Muscle mass and air consumption

[Dr Deco]

Dear DivingDoc:

Men vs. Women (Breathing, that is) :scuba:

I suspect that the main difference between men and women in how quickly they can empty a scuba tank resides in the tidal volume which is body-size dependent. Looking in a physiology text, I find that the average Tidal Volume for men = 600ml and for women = 500 ml. We could then say that if each were calm, relaxed, and taking about 22 breaths/min, the woman would use breathing gas at a slower rate

Gas Consumption

Regrettably, I do not know exactly what the units are on your meter. I believe that they are pounds/min at the depth at which you are breathing. Some divers give this in cubic feet per minute and this will produce the 0.5 values for surface consumption in some of the posts.

Dr Deco :doctor:

 

[ChrisF]

DivingDoc once bubbled...
Dr. Deco once bubbled:

...snip
Dr. Deco -- do you know what units the "airconsump" data is on my Oceanic when uploaded -- numbers range from 11-66. How do I compare that to what the people in this thread are talking about with numbers in the .5 range?

I guessing here, but I'll bet that those are liters per minute. 1 liter = .035 ft3.

11 would be .385 ft3/min
66 would be 2.31 ft3/min

Depending on the sampling rate of the computer, it's concievable that the numbers would be all over the map. It would probably be more interesting to look at the average of those mystery numbers over an entire dive.

<Edit: After re-reading your original post, I take it back. I didn't realize there was only one number per dive. I agree, it seems unlikely that your air consumption rate was that variable across multiple dives.

Are the smaller numbers associated with shallower dives? If the number is an absolute consumption rate, it will vary considerably based on the average depth for the dive. When people talk about "typical" consumption rates, they're talking about a normalized rate based on consumption at the surface.

Does the computer show the average depth for each dive? If so, try calculating the following for a couple of dives:
SAC = mystery number / (avg. depth/33 + 1)

See if this gives you a more stable value across several dives.>

 

[DivingDoc]

Dr Deco once bubbled...
Dear DivingDoc:

Men vs. Women (Breathing, that is) :scuba:

I suspect that the main difference between men and women in how quickly they can empty a scuba tank resides in the tidal volume which is body-size dependent. Looking in a physiology text, I find that the average Tidal Volume for men = 600ml and for women = 500 ml. We could then say that if each were calm, relaxed, and taking about 22 breaths/min, the woman would use breathing gas at a slower rate

Dr Deco :doctor:

But there has got to be more to it than that. O2 consumption has got to vary with RMR plus whatever increase in O2 requirements are resulting from exertion. RMR varies largely with lean body mass -- mainly the muscle. Of course, those who are more cardiovascularly fit will be able to use O2 more efficiently -- thus require less, right?

 

[Se7en]

DivingDoc once bubbled...
do you know what units the "airconsump" data is on my Oceanic when uploaded -- numbers range from 11-66. How do I compare that to what the people in this thread are talking about with numbers in the .5 range?
[/B]

I'll suggest litres per minute, absolute.

To explain, consider this.
At the surface, you may have a tidal volume of 0.5 Litres
At 10m / 30 ft / 2 atmospheres, that 0.5 litres of volume contains 1 litre of air (calculated at 1 atm).

That's why there is the S in Surface Air Consumption - you calculate what you would use at the surface, you air consumption increases with depth (as we all know).

Therefore, I'd suggest that your 11 to 66 range is from 11 litres per minute at the surface, to 66 litres per minute at depth. At 30m, this equates to a SAC of 16.5 litres per minute, at 40m to a SAC of 13.2 litres per minute.

FWIW, when doing gas planning, it is quite common to use 20 litres per minute for the active part of the dive, and 15 litres per minute for deco, with these figures being a bit conservative. 10 litres per minute would be excellent.

Oh - one cuft is 28 litres. Conversion to an antiquated measuring system is left as an excercise for the reader...

Mike

 

[Dr Deco]

Dear DivingDoc:

Open Circuit Systems :scuba:

I believe that the controversy here arises from the fact that most divers are using open circuit equipment. The gas usage (depletion of tank pressure) is governed by the rate of ventilation.

Closed circuit Systems

In a closed system, the true oxygen usage can be observed since nothing is vented off. In this case, I am sure that you would find a relationship to the resting metabolic rate and the efficiency of oxygen extraction by the muscles. Physically fit individuals would give a better account of them selves than couch potatoes would.

Dr Deco :doctor:

 

[Drew Sailbum]

FWIW, there is a set of gas management tables in the IANTD Advanced Nitrox Diver manual. The ranges of covered SAC rates are from .35 to 1.15 ft3/min. The metric version covers rates from 9.91 to 32.57l/min.

The reality is that many variables affect SAC rates. So the same divers that seems to have gills in warm-water, low-stress diving may double or triple their SAC rates in different conditions.

Comfort in the water strongly determines rate of respiration, and that comes with diving experience. I'm no small guy (5'10", 260lbs), but since I dive a truly stupid amount, I am very comfortable in the water. My SAC rate (0.30-0.35) is lower than most of the divers that come out on our boats. I can only ascribe that to a lower respiration rate, since my lungs are presumably much larger than many of the smaller people I dive with.

Extending that, I have one of the higher SAC rates of our staff members. I'd expect that since all of our instructors are very comfortable in the water. The primary difference is physical size, and I am one of the bigger guys.

I've seen divers with SAC rates of 0.20-0.25. I look them over carefully and still think I'll find the gills some day.