Nitrogen/saturation diving

[audission]

There's something I don't really understand about nitrogen narcosis.

Presumably being narked will depend on the amount of nitrogen being absorbed into the body - so the deeper you go and the longer you stay there, the more narked you'll become.

How then do saturation divers avoid being completely narked and "out of it"? Since the principle seems to be "remaining at depth until the body can't absorb any more nitrogen". The answer is probably really simple but I'm obviously missing something.

Thanks :huh:

 

[DA Aquamaster]

Nitrogen narcosis has much more to do with depth and the partial pressure of the nitrogen than it does with the time you spend at depth or the degree of saturation of your tissues with nitrogen. If time or saturation were the causative factors, a recreational diver could get narced at fairly shallow depths.

Deep saturation divers replace some or all of the nitrogen (depending on the depth) with helium to avoid the problem with nitrogen narcosis.

Technical divers do the same thing on deeper dives (anything deeper than 80-150 ft depending on the diver's personal preference) by using trimix - consisting of a mixture of nitrogen, helium and oxygen. The specific mix again depends on depth with the general idea being to limit the amount of nitrogen at depth to an equivalent of what they would have if diving air to a depth of around 80-100 ft.

At depths below about 200 ft, the amount of oxygen is also reduced to avoid problems with oxygen toxicity, and for mixes designed for depths below about 300 ft, the amount of oxygen in the mix is low enough that it will not support an active diver at or near the surface so a travel gas has to be used to get you deep enough to have a high enough partial pressure to use the bottom mix.

Separate decompression mixes are also used with higher percentages of oxygen (32%, 50%, 80% and 100% are common) to maximize the partial pressure of the oxygen at around 1.6 and increase the speed at which the nitorgena nd helium ar offgassed.

With recreational technical diving, the decompression is done entirely in water, while with surface supplied commerical diving and saturation diving some or all of the decompression is done in a decompression chamber where and with either type of surface supported diving the mix can be continuously changed to optimize decompression while wit technical diving the diver is usually limited to 1 or 2 mixes by the need to carry all of their own decompression gas.

 

[Sideband]

There's something I don't really understand about nitrogen narcosis.

Presumably being narked will depend on the amount of nitrogen being absorbed into the body - so the deeper you go and the longer you stay there, the more narked you'll become.

How then do saturation divers avoid being completely narked and "out of it"? Since the principle seems to be "remaining at depth until the body can't absorb any more nitrogen". The answer is probably really simple but I'm obviously missing something.

Thanks :huh:

I disagree that time is an element. The bad guy is partial pressures. If time were a factor then reducing depth would only be a temporary relief of narcosis and since we are still breathing Nitrogen on the surface it should eventually catch up to us here.
Time could come to play in something like a fast decent. I have heard that narcosis can be more intense with a fast decent than a slow one that allows your body to adjust to the higher partial pressures.

Joe

 

[DA Aquamaster]

Or...look at it another way. If you start to feel narc'd on a dive, an ascent of 10 or 20 ft will usually resolve the feeling. That would not happen if it were an issue of time or tissue saturation as the time element would still be increasing and tissue saturation does not decrease that rapidly.

 

[H2Andy]

How then do saturation divers avoid being completely narked and "out of it"? Since the principle seems to be "remaining at depth until the body can't absorb any more nitrogen".

as has been stated, you have one gas (nitrogen) and two different issues:
saturation and narcosis.

saturation is a result of depth and time: the deeper and longer you go, the
more inert gas your body absorbs (nitrogen is an inert gas).
the more inert gas your body absorbs, the slower you have to come up to avoid freeing all that inert gas from solution and having run amock in your blood (and getting you bent or killed).


narcosis is largely the result of the partial pressure of nitrogen (and other
gases as well, but nitrogen is the most abundant narcotic gas in trimix and air).
if you reduce the partial pressure of nitrogen (by adding, for example, helium into the mix), you can largely avoid nitrogen narcosis.

any deep diving (say, past 150 feet) should be done with trimix (oxygen,
helium, and nitrogen), which will reduce the partial pressure of nitrogen to the
level that it won't be narcotic.

however, there will be enough inert gases left (nitrogen and helium in this case)
that if you stay down deep enough and long enough, you will get "saturated"
just as if you were diving with regular air.

 

[audission]

Thanks for the replies. So just to clarify, the "partial pressure" is another way of saying the "ratio" in this instance? Say I were diving on compressed air though the ratio is always going to be 21%/79% though isn't it, even though as I descend my body is absorbing more and more nitrogen (and expelling CO2)?

Suppose I feel narced, level out and remain static at that depth, will the sensation then pass once the ratio of nitrogen in my body increases past a "narcotic" level?

Also is there any rule of thumb for narcosis? Everything I've been reading up suggests fat, dehydrated etc people are more likely to suffer a DCI hit - so is the opposite true of being narced? Is the narcotic effect similar to alcohol, i.e. I'd expect a short, slightly built person to be paralytic long before a large stocky one was?

Sorry if these are dumb questions.

 

[H2Andy]

partial pressure means the ratio of the gas (i.e. mix percentange) times the pressure

for example, at the surface, air has an O2 partial pressure of .21 ( .21 x 1 ATM)

at 33 feet, it will have a partial pressure of .42 ( .21 x 2 ATM) but still it will be 21% O2

at 66 feet, it will have a partial pressure of .63 ( .21 x 3 ATM) (but still will be 21% O2

likewise for Nitrogen: .79 at 1 ATM, 1.58 at 2 ATM, 2.37 at 3 ATM, and so on

Suppose I feel narced, level out and remain static at that depth, will the sensation then pass once the ratio of nitrogen in my body increases past a "narcotic" level?

hmmm... i'm not sure what you are asking, but the answer is no. if you feel
narced, you can ascend and relieve the symptons, but you can't eliminate them
by staying at the same depth. now, you could adjust to the symptons, and
be better able to deal with them, but they'll still be there.

Also is there any rule of thumb for narcosis? Everything I've been reading up suggests fat, dehydrated etc people are more likely to suffer a DCI hit - so is the opposite true of being narced? Is the narcotic effect similar to alcohol, i.e. I'd expect a short, slightly built person to be paralytic long before a large stocky one was?

the best i can describe narcosis is that it makes you "stupid." it slows down
your thinking and your reactions, and it makes you clumsy and "out of it."

factors that increase the effects of narcosis are cold, rapid descent, anxiety, alcohol, sedatives, fatigue, and carbon dioxide excess.

i don't recall reading anywhere that body weight has an effect, though i
guess an overweight diver may have to work harder and get more
CO2 buildup, which could increase the narcosis.

here's a nice link on narcosis:

http://www.emedicinehealth.com/wilderness_nitrogen_narcosis/article_em.htm

 

[mkennedy]

i don't recall reading anywhere that body weight has an effect, though i
guess an overweight diver may have to work harder and get more
CO2 buildup, which could increase the narcosis.

It takes longer for Nitrogen to absorb into muscle then fat. The leaner your are the longer it will take you to be effected.

 

[H2Andy]

dang ... rethinking stuff

well... i don't think narcosis has to do with tissue absorbtion of nitrogen,
rather how it affects the nervous system

i think

i could be wrong, but i haven't seen body weight listed as a factor
for narcosis anywhere. would love to know, though

 

[breals]

It takes longer for Nitrogen to absorb into muscle then fat. The leaner your are the longer it will take you to be effected.

Your talking about nitrogen absorption for the sake of decompression obligations, muscle vs fat most likely has nothing to do with narcosis. Your going to feel drunk if your in your target BMI or if your 50 pounds over.

The best description of narcosis is like feeling drunk, I can always tell I'm getting it, when I think stuff is funny like a fish swimming and then it's time to go up. That's why below 100 I am usually on mix.