Rate of off gassing

[Piscean]

My instructor and I were just discussing whether you off-gas faster at 15ft than on the surface. He says faster at 15 feet, I say at the surface. The reason I say that is because the difference in partial pressures is greater at the surface. However, I wonder if he is right, because it could be that smaller/fewer bubbles form at the safety stop and so the off-gassing is more efficient. Does anyone know if bubbles reduce the efficiency of off-gassing? Or the answer to the question we were discussing?

Thanks for helping me in my quest for scubaknowledge,
Piscean.

 

[chrispete]

hrm... From what I've gleaned in scubaknowledge from here, I'd think that you'd offgas faster on the surface for the reasons that you pointed out. The only exception that I would be able to think of would be with the use of nitrox at 15 feet...

 

[Dr Deco]

Dear Piscean:

Off gassing without bubble formation

This is probably the general case for recreational divers. When few to no gas bubbles are present in your tissues, all of the dissolved nitrogen will diffuse from the tissues to the capillaries and be transported away to the lungs. This will occur with all nitrogen in the dissolved state.

In this situation, the off gassing will occur more rapidly on the surface, when the pressure gradient (differential) is greatest between you tissue and the nitrogen in your lungs.

Off gassing with bubble formation while at depth

This is the true situation for some recreational divers and probably for most deep divers. When a free gas phase is present in gas bubbles in the tissues, it is best to keep this small so that the internally-directed surface tension pressure (the Laplace pressure) will keep the internal pressure high and cause the bubbles to shrink. If one pauses at the deeper stop, the bubble is kept small and the Laplace pressure (and ambient pressure) is greater on the bubble. Whatever free nitrogen is present in the bubble will exit faster.

This process is even more important at deeper depths, since, once the gas bubbles grow, they will sequester (hold) the gaseous nitrogen. The gaseous nitrogen is at the ambient pressure of the diver (since it is not dissolved) and the gradient is considerably reduced. Off gassing is considerably reduced when large bubbles are present in the diver.

Off gassing with bubble formation while at the surface

When the diver reaches the surface, it is important not to exercise heavily in a fashion that will promote the formation or growth of tissue microbubbles. Off gassing of dissolved nitrogen will be hindered when gas bubbles are present. Remember that the decompression is not over when you reach the surface. You are still eliminating nitrogen.


Dr Deco :doctor:

 

[BRW]

Bubbles will offgas faster at 15 ft.

Dissolved gases will offgas faster at the surface.

Why? Simply, gradients for bubble elimination INCREASE with
pressure, and gradients for dissolved gas elimination DECREASE
with pressure.

There's the real task for deco theory -- eliminate BOTH phases
safely and efficiently. Until recent times, we used to JUST
focus on dissolved gases (called the Haldane approach).
But that is changed now (thank God), and modern dual
phase models (like RGBM, VPM, TBDM) do both. And it's
taken a century for this to happen. Many new experiments
on staging for controlling both phases back this up.

Problem has been that physical science community and medical
community don't talk enough. Another problem is certainly
understanding and listening. Like everything else. Control
is also operative here -- canonical medicine.