Larger gas capacity and increased DCS risk?

[Superlyte27]

The easiest way to overcome your increased air consumption is to dive more. If you are fit enough to do a triathlon I'm guessing you are not carrying around a bunch of extra weight to move through the water. I'm betting your lungs perform pretty efficiently and I'm guessing you're not a smoker. So, with all that in mind, I look to your reported number of dives being at around 50'ish-100'ish and it's pretty simple to figure out that as you get more comfortable in the water, you'll use less air. My prescription to you is.... GO DIVE MORE! and call me in the morning.

 

[Mr Carcharodon]

Nitrogen loading will depend on depth, time and the fraction of nitrogen in the inspired gas. Since the fraction of nitrogen will be essentially unchanged with CO2 load both divers will be at equal DCS risk. Also it is usually assumed that ongassing and offgassing rates are the same. So diver with greater lung volume may disolve more inert gas but should be able to unload faster too.

 

[tmassey]

The easiest way to overcome your increased air consumption is to dive more. If you are fit enough to do a triathlon I'm guessing you are not carrying around a bunch of extra weight to move through the water. I'm betting your lungs perform pretty efficiently and I'm guessing you're not a smoker. So, with all that in mind, I look to your reported number of dives being at around 50'ish-100'ish and it's pretty simple to figure out that as you get more comfortable in the water, you'll use less air. My prescription to you is.... GO DIVE MORE! and call me in the morning.

Well, my fitness and weight could be improved, but they're a *lot* better than many divers I see...

As for more diving: I'm working on it. My gas usage is improving. I get enough bottom time for me. But on deep dives I tend to drag my buddy out, and I don't like that. Hence contemplating more gas.

 

[Mike Boswell]

I'm gonna stick my neck out and disagree with conventional wisdom: The theory tells me you will absorb more N if you breathe faster.

According to Henry's Law, gas diffuses across a permeable membrane due to the difference in partial pressures on either side. As gases cross the membrane, the partial pressures change as gas leaves the zone of higher pressure and crosses to the zone of lower pressure: This reduces the difference in pressure and thus reduces the rate of gas diffusion.

As a little thought experiment, imagine that you stopped breathing altogether: Clearly, the nitrogen in your lungs would eventually reach equilibrium with that in your blood, and you would stop absorbing nitrogen into your bloodstream. So IMO, the more rapidly you breathe, the more rapidly you are refreshing the air in your lungs with new air that is higher in nitrogen, and the more rapidly you are absorbing nitrogen.

 

[boulderjohn]

According to Henry's Law, gas diffuses across a permeable membrane due to the difference in partial pressures on either side. As gases cross the membrane, the partial pressures change as gas leaves the zone of higher pressure and crosses to the zone of lower pressure: This reduces the difference in pressure and thus reduces the rate of gas diffusion.

As a little thought experiment, imagine that you stopped breathing altogether: Clearly, the nitrogen in your lungs would eventually reach equilibrium with that in your blood, and you would stop absorbing nitrogen into your bloodstream. So IMO, the more rapidly you breathe, the more rapidly you are refreshing the air in your lungs with new air that is higher in nitrogen, and the more rapidly you are absorbing nitrogen.

I believe you are correct with this, but with reasonably normal breathing the difference would be very minor.

Maybe it will help the OP if we went back to the beginning and stated the whole process in a different way.

When you breathe, the nitrogen in your lungs comes in contact with blood supply there. The molecules move randomly. If you have the same amount of nitrogen in the air you are breathing as in the blood and the tissues, the random motion means that there will be just as many molecules moving in one direction as another. We say the tissues are at equilibrium, and there is just as much nitrogen in the air as in the body.

If you descend to 99 feet of sea water, every breath you take has four times as many air molecules as it did on the surface. That means that four times as many molecules are going into the body as are coming out. In the extreme (and, of course, impossible) situation Mike describes, most of the nitrogen would have left the lungs, and the diver would be at equilibrium again. This, of course, does not happen, for the diver keeps exhaling the (somewhat) nitrogen depleted air and replacing it with a fresh supply. This means that the blood and tissues will not reach equilibrium until the tissues have caught up with the amount of nitrogen in the air supply.

Mike's point is that if the air lingers in the lungs longer in one diver, the rate of transfer slows down more (albeit briefly) before the next batch of nitrogen-filled air comes in to replace it. That will, in theory, make a difference. I doubt if it will make very much of a difference, though, and it is almost certainly not something to be concerned about.

 

[knotical]

The OP's question is also addressed by scuba-doc (Dr. Campbell) at the bottom this page: FAQ - Physics and Gases under:

SAC (Surface Air Consumption)
Nitrogen consumed, nitrogen absorbed

 

[ianr33]

I
As a little thought experiment, imagine that you stopped breathing altogether: Clearly, the nitrogen in your lungs would eventually reach equilibrium with that in your blood, and you would stop absorbing nitrogen into your bloodstream. So IMO, the more rapidly you breathe, the more rapidly you are refreshing the air in your lungs with new air that is higher in nitrogen, and the more rapidly you are absorbing nitrogen.

The problem with that is very little nitrogen is actually absorbed. At SATURATION you only absorb about 1 litre of nitrogen per atmosphere of pressure. The amount of N2 absorbed per breath is tiny,so the amount of N2 exhaled is for all intents and purposes the same as that inhaled,so no change in N2 absorption with a higher breathing rate.

Also its the pressure of N2 that matters. If some N2 is absorbed the volume of your lungs will decrease but they will remain at the same,ambient,pressure.

 

[flots am]

Excellent question.
The concentration of nitrogen may be the same however it is increased through volume. Air in and out is faster than bubbles in and out and despite how quickly breathing occurs the body still requires it's fill to function and nitrogen is there to carry our requirement of oxygen and to fulfil our requirement for volume of inhalation and exhalation.
To many variables, impossible to answer, if the divers are identical then the answer must be yes.
And if the divers are identical there is no question because there is no theory because the divers would end having consumed an identical amount of air.

Maybe I'm having a "stupid attack" but I don't have even the beginning of a clue what that means.

flots.

 

[ianr33]

Its not you!

 

[disbelief]

I'm gonna stick my neck out and disagree with conventional wisdom: The theory tells me you will absorb more N if you breathe faster.

According to Henry's Law, gas diffuses across a permeable membrane due to the difference in partial pressures on either side. As gases cross the membrane, the partial pressures change as gas leaves the zone of higher pressure and crosses to the zone of lower pressure: This reduces the difference in pressure and thus reduces the rate of gas diffusion.

As a little thought experiment, imagine that you stopped breathing altogether: Clearly, the nitrogen in your lungs would eventually reach equilibrium with that in your blood, and you would stop absorbing nitrogen into your bloodstream. So IMO, the more rapidly you breathe, the more rapidly you are refreshing the air in your lungs with new air that is higher in nitrogen, and the more rapidly you are absorbing nitrogen.

Nice thought experiment.

Tissue inert loading is a factor of both diffusion and perfusion. If the tissues are perfusion limited, then increasing diffusion will have negligible effect. This includes transfer across the alveolar interface.

Mark Powell uses a useful analogy in 'Deco for Divers', p33. To paraphrase, imagine the inerts being carried by truck. The frequency of the trucks arriving at a depot is the perfusion, and the rate of loading/unloading the trucks is the diffusion. If the trucks arrive infrequently, the cumulative amount offloaded/loaded will be dictated by perfusion. If they arrive frequently, it'll be dictated by diffusion... the trucks may be arriving faster, but you can't unload them any quicker!

This is why workload is a factor in DCS - the rate of perfusion to tissues changes.

DB