Y.A.A.T. (Yet Another Ascent Thread)
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serambin
Guest
My understanding of off gassing is limited. Having said that please correct me on the following:
1. Slower compartments off gas at a slower rate. Say a compartment rated at a half life of 120 min. that is 20% saturated, will off gas at a much slower rate than a compartment rated at a half life of 6 min. and that is carrying 90% saturation. (By the way, given a rate of descent equal to the rate of ascent, are theoretical desaturation rates the same as saturation rates for a given compartment?)
2. The controlling compartment is the one most saturated with the highest 'potential' for rapid desaturation i.e. bubble formation.
3. The only way for absorbed nitrogen to exit the body is through osmotic pressure from higher saturated tissues into lower saturation areas.
4. All tissue compartments begin to absorb nitrogen at a rate faster than the body can expel it at or above 30'.
Therefore:
1. As you go shallower, all compartments are degassing to the extent allowed by the decrease in tension between that compartment and the transferring tissue around it.
1. That rate of transfer is determined by:
a. The amount of that tissue’s saturation.
b. The amount of the decrease in pressure.
c. The saturation of the tissues acting as a conduit for gas removal.
If the above is correct, then a steady ascent, say from 60 to 30 feet in 2 min., would not have the same effect as a one minute stop at 60 feet followed by a 1 min ascent to 30 feet, or even a 1 min ascent to 30 feet followed by a 1 min stop at 30 feet. The rate of desaturation would be different in each case.
How do you go from a theoretical tissue model to the actual tissues involved? For example, in on gassing and off gassing I would think that first blood and then tissues most involved in gas transfer (say, areas around capillaries) would be the first affected areas, but I have never read a discussion of these issues correlating to the ‘theoretical models’.
In addition, one argument I’ve hear for using an integrated air system computer is that respiratory rate (SAC Rate) is a factor for time allowed at a given depth. That is to say, if a person diving to a depth of say 80 feet has a bottom time of 18:00 min he will absorb the same nitrogen than a lower SAC rate diver whose tank lasts 22:00 min. Doesn’t time at pressure have more to do with nitrogen saturation than the cubic feet of air passed through the lungs?
Stan
1. Slower compartments off gas at a slower rate. Say a compartment rated at a half life of 120 min. that is 20% saturated, will off gas at a much slower rate than a compartment rated at a half life of 6 min. and that is carrying 90% saturation. (By the way, given a rate of descent equal to the rate of ascent, are theoretical desaturation rates the same as saturation rates for a given compartment?)
2. The controlling compartment is the one most saturated with the highest 'potential' for rapid desaturation i.e. bubble formation.
3. The only way for absorbed nitrogen to exit the body is through osmotic pressure from higher saturated tissues into lower saturation areas.
4. All tissue compartments begin to absorb nitrogen at a rate faster than the body can expel it at or above 30'.
Therefore:
1. As you go shallower, all compartments are degassing to the extent allowed by the decrease in tension between that compartment and the transferring tissue around it.
1. That rate of transfer is determined by:
a. The amount of that tissue’s saturation.
b. The amount of the decrease in pressure.
c. The saturation of the tissues acting as a conduit for gas removal.
If the above is correct, then a steady ascent, say from 60 to 30 feet in 2 min., would not have the same effect as a one minute stop at 60 feet followed by a 1 min ascent to 30 feet, or even a 1 min ascent to 30 feet followed by a 1 min stop at 30 feet. The rate of desaturation would be different in each case.
How do you go from a theoretical tissue model to the actual tissues involved? For example, in on gassing and off gassing I would think that first blood and then tissues most involved in gas transfer (say, areas around capillaries) would be the first affected areas, but I have never read a discussion of these issues correlating to the ‘theoretical models’.
In addition, one argument I’ve hear for using an integrated air system computer is that respiratory rate (SAC Rate) is a factor for time allowed at a given depth. That is to say, if a person diving to a depth of say 80 feet has a bottom time of 18:00 min he will absorb the same nitrogen than a lower SAC rate diver whose tank lasts 22:00 min. Doesn’t time at pressure have more to do with nitrogen saturation than the cubic feet of air passed through the lungs?
Stan
OP
DivesWithTurtles
Contributor
There's a lot of stuff here to cover, and I'm not going to come close to covering it all. I'll shotgun some answers in no particular order.
(My usual disclaimer here: My understanding is WAY limited. I may be wrong. Correct me if I am.)
Enough for now. Time to go absorb some inert gas (yeah!). Corrections to my statements requested, please.
(My usual disclaimer here: My understanding is WAY limited. I may be wrong. Correct me if I am.)
Yes. It has everything to do with time and pressure. It has nothing (or virtually nothing) to do with respiratory rate.serambin:
You don't. Compartments in dissolved gas models are used to mathematically simulate (or model) what has happened, but they should not be confused with what is really happening in the body. We don't really know much about the mechanics of what is really going on in the body. Maybe studies such as what was recently mentioned by Thalassamania regarding radioactively tagged gas will eventually tell us more about those mechanics, but such studies don't exist in the public domain yet. Don't try to associate a discreet type of body tissue with specific theoretical compartments of a model. Just accept the model for what it is.serambin:
At a given, unchanging, ambient pressure (neither ascending nor descending), any compartment that is not saturated on-gasses. Everything tries to reach a state of equilibrium with ambient partial pressure of inspired inert gas. So both of the compartments you mention will be on-gassing at a steady ambient pressure. Only when a compartment is super-saturated will it off-gas, and that can only happen when ambient pressure is reduced.serambin:
Most traditional models assume equal rates of on-gassing and off-gassing. Some models assume a slower rate of off-gassing than on-gassing. It's back to 'we don't really know what is happening in the body'. Remember, this is all about modeling the results, not describing what is really going on.serambin:
No. The controlling compartment is the one that will reach an unacceptable level of super-saturation first as ambient pressure is reduced. In Neo-Haldanean models, acceptable amounts of super-saturation vary from compartment to compartment. Faster compartments are assumed to be able to tolerate a higher degree of super-saturation.serambin:
Yes (almost). The tissue must be in a state of super-saturation to off-gas. But, most (but not all) models are based on the assumption that absorbed nitrogen goes directly from the compartment to the outside. Don't pass go, don't collect $200, and don't go into another compartment.serambin:
No. All tissues begin to absorb nitrogen when they have a lower partial pressure than the inspired gas. Nothing goes out until the partial pressure in the tissue is above the inspired partial pressure. And there are no mechanisms to push gas out other than the pressure differentials. Tissues don't reject or eject inert gas. See your statement #3.serambin:
No. Only compartments that have a higher pp of absorbed inert gas than the inspired pp of that gas off-gas. Other compartments will continue to on-gas.serambin:
Enough for now. Time to go absorb some inert gas (yeah!). Corrections to my statements requested, please.
OP
DivesWithTurtles
Contributor
Just one more thing.
Remember the balancing act between off-gassing as quickly as possible, and not off-gassing so fast that we bubble more than the body can handle. The 60 to 30 in 2 min., and the 1 min. at 60 then 1 min. to 30 may be too slow. Or the 1 min. to 30 then 1 min. at 30 may be too fast. What is best? We don't know. We can only base our decisions on either research or seat-of-the-pants results. And researchers haven't even decided what to shoot for, e.g., what degree of bubbling is acceptable, let alone what profile produces what degree of bubbling. And there may be more to it than just the rates, as alluded to by research that DAN misinterpreted (per Charlie
).
So, what do you think is best? Why?
Right, the rates of off-gassing will all be different per the existing models. What were looking for is what is best.serambin:
Remember the balancing act between off-gassing as quickly as possible, and not off-gassing so fast that we bubble more than the body can handle. The 60 to 30 in 2 min., and the 1 min. at 60 then 1 min. to 30 may be too slow. Or the 1 min. to 30 then 1 min. at 30 may be too fast. What is best? We don't know. We can only base our decisions on either research or seat-of-the-pants results. And researchers haven't even decided what to shoot for, e.g., what degree of bubbling is acceptable, let alone what profile produces what degree of bubbling. And there may be more to it than just the rates, as alluded to by research that DAN misinterpreted (per Charlie
).So, what do you think is best? Why?
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