Nitrox/Time to Fly

[DivesWithTurtles]

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?

With further thought, the stuff I've written here may be all wet. I'd really appreciate it if someone who knows this stuff straightened me out.

I guess my confusion or lack of confidence comes from what you mean by "rate at which you offgas nitrogen".

The offgassing is driven by the difference between the tension of the gas within the tissue and the ambient pressure of the gas. We all understand that.

Within a given time, half of that differential will be eliminated. That's one kind of "rate at which you offgas". That doesn't change with a change in ambient pressure. But to eliminate half of that difference you have to get rid of more gas in same amount of time if the gradient is higher. That's another kind of rate. That's the kind of rate that can "speed up decompression" or be "too fast".

So, the answer is no, gc, to your question of "is more nitrogen being offgassed in 15 minutes at 30 fsw than in the first 15 minutes once you reach the surface". More nitrogen is being offgassed at the surface, like Dandy Don said. What that means to your decompression status is a different question.

See? I'm thinking myself into circles.

 

[ianr33]

The fastest way to offgas nitrogen is to make a ballistic ascent to the surface.This will maximise the pressure differential between your tissues and the ambient pressure.It will also get you horribly bent as the nitrogen rushes out of solution and forms bubbles (Just using a simple model here,no bubble models allowing for the fact that once bubbles have formed they slow down the rate of offgassing)

The purpose of safety/deco stops is not to eliminate nitrogen as quickly as possible but rather to eliminate it as fast as possible but WITHOUT bubbles being formed.

And to answer the original question. Yes,nitrox will reduce the time to fly,if you breathe it on the surface for a few hours after your dive,otherwise its your nitrogen loading after diving that matters,whether you got that breathing air or nitrox makes no difference.

 

[Dr Deco]

Hello readers:

There are different concepts at work here.

Halftimes of Compartments

The table designers set the halftimes. They run from about two minutes to about six hundred minutes depending on the particular goal or purpose of the table’s creator. These compartment halftimes will not change with conditions of the dive (e.g., exercise).

The exchange is determined by the partial pressure of nitrogen in the lungs [and arterial system therefore] and the partial pressure of nitrogen in the tissue. The deeper one is, the greater the partial pressure of nitrogen in the breathing mix. Exchange is thus slightly slower at ten fsw than at the surface. [If the diver were to breathe pure oxygen, the exchange is independent of depth.]

Exchange by Perfusions

Since dissolved nitrogen is eliminated from the tissues by the capillaries [then to the veins and finally the lungs], the perfusion [local blood flow] is the true determining factor. This flow rate will change with exercise, and in some cases with body temperature. You will increase the loss of dissolved nitrogen in the tissues by movement of the arms and legs while at the decompression stops. If you remain for a couple of minutes at the surface before reboarding (“the hidden stop”), arms and leg motion will also promote nitrogen elimination.

Tissue Microbubbles:

the problem with microbubbles during decompression is that they can grow by Boyle’s Law-expansion as the pressure is reduced. If bubbles grow too large, then the inward, collapsing pressure of surface tension [Laplace pressure] is reduced so much that dissolved nitrogen now will diffuse into them from the tissue. The internal pressure is ambient [hydrostatic] and the elimination gradient is almost nil.

Slow ascents and pausing at the surface will greatly help the outcome by keeping microbubbles small.

Dr Deco :doctor

 

[DandyDon]

Thanks Doc - always good to have the straight scoop!

... pausing at the surface will greatly help the outcome by keeping microbubbls small.

Hadn't heard that one, and certainly hadn't thought of it - but makes sense. Pause for a couple of minutes on the drift line before going thru the exertion of climbing the ladder. I need to add that to my routine.