Ken: It does seem counter-intuitive. if I understand this correctly (and I may not), it is thought to work like this: When bubbles form, they have a "skin" of molecules around them that form the interface between the gas bubble and the surrounding liquid medium. When the bubble is very small, the molecular "skin" around the bubble is tight and this limits gas diffusion into or out of the bubble. As ambient pressure decreases, the bubble grows and this increases the distance between the molecules in the bubble's skin. This increased distance between the molecules increases the permeability of the bubble. If the ambient pressure rises some, but not too much, the gas in the bubble will diffuse across the bubble skin, redissolve into the liquid, and the bubble will either become stable or collapse altogether. If the ambient pressure rises too quickly, the gas in the bubble cannot diffuse out into the surrounding medium fast enough and the bubble continues to grow.
How did you feel after that Deco ?
- Thread starter Remy B.
- 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
Ken: It does seem counter-intuitive. if I understand this correctly (and I may not), it is thought to work like this: When bubbles form, they have a "skin" of molecules around them that form the interface between the gas bubble and the surrounding liquid medium. When the bubble is very small, the molecular "skin" around the bubble is tight and this limits gas diffusion into or out of the bubble. As ambient pressure decreases, the bubble grows and this increases the distance between the molecules in the bubble's skin. This increased distance between the molecules increases the permeability of the bubble. If the ambient pressure rises some, but not too much, the gas in the bubble will diffuse across the bubble skin, redissolve into the liquid, and the bubble will either become stable or collapse altogether. If the ambient pressure rises too quickly, the gas in the bubble cannot diffuse out into the surrounding medium fast enough and the bubble continues to grow.
I dive 40/80 and on normal profiles I skip deepstops. In caves, the cave dictates the profile.
I feel ok after decodives.
I feel ok after decodives.
Again, with the caveat, that I don't understand this very well...
. This really gets into the realm of bubble/fluid dynamics, which is not my forte.
Prior to making much of an ascent, the molecular skin around the micro bubbles is very tight, so gas molecules cannot enter or leave the bubble from the surrounding tissue. As the diver continues to ascend, the ambient pressure of course decreases. Consequently, the inert gas volume inside a bubble increases as the gas molecules spread apart under decreasing pressure. As the bubble volume increases, it forces apart the molecules that form the bubble "skin." As an analogy, think about a balloon, as the volume of the balloon increases, the thickness of the latex balloon decreases because the material gets stretched apart. This stretching increases the permeability of the bubble. As the bubble becomes more permeable, gas can now enter or leave the bubble. If gas enters, the bubble grows, if it leaves, the bubble shrinks. Whether or not the now permeable bubble will gain or lose gas molecules depends on a complex array of factors including but not limited to: the LaPlace pressure (pressure of the bubble's skin), inert gas pressure inside the bubble, inert gas pressure in surrounding tissue, tissue type (e.g. elasticity), etc. All of these factors interact in complex ways, effectively walking a fine line between bubble growth, stability, or reduction.
Of course if the diver reduces ambient pressure too quickly, then the interacting factors affecting bubble size, quickly tilt towards growth.
. This really gets into the realm of bubble/fluid dynamics, which is not my forte. Prior to making much of an ascent, the molecular skin around the micro bubbles is very tight, so gas molecules cannot enter or leave the bubble from the surrounding tissue. As the diver continues to ascend, the ambient pressure of course decreases. Consequently, the inert gas volume inside a bubble increases as the gas molecules spread apart under decreasing pressure. As the bubble volume increases, it forces apart the molecules that form the bubble "skin." As an analogy, think about a balloon, as the volume of the balloon increases, the thickness of the latex balloon decreases because the material gets stretched apart. This stretching increases the permeability of the bubble. As the bubble becomes more permeable, gas can now enter or leave the bubble. If gas enters, the bubble grows, if it leaves, the bubble shrinks. Whether or not the now permeable bubble will gain or lose gas molecules depends on a complex array of factors including but not limited to: the LaPlace pressure (pressure of the bubble's skin), inert gas pressure inside the bubble, inert gas pressure in surrounding tissue, tissue type (e.g. elasticity), etc. All of these factors interact in complex ways, effectively walking a fine line between bubble growth, stability, or reduction.
Of course if the diver reduces ambient pressure too quickly, then the interacting factors affecting bubble size, quickly tilt towards growth.
Last edited:
Have done many deco dives over the years, especially during the time I was repeatedly diving to 200 fsw on air. Certainly there were days I was tired after 3-4 dives, but nothing unusual. I could be tired after a similar number of dives to 60 fsw. If I tried those same dives today, I'd be narced out of my gourd and exhausted.
DevonDiver
N/A
I'll use the term 'tissue dissolved inert gas pressure' rather than Pamb, as this might confuse people. Pamb actually refers to the combination of dissolved gas pressure in the tissues plus pressure existing in body tissues (i.e. blood pressure), across different compartments, at different levels of saturation.
Factors on ascent:
- Think of a bubble as a flexible container.
- Bubble expands on ascent via Boyle's Law
- Bubble radius increase lowers internal bubble pressure
- Bubble radius increase lowers surface tension
- Diffusion between bubble and surrounding tissue is quick (much quicker than diffusion of inert gas in/out of the body)
- Diffusion into a bubble causes growth much quicker than Boyle's Law.
- Oxygen widow effect (metabolism of O2 in tissues) increases diffusion out of the bubble, at a rate determined by the O2 content of the gas breathed.
- At significantly deep depths (~10ata+) it's theorised that bubble surfactant density lowers or prevents diffusion through the bubble surface.
- Diffusion speed of dissolved gas in the tissues is dependant on a tissue-ambient gradient. The higher the gradient, the swifter the diffusion (off gassing speed), up until the M-value.
1. If ascent is too fast, Boyle's law and inert gas diffusion into the bubble increases bubble volume. As a bubble grows its internal pressure reduces, allowing greater diffusion into the bubble from the dissolved inert gas in surrounding tissues. At the same time, the increasing bubble radius also decreases surface tension, further contributing to bubble growth.
In a nutshell, if bubble pressure is less than the combination of tissue dissolved gas inert pressure and bubble surface tension, then the bubble will grow.
This is bubble growth.
2. If ascent is too slow, a bubble might otherwise diffuse to match surrounding tissue pressure as it decreases, causing the bubble to shrink in line with reducing pressure. However, the bubble surface tension serves to keep bubble pressure just marginally higher than the tissue diffused inert gas pressure. There's little/no diffusion out of the bubble.
In a nutshell, if bubble pressure equals the sum of tissue dissolved inert gas pressure and surface tension, the bubble will maintain volume.
This is bubble persistence.
3. If ascent rate is optimal, you're keeping bubble pressure greater to the sum of tissue dissolved inert gas pressure and bubble surface tension. The ascent speed keeps tissue dissolved inert gas pressure decreasing rapidly enough to 'outpace' the decrease of pressure inside the bubble, even accounting for bubble surface tension. This permits a favorable positive gradient for gas to diffuse out of the bubble into surrounding tissues.
Collapsing bubbles during ascent (not stops) is about a fine balance, dictated by a pressure differential influenced by bubble surface tension.
In a nutshell, if bubble pressure is greater than the combined pressure of tissue dissolved inert gas and surface tension, then the bubble will shrink.
This is bubble collapse.
It all seems very counter-intuitive right?
We're used to things growing when ambient pressure reduces or shrinking when ambient pressure increases. However, bubble volume is FAR more influenced by gas exchange / diffusion than Boyle's Law.
Summary of Principle:
We ascend quickly to our first stop because bubble-to-tissue diffusion is very quick, but tissue-ambient diffusion is much slower. Boyle's Law is relatively insignificant compared to diffusion.
We need to maximise the tissue-ambient gradient to increase diffusion and get tissue dissolved inert gas pressure lower than bubble pressure. That facilitates bubble collapse.
Context:
On recreational dives, this is important because stops (where bubble collapse can occur easier) are limited...and there's a limited oxygen window effect. There is, however, a more pronounced ambient pressure change on ascent in shallowee water. Nonetheless, you can be too slow.
On technical dives, we're talking about ascent through a depth range where there's less pronounced ambient pressure change. We want to get ourselves moving up so that we can create a positive gradient from the bubble to surrounding tissue. Deep stops can kick in when the bubbles reach 'critical radius' for collapse...or they can be ignored and resolved on shallower stops.. all aided by a substantial oxygen window.
(I think I've got that right... it's early am here and I'm distracted by watching GoT re-runs) LOL
Last edited:
DevonDiver
N/A
Tissue Inert Gas Pressure rise/fall in relation to Bubble Pressure. i.e. differential
Ambient pressure (outside the body) decreases on ascent via Boyle's Law.
Dissolved gas pressure (in the tissues) also decreases on ascent, but more slowly, via diffusion through the lungs.
Free gas pressure (in the bubbles) decreases, via diffusion into the tissues as dissolved gas, when the gradient allows it. There are, as explained, factors associated with bubble dynamics that influence the gradient.
Now it sounds like Pamb refers to the inert gas pressure surrounding the bubble. Is that what you mean? I took Pamb to simply be the ambient pressure (e.g. 2 bar at 10m, 4 -5 30 etc). The inert gas pressure I'd call Pinert/Pnitrogen or some such.
DevonDiver
N/A
If we're talking about bubbles, 'ambient' is the external pressure of the medium in which the bubble exists.
If we're talking about the body, 'ambient' is the external pressure of the medium in which the body exists.
That's an important difference because, as you know, tissue inert gas pressure doesn't correspond directly to ambient pressure outside the body. We predict tissue saturation to be variable according to on/off-gassing speeds in different compartments. Hence, why bubble models still need compartments.
Pamb (or Pambient) is used in that context in scholarly articles. I understand how it can be confusing.
I have some time lined up with a proper expert in this in a few weeks. I will poke him until he makes me understand. At the moment I am still troubled by the 'ambient' vs local inert pressure.
If you have a link to an appropriate paper I will give it a read too.
Similar threads
