Bubble mechanics confusion

[IWantToBeAFish]

Hi all. So I'm trying to understand the mechanics of bubble behaviour as put forward in discussions of dual phase models. I've read the TDI documentation (deco procedures course) and Mark Powell's Deco for Divers and I'm missing something.

The docs put forward the following rules:
1) To be stable (for size), the bubble pressure must equal ambient plus Laplace pressures.
2) if the bubble pressure is different from tissue partial pressure (for a given inert gas), diffusion occurs between the two (in or out) and the bubble size changes accordingly.

I have a couple of questions regarding the different pressure change scenarios:

- On rapid ascent or descent, ambient P changes and so it says the bubble P changes accordingly to satisfy (1), which then causes a size change via diffusion due to (2).
What actually causes the initial pressure change? It seems to make more sense to me that the ambient pressure change causes an imbalance in (1) and this changes the size of the bubble which then changes the bubble pressure according to boyles law. Then (2) comes into play which causes further change in size until stability is reached. Am I right here?

- On gas switch to deco gas for given depth, ambient P stays the same, (1) is invariant but now shrinkage can still occur due to (2).
But when that shrinkage occurs, and the bubble pressure is lower, has the Laplace pressure changed by the same amount to satisfy (1) because if boyles were in effect then the bubble volume would have to increase?

A little confusing - to me at least. I'd really appreciate it if someone can help shed some light on this subject. Many thanks!

 

[Duke Dive Medicine]

Bubbles in the body follow Boyle's Law, i.e. if the ambient pressure is decreased, they will tend to increase in size. Depending on what you mean by rapid ascent, and at what point in the ascent the bubbles form, this will probably have a greater effect on bubble size than gas diffusion. For gas switches, if you're assuming a constant depth during the gas switch, Boyle's Law will not be an influence.

Hopefully this answers your question; if not, please feel free to clarify. Also, Ross Hemingway (RossH) is a ScubaBoard member; he may respond to a PM.

Best regards,
DDM

 

[wedivebc]

This may help

[video=youtube;qNBTygWcy0s]http://www.youtube.com/watch?v=qNBTygWcy0s&feature=youtu.be[/video]

 

[Duke Dive Medicine]

After a pint and a brief nap I think I understand the OP's questions. Let me take another crack at it.

The docs put forward the following rules:
1) To be stable (for size), the bubble pressure must equal ambient plus Laplace pressures.
2) if the bubble pressure is different from tissue partial pressure (for a given inert gas), diffusion occurs between the two (in or out) and the bubble size changes accordingly.

I have a couple of questions regarding the different pressure change scenarios:

- On rapid ascent or descent, ambient P changes and so it says the bubble P changes accordingly to satisfy (1), which then causes a size change via diffusion due to (2).
What actually causes the initial pressure change? It seems to make more sense to me that the ambient pressure change causes an imbalance in (1) and this changes the size of the bubble which then changes the bubble pressure according to boyles law. Then (2) comes into play which causes further change in size until stability is reached. Am I right here?

An inert gas bubble in a diver is never stable under normal diving conditions because we have oxygen in our breathing gas. There are a few things that can cause a bubble to grow. Decreasing the ambient pressure is one. If an in-situ bubble is subject to a decrease in ambient pressure, its size will increase. This growing bubble exerts pressure against the surrounding fluid in order to grow and "keep up" with the surrounding ambient pressure. The lag time is negligible, but you're right, the reason for initial bubble growth is the decrease in ambient pressure. Since the Laplace pressure is based on the size of the bubble (the larger the bubble, the smaller the Laplace pressure), the Laplace pressure will decrease accordingly. Once the ambient pressure change stops, the bubble will begin to decrease in size.

- On gas switch to deco gas for given depth, ambient P stays the same, (1) is invariant but now shrinkage can still occur due to (2).
But when that shrinkage occurs, and the bubble pressure is lower, has the Laplace pressure changed by the same amount to satisfy (1) because if boyles were in effect then the bubble volume would have to increase?

A little confusing - to me at least. I'd really appreciate it if someone can help shed some light on this subject. Many thanks!

I think what you're asking is whether Laplace pressure changes with bubble size, and yes, it does (see above). With a switch to a higher pO2, a bubble will decrease in size more rapidly at the decompression stop at which the gas switch occurred, though there may be a transient increase due to diffusion of O2. This decrease in bubble size results in a corresponding increase in the Laplace pressure, which in turn means that the bubble pressure must be lower. This is not due to Boyle's Law, however, as there is no change in ambient pressure at a decompression stop.

 

[Duke Dive Medicine]

...This decrease in bubble size results in a corresponding increase in the Laplace pressure, which in turn means that the bubble pressure must be [-]lower[/-] higher. This is not due to Boyle's Law, however, as there is no change in ambient pressure at a decompression stop.

Please note typo correction in the above section of the last post.

 

[IWantToBeAFish]

Ok thanks for the clarification Duke! So really then, with regards bubble mechanics, Boyle's law only applies to the relationship between bubble volume and the external ambient pressure? This is what I was missing. Thanks