Gas Consumption Question

[Jarrett]

Let's say:

Diver A is a newbie weighing 250lbs and goes down with an Steel120 filled to 3500psi
Diver B is a seasoned DM weighing 98lbs and goes down with an AL80 filled to 3000psi

Both are diving air at a depth of 50 feet. After 50 minutes they both surface. Diver A has 500psi in his tank. Diver B has 1500psi in her tank. They have both been at 50 feet for 50 minutes, but Diver A has consumed twice as much compressed air as Diver B.

Even though a PADI RDP table would put them in the same pressure group, is Diver A at any greater risk than Diver B at this point? Or is it strickly bottom time and not total air consumption that adds to things like nitrogen loading and potential for DCS?

 

[jeraldjcook]

No, not as I understand it. The relevant issues are pressure and time. They were at the same depth for the same amount of time. Their air had the same amount of N2 in it so it would dissolve into their blood at the same rate. The amount of air breathed is irrelevant.

 

[Rick Inman]

Interesting question, but the more relevant issue - regarding DCS - would be physical condition (250 pound diver, if he/she is overweight) and exertion during the dive. Both would put diver A at a higher risk of DCS.

I'm talking, in gereral, because I do not know the dive A.

 

[jeraldjcook]

Agreed. Body composition and exertion would be a factor.

But I think the question is does the "amount" of air breathed matter. Right?

 

[Jarrett]

Let's say they are relatively in the same physical condition.

 

[Melicertes]

Let's say they are relatively in the same physical condition.

Clayjar hit the nail on the head here http://www.scubaboard.com/showpost.php?p=2361416&postcount=34

N₂ (or any gas for that matter) is absorbed at a rate and to a (quantity) level based on the the partial pressure difference (gradient) between the N₂ already dissolved into both divers' tissues and the N₂ in the breathing gas that is circulating through their lungs. Since both divers were exposed to similar (I'll assume the same for the argument's sake) ambient pressures throughout the dive; i.e. they both dived exactly the same depth profiles, both divers are thus subjected to the same pressure gradient between the absorbed and circulating N₂. Therefore both divers will absorb the same amount of N₂ up to the point where the pressure differences are zero; i.e. when they reach a state of equilibrium. Diver A might just reach that equilibrium faster than diver B, because he breathed faster and circulated more N₂, but he will not absorb more than diver A. Of course this would mean that both divers would have to be exactly at the same depths for exactly the same times which is highly improbable, and is the exact reason why two individual divers cannot use a shared dive computer to track their individual N₂ absorption levels.

The pressure group symbols are simply abstract indicators of absorbed N₂ and while diver A might reach their specific pressure group before diver B does, that does not mean they absorb different amounts of N₂. As mentioned by other posters, there are other conditions that may predispose a diver to DCS and that might include how much N₂ is absorbed. Just to note, the more N₂ is absorbed and the faster that absorbed N₂ comes out of solution when ambient pressure is reduced, e.g. ascending, the higher the probability of microbubble formation that could lead to symptoms we generically refer to as DCS.

 

[gcbryan]

Let's say they are relatively in the same physical condition.

It's strictly a matter of time and depth.

Just to be accurate as to your original statement, diver B didn't use half as much as diver A. She used much less than that.

I'm just trying to point out that you were not making accurate comparisions as you were comparing one diver using 1,500 psi from an aluminum 80 and another diver using 3,000 from a steel 120. This is apples and oranges since the two tank sizes are different. You don't compare psi between tanks with different volumes.

Diver A used 102 cu ft and diver B used apporximately 39 cu ft.

 

[Jarrett]

It's strictly a matter of time and depth.

Diver A used 102 cu ft and diver B used apporximately 39 cu ft.

I didn't know that. I learned two things in this thread Thanks.

 

[Melicertes]

...the more relevant issue - regarding DCS - would be physical condition (250 pound diver, if he/she is overweight) and exertion during the dive.

Overweight means more body tissue to absorb more N₂, especially fat (that absorbs/releases N₂ relatively quickly, I think - anybody got a good scientific grip on this?) Physical exertion means breathing faster thus circulating more N₂ through the lungs' alveolar spaces thus increasing the N₂ uptake rate. The reason this might predispose this particular diver to DCS is simply because there's more N₂ that will come out of solution when he/she ascends and this may increase the formation probability of microbubbles and lead to the onset of DCS. DCS is of course a highly studied subject, and much is still unknown about the topic. My response here, I very definitely suspect, is an overly simplified explanation and there very probably are other more complex issues also at play with regards to the occurence of DCS.