Nitrox/Time to Fly

[gcbryan]

Dr. Deco,

Is the Rate at which you offgass nitrogen affected by depth? In other words is more nitrogen being offgassed in 15 minutes at 30 fsw than in the first 15 minutes once you reach the surface or is it the same?

 

[DivesWithTurtles]

Dr. Deco,

Is the Rate at which you offgass nitrogen affected by depth? In other words is more nitrogen being offgassed in 15 minutes at 30 fsw than in the first 15 minutes once you reach the surface or is it the same?

I'll take a crack at this question. I'm no expert (to say the least!), and I may be way off-base. No one should hesitate to correct me!

The rates at which various body tissues off-gas is not really known. They are hypothesized by various modelers.

Many models assume off-gassing rates are the same as on-gassing rates, i.e., halftimes. Some other models assume slower off-gassing rates of various theoretical tissues. Pressure gradients (difference between ambient pressure and the pressure of gases in solution within tissues [gas tension]) are not assumed to affect these rates. So, in other words, the rates of off-gassing are assumed to be the same at various depths.

When the gas tension within a tissue is the same as ambient pressure the tissue is saturated. Off-gassing occurs when the tissue is supersaturated, i.e., the gas tension in the tissue is higher than the ambient pressure. When a tissue is supersaturated gas will either migrate to other tissues in which the level of saturation is lower and so stay in solution, or come out of solution and become free-phase gas (bubbles). Gas migrating to the bloodstream and then out through the lungs is off-gassing properly.

The body can tolerate a certain number and size of bubbles, and therefor certain levels of supersaturation of various tissues. By controlling the degree of supersaturation we can control the number and size of bubbles. These are described by Haldane's ratios, Yount's and Wienke's critical radii, etc. And, the degree of supersaturation a tissue can tolerate is thought to vary by depth for a given tissue (Workman's and Buhlmann's M-values).

The objective of decompression strategies are to control the supersaturation of the tissues with the highest tensions (thereby limiting number and size of bubbles) while at the same time minimizing the continued on-gassing of tissues whose tensions are still below saturation.

So, back to the premise of your question, although the rate that gas comes out of solution of a given tissue may be the same at the surface as at 30 feet, the degree of supersaturation must be taken into consideration.

 

[gcbryan]

Yes, I understand that the purpose of deco is to keep the smaller bubbles from becoming larger bubbles.

Some people seem to be under the impression that extented time at shallow depths "cleans up your body" regarding eliminating nitrogen faster than your surface interval would. I understand what it does for reducing bubble formation due to decreasing pressure.

According to what you've said however it shouldn't make one feel better as in wow, I've eliminated a lot of nitrogen during those last 15 minutes. One would be eliminating no more nitrogen than one would eliminate during the first 15 minutes of a surface interval.

 

[DandyDon]

So, back to the premise of your question, although the rate that gas comes out of solution of a given tissue may be the same at the surface as at 30 feet...

One would be eliminating no more nitrogen than one would eliminate during the first 15 minutes of a surface interval.

I'm not following this...??

One will off gas faster on a surface interval than at 20 or 30 feet. The reason for the just-in-case deco-stop/safety-stop is to off gas for a few minutes at a rate faster than the half-depth-stop rate, but slower than on the surface.

You would off gass even faster on a jet liner an hour later, which is why we don't - too fast.

 

[Peo]

Dr. Deco,

Is the Rate at which you offgass nitrogen affected by depth? In other words is more nitrogen being offgassed in 15 minutes at 30 fsw than in the first 15 minutes once you reach the surface or is it the same?

It depends on what you are breathing. The effect called "Oxygen Window" or "Inherent Unsaturation" helps you make off-gassing faster under hyperbaric conditions, under the 'right' circumstances. Breathing pure O2 at 20' can actually be better than breathing pure O2 on the surface. See for example The effect of hyperbaric oxygen on the oxygen window for more detail.s

 

[DivesWithTurtles]

Some people seem to be under the impression that extented time at shallow depths "cleans up your body" regarding eliminating nitrogen faster than your surface interval would. I understand what it does for reducing bubble formation due to decreasing pressure.

According to what you've said however it shouldn't make one feel better as in wow, I've eliminated a lot of nitrogen during those last 15 minutes. One would be eliminating no more nitrogen than one would eliminate during the first 15 minutes of a surface interval.

Holiday Inn Express rule applies. I am a student of this stuff, and a novice student at that; I am no authority.

It may "clean up you body" better and it may "make one feel better as in wow..." for two reasons. One, fewer and smaller bubbles means less insult to your body. (There are still a severe lack of understanding what bubbles do to the body both physically and chemically.) And, two, free-phase gas in bubbles beyond a certain number and size is not eliminated as efficiently as gas remaining in solution. You do eliminate gas faster if you reduce bubbling (via a lower pressure gradient), regardless of ambient pressure.

(And Dr. Powell writes frequently about other important factors in reducing bubbles, other than controlling gradients.)

One will off gas faster on a surface interval than at 20 or 30 feet. The reason for the just-in-case deco-stop/safety-stop is to off gas for a few minutes at a rate faster than the half-depth-stop rate, but slower than on the surface.

I think you're wrong. The reason for both the deep stop and the safety stop (as well as not going to altitude) is to reduce bubbling. Gas is always assumed to move out of a supersaturated tissue at a certain (exponentially varying) rate (think half-times), again without regard to ambient pressure. I believe this is true for all of the popular models.

 

[gcbryan]

Peo, I was referring to air. There are those that claim to feel better by staying shallow for a while before getting out. I understand that it is more conservative to stay shallow for a while (which I do) to allow for more tiny bubbles to come out of solution before heading for the surface. But to "feel better" you would think that would have to involve more nitrogen coming out than would come out during the same period of time on the surface.

I was just trying to logically determine how or why this could be so.

 

[DandyDon]

I think you're wrong. The reason for both the deep stop and the safety stop (as well as not going to altitude) is to reduce bubbling. Gas is always assumed to move out of a supersaturated tissue at a certain (logarithmically varying) rate (think half-times), again without regard to ambient pressure. I believe this is true for all of the popular models.

Perhaps I am wrong. Got the Dec Class resceduled now that my Inst is back in good health - much to learn.

True, the foremost reason to stop is reduce bubbling, but I'd still think you'd offgas faster at less ambient pressure.

 

[gcbryan]

Holiday Inn Express rule applies. I am a student of this stuff, and a novice student at that; I am no authority.

It may "clean up you body" better and it may "make one feel better as in wow..." for two reasons. One, fewer and smaller bubbles means less insult to your body. (There are still a severe lack of understanding what bubbles do to the body both physically and chemically.) And, two, free-phase gas in bubbles beyond a certain number and size is not eliminated as efficiently as gas remaining in solution. You do eliminate gas faster if you reduce bubbling (via a lower pressure gradient), regardless of ambient pressure.

Good points. Thanks.

 

[PaulChristenson]

Well you really gotta discount such based on the source. You can tell from my profile that I am not an expert on anything.
/QUOTE]

Except HATS!!!


Paul in VT