets the farthest is generally the one who is willing to do and dare. The sure thing boat never gets far from shore. Charles A. Lindbergh

www.hugsac.org.uk

SBOD

thank you all for the replies. i have learnt quite a lot from them which have made me question (in hindsight and retrospectively), my own narcotic tolerance between 30-40M on my re-breather and why.
i am not qualified to use helium in my dil yet but hopefully this will be addressed in the near future.

Mdemon

Ditto. I stopped using air immediately I finished MOD1 and went straight to trimix. I only used air again a year or so after that, when I had a better idea how the thing worked (and trimix was too expensive for Red Sea reefs!).

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"small minds talk about people, Average Minds Talk About Events, GREAT MINDS TALK ABOUT IDEAS!"

The WRONG Attitude will get you killed.

"Once the agenda-monkeys and perfect-worlders have moved on, perhaps we can do some diving?"

Mark Chase

As above.

I quite often have a equivilent narcotic depth of sitting on or just above the dive boat


Tuesdays dive was 46m on 16/55 END about 10m

Gas cost about £1.50




ATB

Mark

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