Average Gas Consumption

[JMarc]

And for those with great consumption on an average low effort dive, you shouldn't even try for less than 0.35 CFM. The required breathing habit (and baseline hypercarbia) will not put you in a good place on a deep dive where you're suddenly required to work with high gas density.

Interesting. I'm out of my depth here and I'd appreciate if you could expand your reasoning here, it usually helps me to better remember those kind of things. And I'd like to ensure that I've not developed a breathing pattern which would put me at risk while scuba diving (I'm also also doing breath-hold diving which has probably developed some kind of accoutumance) although at an average of 13l/min (male, 55yo, 80kg for 1.83m) I don't seem to be that close to the limits.

 

[rsingler]

I'd appreciate if you could expand your reasoning here...

During 40 years of "passing gas" as an anesthesiologist, I have round numbers in my head for the breathing pattern required to maintain normal CO2, which we monitor during every general anesthetic. Someone will doubtless jump in to correct me regarding "proper" numbers taught since I was on the UCSF faculty, but it's common to ventilate the "average" patient 4.8 l/m to generate an end-tidal pCO2 around 35 mmHg during an anesthetic (adjusting breath size and/or rate based on the monitored CO2). That "end of breath" CO2 of 35 correlates pretty well with a normal arterial pCO2 of 40 (absent emphysema or lung problems).

Using 4.8 lpm as an average starting point we can extrapolate.

First, an anesthetized patient does not have a lower metabolic rate from anesthesia. However, he/she is completely motionless, resulting in about a 10% reduction in CO2 generation at normal body temperature.

Therefore, if 4.8 lpm is 90% of normal resting awake ventilatory requirement, we can translate that to a minimum awake ventilation of 5.3 lpm.
Transcribing that to Imperial measure yields 0.19 CFM.

That corresponds very nicely with a test I performed on myself in an operating room discussed here:
Post in thread Overshooting NDL and mandatory deco stops

Extrapolating further, I have experimented on myself using Shearwater's SAC readout on segments of a dive where my effort has been zero, just drifting along. With the minimal effort required to maintain trim on a drift dive in the best of conditions, my minimum SAC rose from 0.18 CFM sitting quietly in a chair in a cool operating room, to 0.23 CFM floating motionless in the water. I couldn't maintain that for more than a couple of minutes during any dive.

Comparing that to my average "good" SAC on an easy dive of 0.4 CFM where I was trying a bit of "skip breathing" to minimize my gas consumption, I quickly realized the contribution of any activity on CO2 generation, and required ventilation.

I would note that I am very sensitive to elevated CO2, as proved in that operating room test. About 25% of divers do not respond and perhaps another fraction are less sensitive. That doesn't mean they are immune to elevated CO2 and its toxicity. It means their receptors don't function the same way as mine. But with a given elevated CO2, we're both at the same risk.

So my final assumption in all this was that I was "normal". My friends may disagree, but at least I'm 70kg in good health with normal body mass index.

And with that assumption, I suggested that my very best (with a hint of cheating by skip breathing) was the lowest your SAC could be at 70kg without hypercarbia. And from my observations of my best days at 0.4 CFM with modest activity, and adding a little fudge factor for divers that are better than I, I suggested that 0.35 CFM is as low as you should try to go.

Translating that back from Imperial, I wouldn't recommend trying for a SAC lower than 10 lpm. If you're less than that, and weigh 70 kg, I'll bet you are retaining CO2, and are at risk for a CO2 hit if you suddenly have to exert yourself at depth with elevated gas density.

Aren't you sorry you asked?

 

[JMarc]

Aren't you sorry you asked?

It was the answer I desired and more complete than I hoped. I thank you very much for taking the time to write it.

 

[atdotde]

Aren't you sorry you asked?

I found that specific SAC (I mean SAC divided by body weight) to vary much less amongst divers (at least somewhat experienced) than SAC alone does. Do you take body weight into account for ventilating in the operating room?

 

[rsingler]

Do you take body weight into account for ventilating in the operating room?

Absolutely

 

[scubadada]

Gender and/or body weight do not fully explain differences in RMV. I am a 5' 10"/178 cm, 185 lb/84 kg man. I am in good shape but am hardly svelte. I posted my RMV data a few posts ago:

Average Gas Consumption

No problem with your ears too? No problem with ears. Occasionally I have to hesitate on the initial descent - not nearly as much as my husband. Never any problems after a dive (hope I didn't just jinx it).

scubaboard.com

In an ideal world, the data from this thread's poll would be available by gender and body weight. Too late now, the poll has been going for 7 2/3 years. I am grateful for the 783 divers who have contributed their average RMV

 

[WillieJustice]

During 40 years of "passing gas" as an anesthesiologist, I have round numbers in my head for the breathing pattern required to maintain normal CO2, which we monitor during every general anesthetic. Someone will doubtless jump in to correct me regarding "proper" numbers taught since I was on the UCSF faculty, but it's common to ventilate the "average" patient 4.8 l/m to generate an end-tidal pCO2 around 35 mmHg during an anesthetic (adjusting breath size and/or rate based on the monitored CO2). That "end of breath" CO2 of 35 correlates pretty well with a normal arterial pCO2 of 40 (absent emphysema or lung problems).

Using 4.8 lpm as an average starting point we can extrapolate.

First, an anesthetized patient does not have a lower metabolic rate from anesthesia. However, he/she is completely motionless, resulting in about a 10% reduction in CO2 generation at normal body temperature.

Therefore, if 4.8 lpm is 90% of normal resting awake ventilatory requirement, we can translate that to a minimum awake ventilation of 5.3 lpm.
Transcribing that to Imperial measure yields 0.19 CFM.

That corresponds very nicely with a test I performed on myself in an operating room discussed here:
Post in thread Overshooting NDL and mandatory deco stops

Extrapolating further, I have experimented on myself using Shearwater's SAC readout on segments of a dive where my effort has been zero, just drifting along. With the minimal effort required to maintain trim on a drift dive in the best of conditions, my minimum SAC rose from 0.18 CFM sitting quietly in a chair in a cool operating room, to 0.23 CFM floating motionless in the water. I couldn't maintain that for more than a couple of minutes during any dive.

Comparing that to my average "good" SAC on an easy dive of 0.4 CFM where I was trying a bit of "skip breathing" to minimize my gas consumption, I quickly realized the contribution of any activity on CO2 generation, and required ventilation.

I would note that I am very sensitive to elevated CO2, as proved in that operating room test. About 25% of divers do not respond and perhaps another fraction are less sensitive. That doesn't mean they are immune to elevated CO2 and its toxicity. It means their receptors don't function the same way as mine. But with a given elevated CO2, we're both at the same risk.

So my final assumption in all this was that I was "normal". My friends may disagree, but at least I'm 70kg in good health with normal body mass index.

And with that assumption, I suggested that my very best (with a hint of cheating by skip breathing) was the lowest your SAC could be at 70kg without hypercarbia. And from my observations of my best days at 0.4 CFM with modest activity, and adding a little fudge factor for divers that are better than I, I suggested that 0.35 CFM is as low as you should try to go.

Translating that back from Imperial, I wouldn't recommend trying for a SAC lower than 10 lpm. If you're less than that, and weigh 70 kg, I'll bet you are retaining CO2, and are at risk for a CO2 hit if you suddenly have to exert yourself at depth with elevated gas density.

Aren't you sorry you asked?

Nope, not sorry at all. And I didn't even ask. Considering my body mass and muscle mass, I should not expect to lower air consumption all that much. It's exhaling that causes me to use air as much as anything, and you just explained why that is. Thanks.

 

[scubadada]

Another quick 1,000 views, now more than 60,000. Nine more votes in the poll, now 785.

The median, mode, and weighted average remain 0.5-0.59 cu ft/min or 14.2-16.9 l/min

If you have not voted in the poll, please consider doing so. If your average RMV has changed, please change your vote.

Best of diving

 

[Phteven]

Just record your psi used, average depth, and dive time and you can calculate your RMV. Early in your dive career you stand a very good chance of improving your gas consumption with improved buoyancy, trim, propulsion, relaxation... Participate in the poll when you have an average number. You can always change your vote as you improve.

By the end of the trip it looks like I was hanging around the mid 0.6X mark, though a night dive with a decent current was at almost 0.8. On the other hand I had two dives that got into the 0.5X range, so that's good.

For the first 3-4 dives I was decently overweighted (by about 6 pounds) and then finally had an opportunity to calibrate against a tank that was at the end of a dive, and that made a decent difference by itself.

I can definitely improve more, but now with my own BC and the right amount of weight, I feel like it'll be getting easier from here.

Oh and for the sex and size data: Male, about 260lb currently (oof covid. I should be more like 230 at my height and build)

 

[scubadada]

Over 61,000 views, 788 votes in the poll

The median, mode, and weighted average remain 0.5-0.59 cu ft/min or 14.2-16.9 l/min

If you have not voted in the poll, please consider doing so. If your average RMV has changed, please change your vote.

Good diving to all