accuracy of computers/dive tables

[Travis*N]

I know this is a strange question, and I don't know if I can properly explain what I am trying to ask, but here goes.
The situation is: Two divers (divers A & B)go down to 100'. Both are using Al80 tanks and both are using the same computer. According to the PADI dive planner, the max bottom time at 100' before having to make a deco stop is 20 mins. Let's say these divers stay at exactly 100 feet for 20 mins, (including the decent time) thus they should hit their max NDL. Diver A has very good air consumption and diver B breathes extremely hard and burns alot of air. Assuming each diver started the dive with exactly 3000 lbs of air in their tank, and upon surfacing, diver A has 1600lbs of air in his/her tank, and diver B has 300lbs of air, the computers they are using should show the same level on the tissue loading bar graph, however, diver B has breathed nearly twice as much nitrogen into his/her body. Would diver B actually have twice the level of nitrogen in their body and be more susceptilble to getting DCS than diver A, or does it not make that much difference?
any thoughts appreciated
thanks, travis

 

[Travis*N]

I know this is a strange question, and I don't know if I can properly explain what I am trying to ask, but here goes.
The situation is: Two divers (divers A & B)go down to 100'. Both are using Al80 tanks and both are using the same computer. According to the PADI dive planner, the max bottom time at 100' before having to make a deco stop is 20 mins. Let's say these divers stay at exactly 100 feet for 20 mins, (including the decent time) thus they should hit their max NDL. Diver A has very good air consumption and diver B breathes extremely hard and burns alot of air. Assuming each diver started the dive with exactly 3000 lbs of air in their tank, and upon surfacing, diver A has 1600lbs of air in his/her tank, and diver B has 300lbs of air, the computers they are using should show the same level on the tissue loading bar graph, however, diver B has breathed nearly twice as much nitrogen into his/her body. Would diver B actually have twice the level of nitrogen in their body and be more susceptilble to getting DCS than diver A, or does it not make that much difference?
any thoughts appreciated
thanks, travis

I forgot to add, hypothetically, these two divers are about the same height, age, physical fitness level weight and sex.

 

[Wildcard]

Thats a very intresting question. Got me thinking. Don't have an answer for sure, but I think it's more time related than Breath per minute. I could be wrong, it happened to me once before

 

[neil]

I believe the answer is that, according to Henry's Law, the amount of gas dissolved is proportional to the partial pressure of the gas, not how much gas is breathed. The only way for a person to absorb more or less N2 than another would be to change the partial pressure of N2 by increasing or decreasing its percentage in the mix. Does that make sense?

Neil

 

[The Kraken]

Good question. The Henry's Law response has me confused, however.

If one guy drank 14 beers (3000 psi - 1600 psi) and the other guy drank 27 beers (3000 psi - 300 psi) which one would have a higher blood alc content???

If you have aspirated almost twice as much breathing gas as your buddy then wouldn't you have taken in almost twice as much nitrogen?

 

[Charlie99]

Good question. The Henry's Law response has me confused, however.

If one guy drank 14 beers (3000 psi - 1600 psi) and the other guy drank 27 beers (3000 psi - 300 psi) which one would have a higher blood alc content???

If you have aspirated almost twice as much breathing gas as your buddy then wouldn't you have taken in almost twice as much nitrogen?

The better analogy would be: one guy has 14 beers poured over his head, another guy has 27 beers poured over his head. Which one has absorbed more alcohol through his scalp?

Only a minute fraction of the inhaled N2 gets dissolved into your blood. How much goes into your blood depends on the absolute pressure of N2 in the lung, not how much has gone into the lungs, and then exhaled back out.

In simpler terms; the amount of N2 loading is almost entirely controlled by the pressure (depth) and time.

 

[The Kraken]

Charlie, I'm not trolling here, just trying to understand more of the physics/physiology involved . . .
But, wouldn't the amount of N absorbed into the blood stream be proportional to amount of O2 absorbed into the blood stream?

 

[jhelmuth]

Time does play a factor (we'd all admit that). So ask yourself - why does time play a factor? Is it because of the amount of gas ingested? Or is it the half times of the tissues?
Answer: Primarily, it's the half time of the tissues (AKA compartments). But there is some minor factor attributable to the amount of gas ingested. We metabolize the O2, but the Nitrogen is inert. Still, it's a very light gas and is absorbed easily into our tissues. It's always there (in your body), but at equilibrium. If you don't inhale any gas, and stay down for a long enough time at depth, then shoot to the top, you won't likely get bent. Why? Because ALL gases obey the gas laws and is compressed at depth. Only drastically rapid changes in depth could cause a problem because the gas will want to come out of solution at rapid changes in pressure. Breath just one breath of gas at depth, and you're going to have that gas expand as you decrease pressure. It's the expansion of the gas that is the problem. Clearly, the more gas that expands, the worse off you are.

 

[miketsp]

We're obviously going to get some more scientific explanations but the first thing that comes to mind is breathing efficiency. The diver doing steady deep breathing with pauses at the right times is moving less air but absorbing a higher percentage of the gas. The diver moving a lot more air is effectively wasting the gas.
I think neil's answer is quite correct, the gas absorbed depends on the pressure gradient at the alveoli/air interface and not on whether there is a turbulent air flow above the interface.
Turbulence in itself requires you to do more work moving the air, which in turn requires more air and remember this gets worse at depth where the air is denser.
Also shallow breathing will not clear out the CO2 which will also cause faster inefficient breathing.
Conclusion, there may be slightly higher N2 levels in the diver that used more gas due to the higher average partial pressure of N2 in the lungs but my first impression is that it will be negligable since the quantity of N2 absorbed per breathing cycle is negligable and the reduction of the N2 PP should be minimal.
Henry forever.

 

[eab]

Let me see if I can explain this correctly.

The reason you take on nitrogen at depth and off-load it as you ascend is due to the changes in pressure. The more pressure, the more nitrogen that is forced into your blood. The less pressure, the less nitorgen your blood can hold so it is released.

There is a point, called supersaturation, where your blood cannot take on any more nitrogen. The theory of this is upon which the tables are based. So breathe all you want at this point, your nitrogen load is not going to change as long as the pressure doesn't change.

So, yes, it's a matter of pressure rather than the amount of air breathed.

Does that help?