Larger gas capacity and increased DCS risk?

[tmassey]

In calculating NDL limits for a dive, there are only two variables that I can see (at least with, say, the RDP tables): depth, and the amount of time spent at that depth. In any case, even with a non-air-connected computer, there is no consideration given to the volume of gas used during that time.

Consider two identical divers. One is diving with 77 cu ft of gas, and the other is diving with 120 cu ft of gas. Both perform a dive with an identical profile, and both end up on the boat with the same volume of air in reserve (say, 12 cu ft), meaning that one diver used 65 cu ft and the other used 108 cu ft.

My question is this: is the diver who went through the much larger volume of air at an increased risk for DCS?

Given that I've never seen this mentioned, I would assume no. And I can even rationalize a mental picture of why: the concentration of nitrogen that a diver is exposed to during the entire dive is not really altered by how fast they inhale and exhale, so the fact that "more" nitrogen passes through their lungs doesn't affect how much nitrogen they're going to absorb. They just exhale away more gas.

However, I thought I'd ask the question. Does using a larger volume of gas for the same profile put a diver at increased risk of DCS?

 

[Kevrumbo]

Barring any lung disease/disorder or shallow, rapid, inefficient "dead-space" breathing . . .there should be otherwise no increased risk of DCS.

 

[ZenDiver.3D]

Tissue loading happens depending on depth and time length at that depth. You won't load more because you breathe more volume. Changing mix will change the load times, but volume of gas breathed should not.

 

[lamont]

the diffusion equation is only going to care about pressure and tension. how many atoms of gas your lungs are exchanging with your regulator isn't going to matter.

 

[disbelief]

If they are 'identical divers' then yes, *theoretically* there is an increased risk of DCS... but for which diver?

1. Diver 1 was purposefully skip-breathing, shallow-breathing or otherwise artificially reducing gas usage (rather implausible over an entire dive, but let's presume it was successful in reducing overall consumption). This could lead directly to hypercapnia and indirectly to DCS. But I think you actually meant:

2. Diver 2 conducted a rather more strenuous dive than diver 1, leading to greatly increased respiration and therefore perfusion of inerts. Note that all decompression methodologies (that I am aware of) will advise more conservative limits if the dive is strenuous, cold or stresses the body in some other fashion.

 

[tmassey]

If they are 'identical divers' then yes, *theoretically* there is an increased risk of DCS... but for which diver?

1. Diver 1 was purposefully skip-breathing, shallow-breathing or otherwise artificially reducing gas usage (rather implausible over an entire dive, but let's presume it was successful in reducing overall consumption). This could lead directly to hypercapnia and indirectly to DCS. But I think you actually meant:

2. Diver 2 conducted a rather more strenuous dive than diver 1, leading to greatly increased respiration and therefore perfusion of inerts. Note that all decompression methodologies (that I am aware of) will advise more conservative limits if the dive is strenuous, cold or stresses the body in some other fashion.

No, I actually meant that both divers performed *identical* dives, meaning that they both exerted themselves to the same extent. And neither diver used any artificial means to reduce (or accelerate, for that matter) their use of gas. They both breathed normally, but used differing amounts of gas.

However, I'm glad you brought this up. My question (intentionally) ignores *why* each diver is using a different amount of gas. It only asks if there is an increased risk in consuming a larger amount of gas over the same time and depth (and exertion, and breathing, etc.) profile.

But the answer to *why* someone is using more or less air does have an effect. It was not the point of my question, but I appreciate the reminder of this. It's certainly important to consider.

I was motivated to ask the question after thinking about increasing the amount of gas that I dive with, to compensate for my larger air consumption. I am in decent shape (I am currently training for a triathlon) and comfortable in the water, but keeping my air consumption down is a lot of work. It's not something I'm overly worried about (I'm in the middle of the pack in bottom time), and it's *definitely* not something I would be willing to do if it increased my risk.

 

[knowone]

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.

 

[tmassey]

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.

You know, that's *exactly* why I asked the question.

I agree with what you seem to be saying: the fact that two divers consume different amounts of gas means that they did not do the same dive in the same way, and therefore their risks will be different. And like disbelief described, the why of the extra gas consumption matters. However, in spite of this, tables and computers seem to only care about two numbers: depth and time.

I understand why specific adjustments for things such as physical fitness and exertion level aren't practical: they're too subjective. But it would seem that such an adjustment for rate of gas consumption *could* be practical--if it has an effect in and of itself. Or even as a proxy for all of those other subjective factors like fitness and exertion. That is the reason for my question.

 

[DCBC]

...other subjective factors like fitness and exertion. That is the reason for my question.

Yes. Part of the problem in DCS is that the tables are based upon a minimal amount of exertion on a fit diver. When it comes to poor fitness & excess body weight, older age and exertion (for example), some people feel they can dive to these limits and many push the envelope too much (even by following the rules that were not designed for them).

Military and commercial divers routinely change the table configurations if they have a need to exert themselves more than normal. Some recreational divers don't plan for such contingencies although some dive computers allow you to build in a safety factor for such considerations.

 

[disbelief]

No, I actually meant that both divers performed *identical* dives, meaning that they both exerted themselves to the same extent. And neither diver used any artificial means to reduce (or accelerate, for that matter) their use of gas. They both breathed normally, but used differing amounts of gas.

However, I'm glad you brought this up. My question (intentionally) ignores *why* each diver is using a different amount of gas. It only asks if there is an increased risk in consuming a larger amount of gas over the same time and depth (and exertion, and breathing, etc.) profile.

But the answer to *why* someone is using more or less air does have an effect. It was not the point of my question, but I appreciate the reminder of this. It's certainly important to consider.

I was motivated to ask the question after thinking about increasing the amount of gas that I dive with, to compensate for my larger air consumption. I am in decent shape (I am currently training for a triathlon) and comfortable in the water, but keeping my air consumption down is a lot of work. It's not something I'm overly worried about (I'm in the middle of the pack in bottom time), and it's *definitely* not something I would be willing to do if it increased my risk.

Haemoglobin in arterial blood is normally around 97.5% saturated; venous blood around 75%. Increased inspiration rate is driven more strongly by the need to expire CO2 than it is to increase O2 to the alveolar interface and through the O2 cascade.

Some computers include a heart rate monitor and factor this into their deco calculations, but if you are fit, efficient in the water, breathing effectively, relaxed and using efficient regs, there's not a great deal more you can do to lower your gas consumption (other than continue to improve the above).
Grab a bigger tank and don't sweat it.

I can't recommend too highly "Deeper into Diving" by Jon Lippmann & Simon Mitchell as the definitive first point of reference for all questions on diving physiology.