Oxygen solubility and half times

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In the prone position, the gas in the lungs(chest mounted) is at a slighty higher pressure.. Most divers agree its easier to exhale into higher pressure than inhale from a lower pressure to a higher pressure. On chest mounted lungs you have gas below you, and some above (the canister,hoses) this seems to be a good balance, the positioning of the "t"s which the hose connects to is very important (normally the ts are at approximate lung centroid) as is the counter lung positioning.. if the lungs are not firmly attached to ones chest and you go in the surpine position the inhalation resistance becomes noticable trying to breathe against lungs just a few cms above your chest.


On back mounted counter lungs bag placement is critical. If I remember correctly the max recommended distance is 11cm from lung centroid.. I haven't dove a back mounteed CCR (I'll own one shortly) but my guess is that in the prone position the buoyancy change would be even more noticable..

A book I would recommend is
Life Support Systems Design
Diving and Hyperbaric applications
By M.L. nuckols/ W.c. tucker and A.j. sarich
I believe its available from best publishing.

This is a great text for those with a mathematical and scientific background, for other most of the material will probably be uncomprehendable.

In the prone position(front mounted) the buoyancy change is less noticable than when head up since the gas will now be at the highest points (a reasonable amount above the lung centroid)possible therfore the largest volume displacement.

I'm not sure how to figure out what the buoyancy change in reality is but Its probably about .5kg or less (from perception.. I could be totally wrong) Its definately more pronounced in shallower water where our bodies are less compressed, but somewone who is really paying attention and is truely "neutral" you can see the effect.


Rebreathers without an auto diluent addition valve don't vent large amounts of gas when the hose is above the head or make you look like a chipmunk when in less than optimum a position (with an auto diluent valve it will act like an old 2 hose reg since it will constant add gas as long as the pressure at the ADV is less than ambient)
 
I really appreciate the information you have given. I certainly understand it a lot better now. You certainly seem to know your subject!

I'll see if I can borrow the book from my library. I do not think it would be worthwhile for me to buy it.

As for buoyancy change. Its all to do with Boyles Law.

V2 = P1V1/P2

If P1 is the pressure at the centre of buoyancy of the lungs and P2 the pressure in the counterlung at say 30 cms shallower.

AT 3 metres P1 = 1.3 bar, P2 = 1.27 bar

Thus if tidal volume, V1, is 1 litre V2 = 1 x 1.3/1.27 = 1.024 litres

If my maths is right he difference is only 24 mls or 24 grams!

If you are wondering why I stated asking these questions it is because I am considering getting a rebreather sometime soon and was a little concerned about some of the stories I was hearing about them.

Perhaps I should come over to NY and do my training there?

It is certainly a fact that the more air you carry around you the more difficult it is to retain neutral bouyancy. When I wear my thick (200g/M) thinsulate undersuit I feel much less comfortable than when wearing a thinner undersuit or semi dry. It's all to do with the positive feedback in bouyancy with small changes in depth.

Kind regards, :)
 
Originally posted by Dr Paul Thomas
I really appreciate the information you have given. I certainly understand it a lot better now. You certainly seem to know your subject!

I'll see if I can borrow the book from my library. I do not think it would be worthwhile for me to buy it.

As for buoyancy change. Its all to do with Boyles Law.

V2 = P1V1/P2

If P1 is the pressure at the centre of buoyancy of the lungs and P2 the pressure in the counterlung at say 30 cms shallower.

AT 3 metres P1 = 1.3 bar, P2 = 1.27 bar

Thus if tidal volume, V1, is 1 litre V2 = 1 x 1.3/1.27 = 1.024 litres

If my maths is right he difference is only 24 mls or 24 grams!

If you are wondering why I stated asking these questions it is because I am considering getting a rebreather sometime soon and was a little concerned about some of the stories I was hearing about them.

Perhaps I should come over to NY and do my training there?

It is certainly a fact that the more air you carry around you the more difficult it is to retain neutral bouyancy. When I wear my thick (200g/M) thinsulate undersuit I feel much less comfortable than when wearing a thinner undersuit or semi dry. It's all to do with the positive feedback in bouyancy with small changes in depth.

Kind regards, :)

I trying to think of a physical way to verify/measure it.. since neither your body or the bags are totally rigid and we really don't know how much each changes .. Body positioning and counterlung placement will all be factors.. The change is small but if you really pay attention you can sense it... I don't think I could sense 24 grams though.. I'll have to ponder a bit more...

Training makes all the difference when it comes to RBs. When I get into something I always try abd learn as much as I can about a subject... I'm not the most experienced RB diver around but once I chose to dive a RB I put in alot of hours and practiced skills.. With all the Inspirations on your side of the pond there has to be some good trainers around.. I would seriously question any instructor before you chose.. I have friends I can ask about recommendations for your part of the ocean.. Really find out what he/she knows about the subject other than just the basics he needs to know.. I have dove the unit in both warm and cold water.. you perfect differnet skills in each.. It's much easier getting the basics down in warm water, but you wount learn anything about trying to do a free ascent with a drysuit.. remember now you have THREE airspaces to control rather than two. I remember my first free ascent in my druysuit (I learned it in cold water) it wasn't pretty.. I ended up on the surface against my wishes..




I guess I was lucky in that my trainer was the Founder of ANDI.
 
Two kinds of oxtox are of concern -- full
body and CNS. Halftimes for CNS oxtox are not employed in the tec diving community, but for full body oxtox, 70 - 90 minutes is commonly used.
Fill body oxtox is measued in OTUs and is limited
by 450 - 750 OTUs depending on diver discretion.
A CNS clock, that takes ratios of actual time
divided by oxtox limit for a given ppO2 (in
atm), is limited to be less than one (summing
up all ppO2 levels/ratios during a dive). On OC,
this is work and requires planning, for both
OTUs and CNS clock. On a RB, working mostly
at fixed ppO2, this is easy. So get an RB and
use it with a trimix diluent (RB = rebreather).
This helps your deco too -- maximizes O2 and minimizes N2. But you will need training

How to calculate CNS oxtox and full body oxtox
is given in TDID -- pages 89 - 94. Equations and
tables are listed. Limits for CNS and full body
oxtox depend on ppO2, obviously, But BELOW
0.50 ATM (PPO2), NEITHER ARE A CONCERN.

Bruce Wienke
Counterterror Dive Team Ldr
 
Thanks Bruce,

I do appreciate how to calculate the accepted exposure limits for both types of oxygen toxicity but what I cannot understand is why CNS toxicity is so variable . Indeed as it is a one-off, all or nothing phenomenon it suggests that a trigger mechanism is at play rather than an accumulated exposure as with whole body toxicity.

Lost Yooper started a thread on Oxygen Narcosis to which I contibuted a speculative article on the biochemical causes of CNS oxtox at http://www.scubaboard.com/showthread.php?threadid=4425&highlight=oxygen+narcosis

Have you any ideas?

For instance do you think CO2 could be a major player?

kind regards,
 
is there a lifetime limit on the amount of Nitrox exposure one can have? I know there are limits to how much hyperbaric oxygen therapy a patient can have for treating non-healing wounds, etc.

Is there a point at which someone can get pulmonary (i.e. whole body) O2 toxicity from repeatedly diving Nitrox -- eg a divemaster using Nitrox every day for every dive -- or a tech diver who is exposed to hyperbaric O2 just breathing air or even trimix at greater depths?

ET
 
Dear Diving Doc:

I am not aware of any long-term oxygen limits. This would apply also to nitrox.

From research, as I understand it, the body is capable of recovering from many assaults (assuming that one does not die). Recovery from oxygen toxicity is common and the effects are reversible. [Serious effects such as the formation of fibrotic tissue in the lungs during oxygen therapy is a different matter.]

Dr Deco

:doctor:
 
The basis for my thinking there was some lifetime exposure limit was that I have a friend whose financee has had horrendous disfiguring mouth and jaw surgery due to squamous cell CA. 2/3 of her jaw and part of her tongue was removed. After a bone graft implant, she developed a wound infection requiring removal of the graft. She had had previous radiation therapy, which was thought to contribute to the poor wound healing and infection.

When I suggested they might consider HOT, my friend told me that she had had some previously and was told she had reached the limit and couldn't have any more.

I accepted this, since at the time, she was being treated by Dr. Marx in Miami, an oral maxillofacial surgeon who has written many of the protocols for HOT.

Anyway -- are you aware of any possibility of pulmonary oxygen toxicity that could be incurred by diving Nitrox 2-3 dives per day for a week, staying within suggested limits on computer?

Elaine
 
Dear DivingDoc:

I must admit that I am at a loss to explain why Dr Marx eliminated retreatment. I am only aware of the possibility that fibrotic tissue could be forming in the lungs. This would be indicated by pulmonary function tests.

I do not have any figures or info on Nitrox diving, but I have not heard of any problems with that mix for the time limits mentioned. [Pure oxygen for decompression is entirely another matter.]

Dr Deco
:doctor:
 
https://www.shearwater.com/products/perdix-ai/

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