Overshooting NDL and mandatory deco stops

[scubadada]

@scubadada , I can't really argue with either of you. The numbers you summarized, and the dive you posted is quite impressive, as are @BLACKCRUSADER 's numbers.

A standard 70kg male in my operating room will not retain CO2 with an RMV of about 5 liters/min (0.18 cfm). That's at rest, anesthetized. Studies say metabolic rate drops ~10% under anesthesia, so we might extrapolate to say, absolute basal requirements at complete rest are RMV 5.5 lpm, or 0.2 cfm.

Both you and Crusader could be doing this with no retention, a boatload of experience and lots of Zen.

Not many can match this without skip breathing. Now, if only we could measure end-tidal CO2 underwater.

Thanks @rsingler

It's quite interesting seeing what the lower limits looks like

There have been 2 polls on SB, asking divers for their average RMV. Of course, there are many caveats in interpretation. About 2% of respondents reported an average RMV of <0.3 cf/min. Only 1.75% of my dives, for which I have a RMV, are <0.3 cf/min.

 

[Storker]

View attachment 563096

Could I ask you to add a 2nd x axis showing metric units? As a favor for those of us who aren't very familiar with y'all's weird units?

Yes, I know. I could easily calculate. But it'd be even easier if the x axis showed both Imperial and metric units.

 

[atdotde]

Thanks @rsingler

It's quite interesting seeing what the lower limits looks like

There have been 2 polls on SB, asking divers for their average RMV. Of course, there are many caveats in interpretation. About 2% of respondents reported an average RMV of <0.3 cf/min. Only 1.75% of my dives for which I have a RMV are <0.3 cf/min.

View attachment 563096

I would love to see a scatter plot of RMV vs weight (of diver)

 

[scubadada]

I would love to see a scatter plot of RMV vs weight (of diver)

That is an interesting idea, the data is not available in the 2 RMV polls. You could start over
Here is 2016 Average Gas Consumption
Here is 2009 SAC Rate

 

[scubadada]

Could I ask you to add a 2nd x axis showing metric units? As a favor for those of us who aren't very familiar with y'all's weird units?

Yes, I know. I could easily calculate. But it'd be even easier if the x axis showed both Imperial and metric units.

I cannot easily add a 2nd x-axis to my graph. Here is a translation to metric units:

1 cu ft = 28.3 liters
1 liter = 0.035 cu ft

< 0.3 cu ft/min = <8.5 l/min
0.3-0.39 cu ft/min = 8.5-11.0 l/min
0.4-0.49 cu ft/min = 11.3-13.9 l/min
0.5-0.59 cu ft/min = 14.2-16.7 l/min
0.6-0.69 cu ft/min = 17.0-19.5 l/min
0.7-0.79 cu ft/min = 19.8-22.4 l/min
0.8-0.89 cu ft/min = 22.6-25.2 l/min
0.9-0.99 cu ft/min = 25.5-28.0 l/min
>1.0 cu ft/min = >28.3 l/min

 

[Storker]

I cannot easily add a 2nd x-axis to my graph.

Don't you just love Excel graphs? Made by accountants, for accountants. They suck big time for scientific (or any STEM type) graphing.

 

[scubadada]

Don't you just love Excel graphs? Made by accountants, for accountants. They suck big time for scientific (or any STEM type) graphing.

You got that right, I'm happy if I can get a graph to simply display what I want

 

[Storker]

I'm happy if I can get a graph to simply display what I want

If you can afford it, or can get it through your employer, look at Sigmaplot. The learning curve is abysmal (albeit better than when I first started using it), but you can basically get just the display you want. No matter how weird. And these days, it comes with Excel integration (FWIW).

If you don't use Windows, there are probably better options, but I don't know crap about those.

 

[dmaziuk]

I would love to see a scatter plot of RMV vs weight (of diver)

Also height, pants size, and gender.

 

[rsingler]

...are there instances of sleepers retaining CO2, or does the body subconsciously regulate breathing more efficiently as opposed to a person consciously skip-breathing or overexerting?

Autoregulation of carbon dioxide is extremely precise in the mammalian organism. What should surprise you is how little carbon dioxide production decreases under anesthesia. It would be even less during sleep. Metabolism is primarily affected by body temperature and exercise, with the latter dominating.
Hence, the mandate to relax while diving. Given the various inefficiencies in our equipment, it becomes easier to see why exertion on a rebreather can be fatal.

Like many biologic systems, RMV data follows a bell curve of one shape or another. I wish my SAC were lower, but it just ain't. So I work really hard on streamlining, finning technique and resting. I am really tempted to try skip breathing since I know I won't get hypoxic, but as a physician I know that way leads to catastrophe. And as a diver wanting to start CCR, I absolutely have to resist the temptation.

My comments above were prompted by seeing on a daily basis the adjustment in end-tidal carbon dioxide that occurs when I change ventilation in my patients. In the early days of end-tidal CO2 monitoring, truly frightening instances of hypercarbia were seen with what were assumed to be "adequate" ventilation, along with more than one cardiac arrest.

So do all those things you were taught, to relax and streamline. Breathe fully (not shallowly) to minimized dead-space ventilation, and accept what you get. We just can't all be like @scubadada and Crusader, sad to say.