Years ago I was told that we offgass quicker while on a safety stop (under pressure) than sitting on the boat during a surface interval. If I remember the correct explanation, it was because of the higher partial pressure of the O2 content of the breathing gas while at depth. Am I correct, or all wet...so to speak?
Offgassing at depth VS. on the boat
- Thread starter ranger
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
ianr33
Contributor
Huh? Surely not!
You will offgas faster at the surface as the nitrogen pressure gradient will be larger. You would offgas even faster if you drove to the top of a high mountain.
Problem is that you might then be offgassing so quickly that bubbles would form and you would get bent. My understanding is that you want to offgas as fast as possible,but without significant bubble formation.
The increased pressure of O2 in air or 32% at a safety stop depth is not going to make any significant difference. (The wrong oxygen window as some might say!)
(The above ignores the fact that once bubbling occurs it will slow down offgassing )
You will offgas faster at the surface as the nitrogen pressure gradient will be larger. You would offgas even faster if you drove to the top of a high mountain.
Problem is that you might then be offgassing so quickly that bubbles would form and you would get bent. My understanding is that you want to offgas as fast as possible,but without significant bubble formation.
The increased pressure of O2 in air or 32% at a safety stop depth is not going to make any significant difference. (The wrong oxygen window as some might say!)
(The above ignores the fact that once bubbling occurs it will slow down offgassing )
Decompression is imperfectly understood. It tolerates variance - e.g. different divers use different gasses and methods to decompress, yet achieve similar success rates.
Universally, however, the rate of molecular diffusion across a semi-permiable membrane is determined by a number of different factors. Collectively the factors describe the "gradient", or the relative partial pressures of gasses (usually N2 is of primary interest) on both sides of the semi-permiable membrane.
When there is a significant variance - one side is high and the other low - then rates of transfer will tend to increase. When there is little variance - both sides are relatively equivalent - then rates of transfer will tend to slow.
Ideally, generally, the diver designs their ascent so as to maintain a steep gradient throughout their ascent, so as to enable diffusion of ongassed N2 out via the lungs most efficiently.
Some factors that determine PPN2 (on both sides of the semipermiable membrane) include rate of ascent, which influences the rate at which N2 comes out of solution in the blood; and the gas mix the diver is breathing, which influences the PPN2 within the lungs. For example, if the diver is breathing pure O2 there is a lower PPN2 in the lungs than if the diver is breathing a gas mix containing N2.
Because theoretically molecular diffusion occurs most efficiently at depths where the gradient is steepest, (for example, where the diver changes from one gas mix to another,) divers pause at these depths to allow N2 diffusion out of the blood to occur most efficiently at these points along their ascent. Diffusion continues as the diver continues to ascend, however, the gradient becomes less steep as the diver ascends - hence diffusion occurs more slowly. EAN50 is most efficient, for example, at 70 fsw. Relative variance between PPN2 levels is high. Diffusion still occurs at 40 fsw, but less efficiently than at 70 fsw. Relative PPN2 levels are less steep at 40 fsw.
To maintain high rates of gas transfer (alternately to maintain a steep gradient) the diver must ascend at a rate of ascent that maintains a high PPN2 on the blood side, and switch gasses so as to maintain a low PPN2 on the lungs side of the semi-permiable membrane (the alveoli).
Divers use software such as the popular V-Planner to determine the points along their ascent where offgassing begins and occurs with maximum efficiency.
There are other variables as well, however, I believe this to be an accurate response to the OPs question.
Respectfully,
Doc
Universally, however, the rate of molecular diffusion across a semi-permiable membrane is determined by a number of different factors. Collectively the factors describe the "gradient", or the relative partial pressures of gasses (usually N2 is of primary interest) on both sides of the semi-permiable membrane.
When there is a significant variance - one side is high and the other low - then rates of transfer will tend to increase. When there is little variance - both sides are relatively equivalent - then rates of transfer will tend to slow.
Ideally, generally, the diver designs their ascent so as to maintain a steep gradient throughout their ascent, so as to enable diffusion of ongassed N2 out via the lungs most efficiently.
Some factors that determine PPN2 (on both sides of the semipermiable membrane) include rate of ascent, which influences the rate at which N2 comes out of solution in the blood; and the gas mix the diver is breathing, which influences the PPN2 within the lungs. For example, if the diver is breathing pure O2 there is a lower PPN2 in the lungs than if the diver is breathing a gas mix containing N2.
Because theoretically molecular diffusion occurs most efficiently at depths where the gradient is steepest, (for example, where the diver changes from one gas mix to another,) divers pause at these depths to allow N2 diffusion out of the blood to occur most efficiently at these points along their ascent. Diffusion continues as the diver continues to ascend, however, the gradient becomes less steep as the diver ascends - hence diffusion occurs more slowly. EAN50 is most efficient, for example, at 70 fsw. Relative variance between PPN2 levels is high. Diffusion still occurs at 40 fsw, but less efficiently than at 70 fsw. Relative PPN2 levels are less steep at 40 fsw.
To maintain high rates of gas transfer (alternately to maintain a steep gradient) the diver must ascend at a rate of ascent that maintains a high PPN2 on the blood side, and switch gasses so as to maintain a low PPN2 on the lungs side of the semi-permiable membrane (the alveoli).
Divers use software such as the popular V-Planner to determine the points along their ascent where offgassing begins and occurs with maximum efficiency.
There are other variables as well, however, I believe this to be an accurate response to the OPs question.
Respectfully,
Doc
underwasser bolt
Contributor
Doc Intrepid is correct; for any curve there is a maximum gradiant, or rate of transfer in diving lingo. The process is NOT linear; if it were, the higher you are the faster you offgas; to assume you have maximum offgassing at the surface is to essentially assume offgassing is linear.
From bubble theory, look at mean bubble size. Bubbles may come off faster at the surface, but if they are larger, you are in deep trouble. So, at what depth can you off-gas bubbles of a size that are managable? That is the question. And that is perhaps another reason why off-gassing at depth is safer. Just giving opinion in this paragraph; I am a physicist, but I am not a bubble diffusion expert.
From bubble theory, look at mean bubble size. Bubbles may come off faster at the surface, but if they are larger, you are in deep trouble. So, at what depth can you off-gas bubbles of a size that are managable? That is the question. And that is perhaps another reason why off-gassing at depth is safer. Just giving opinion in this paragraph; I am a physicist, but I am not a bubble diffusion expert.
ianr33
Contributor
No that was not the question.
The question was where do we offgas the FASTEST.
We offgas the fastest AT THE SURFACE
At what depth can you offgas safely? That depends on the profile but for an NDL dive something in the 10 to 20 foot safety stop range is a good place.
Charlie99
Contributor
- Messages
- 7,966
- Reaction score
- 169
- # of dives
- 500 - 999
As ianr33 says, dissolved N2 will offgas faster while on the boat than at a safety stop.
OTOH, if the N2 has formed bubbles, then it offgasses much more slowly than when dissolved.
Decompression is the balance between ascending too early and forming bubbles, vs. staying deep and not offgassing.
=======================
To the first approximation, bubble formation or lack thereof is set by the relative pressure of the sum all inert dissolved gases vs. the ambient pressure. Staying deeper increases ambient pressure and reduces bubble formation.
Dissolved gas offgassing is set by the relative pressures of each separate dissolved gas and each separate inspired inert gas partial pressure. When breathing air at a safety stop, the inspired ppN2 is higher than while on the boat, and therefore offgassing of dissolved N2 is slower at the safety stop than on the boat.
=========================
In practice, what happens when coming up from an NDL dive, at first staying at a safety stop helps, because you are rapidly offgassing the relatively heavily loaded faster compartments. After some period of time -- a couple of minutes to 10 minute time frame, the compartments will all be offgassed enough that you can safely ascend without causing much bubbling. After that point, staying at the safety stop is counterproductive.
OTOH, if the N2 has formed bubbles, then it offgasses much more slowly than when dissolved.
Decompression is the balance between ascending too early and forming bubbles, vs. staying deep and not offgassing.
=======================
To the first approximation, bubble formation or lack thereof is set by the relative pressure of the sum all inert dissolved gases vs. the ambient pressure. Staying deeper increases ambient pressure and reduces bubble formation.
Dissolved gas offgassing is set by the relative pressures of each separate dissolved gas and each separate inspired inert gas partial pressure. When breathing air at a safety stop, the inspired ppN2 is higher than while on the boat, and therefore offgassing of dissolved N2 is slower at the safety stop than on the boat.
=========================
In practice, what happens when coming up from an NDL dive, at first staying at a safety stop helps, because you are rapidly offgassing the relatively heavily loaded faster compartments. After some period of time -- a couple of minutes to 10 minute time frame, the compartments will all be offgassed enough that you can safely ascend without causing much bubbling. After that point, staying at the safety stop is counterproductive.
Charlie99 is right. Doc is guilty of 'reading-into' the question. (He apologizes...)
I assumed the poster was referring to offgassing during a decompression dive breathing gas mixes other than air. (That is, aiming to achieve a curve maintaining a gradient characterized by maintaining PPO2 at 1.6, maintaining PPN2 in the blood at highest possible PP just under the point where bubble nucleation occurs, and maintaining PPN2 in the lungs at lowest possible PP by breathing hyperoxic gas mixes other than air, specifically 100% O2 at 20 fsw).
If breathing 100% O2 at the safety stop, the inspired PPN2 is zero.
According to Wienke, the fastest gas transfer rates possible are a constant: "the rate at which a gas diffuses is inversely proportional to the square root of its atomic weight" (2003, p.268).
The diver seeks to most efficiently achieve highest transfer rates by maintaining a high pressure gradient. This can most effectively be done when lowering the PPN2 in the lungs from levels which would occur breathing air.
The rate of N2 offgassing will be faster if the diver is breathing 100% O2 regardless of whether this occurs at a safety stop or on the boat, but "the driving force for the transfer of any gas is the pressure gradient" (ibid, 268).
A higher pressure gradient may be maintained over the duration of the ascent while underwater breathing gasses other than air, as opposed to on the boat where the pressure gradient equilizes fairly rapidly (thus slowing gas transfer), particularly if breathing air. (So the diver offgasses large amounts of N2 in solution during the ascent when the pressure gradient is highest.)
But apparently that was not what the OP was asking! Thats what happens when I assume.....
I stand corrected.
That's okay, Doc; it's easy to forget that the majority of the diving world doesn't get in the water with at least three tanks . . .
Similar threads
- Question
