Actual oxygen consumption on CCR

[nigelH]

I've always used the 1L/minute rule of thumb for guessing how much O2 I burn but on Tuesday I was messing about in the pool....

I flushed the lungs with oxygen, well 1.08 ish on the handsets at 2m or less so quite a good flush, and then turned all the gas valves and the ADV off and just carried on. It took me 19 minutes to use all the gas in the HP section and then breath the loop down to the point where I needed some more gas to have volume to breath.

I think I had about 120bar of O2. Watching the O2 SPG drop as I was pushing the O2 button leads me to think I did not use 20L, probably not even 10. I would guess it fell 30 or 40 bar for a litre in the lungs. The handsets were reading 0.85 ish at the end.

Any ideas how I can calibrate how much I breath other than just sitting in front of the computer wearing the darn thing and a nose clip? I'd rather do it in the pool as I know nothing can cheat there. Leaks get noticed but the SPG is so crude that it would take a couple of hours to get a meaningful reading and I can't go up and down or that introduces other factors.

Yes, I'll still top up the O2 tank if it's below 180bar, I'm like that, but I have this thing about numbers...

[DaveB]

I agree the SPG is crude, but I tried to counter that with time. Poddling about at 20m for 20 mins I used less than a 5 bar division, estimated at 4 bar. Which worked out at about 0.6lt/min. This was a very low work rate and on minimum loop volume. It will not be super accurate, but should be within 15% or so.

[Padowan]

I did 20 minutes bimbling about at about 23m in the NDC a while back, and used something like 8bar from my calcs...which gave me a figure of about 1.2l/min which is the right order of magnitude, but also a bit higher than I would have thought for low exertion. If I'm bored this weekend, I'll spend an hour at constant depth, or do the Internet and noseclip thing...

[beanie]

hmm, thing is even if your O2 is say 1 you end up using more than that on the dive anyway due the up and down bits

[MAD]

Dave's method strikes me as being probably the best indication for a CCR that's eaily achieved without doing anything silly.

It doesn't rely upon guessing the volume of HP O2 stored in the hoses and SPG... it also doesn't relate to the volume of gas in the counterlungs.

If you do it at a constant depth... (20m or was it 21m in Stoney????) for a measured period at normal exercise rates then should be close enough.

Here's the other part... those of us with access to a SCR can work it out much easier if we have a VR3 measuring the inspired fO2... it's very simple to then back calculate from the FO2 of the bottle mix and the flow rate.

If you're up in Stoney some time and want to give it a go on the Azimuth... let me know and we'll run your metabolic rates.

Here's the odd thing about O2 consumption... the fitter you are, the more O2 you are likely to use as your ability to metabolise it is higher.

Those of us who are unfit will get better gas usage... (OK I'm prepared for the battering on that one if I'm talking B*lls)

I come out at 0.7lpm at moderate exercise with 1lpm under extreme loading...

Fairly convinced that the 0.7lpm level and below puts both Dave and myself in what would be known as the "Homer Simpson" catagory.

[DaveB]

Originally Posted by MAD

Fairly convinced that the 0.7lpm level and below puts both Dave and myself in what would be known as the "Homer Simpson" catagory.

Doh! You're probably right, but I'd like to think that being fitter means you can burn more O2 at high exercise rates, however, I can't see why a fit sloth should use more than an unfit sloth doing the same thing. The thing to do is spend various times at different work rates to get a range of cumsumption data. Not looking forward to the 20 minutes of finning like a maniac though!

[divetheworld]

http://homepage.ntlworld.com/brent.h...Calculator.xls

This help anyone?

[MAD]

Excellent... not really seen the graphical representation...

The sheet below allows you to input the variables and come out with the values... It's what I use to select the flow rate...


http://www.divejunkie.com/SCR_Equations.xls

There may well be errors in this... if you use it... it's at your own risk... blahdy... blah... etc


Oh and if you want to try a range of workrates... how about wearing the unit whilst riding an exercise bike... Did this on my SCR course... and someone has got the pictures to prove it...

[rooster]

"Here's the odd thing about O2 consumption... the fitter you are, the more O2 you are likely to use as your ability to metabolise it is higher.

Those of us who are unfit will get better gas usage... (OK I'm prepared for the battering on that one if I'm talking B*lls) "

I'm not a Scientist or a Doctor so this may be just as much b*ll!

I thought that fitter people were MORE EFFICIENT with metabolising O2 and therefore used less under duress as they converted it more easily than us wheezing fatties

the real proof is in the pudding though I guess, i have seen plenty of overweight smokers with impressive air consumption, and a fair few gym-rats with similar experience who are gas-hogs

Anyone smarter than me prepared to confirm or deny? (about the efficiency thing, not smoking divers!)

[MAD]

And there Rooster is the nub of the matter...

If it were OC we were talking about you are absolutely spot on... that's because those of us who are unfit need to take a greater volume (more breaths) to try to get the O2 we need out of the air (nitrox or whatever) and into our systems... The exhaled breath in an unfit diver contains a higher fO2 than a fit diver 'cos the fit diver has "extracted" it more efficiently.

This is where the CCrebreather behaves a little differently... It only replaces the O2 that has been metabolised... you can breath as fast as you like and as heavey as you like (OK so I'm ignoring CO2 issues, dwell time in both scrubber and lungs etc etc) but you use what you metabolise and nothing else. (Assuming constant depth and no leaks... if only... )

Hence if you can't get it into your system as quickly... then you'll use less...

This does not take the "under duress" bit into account... where I suspect in absolute terms you might be right. But in normal diving conditions...

The efficiency piece does come into play however 'cos unfit people will tire more quickly because they are not as good at turning the O2 & sugars into energy within the muscles...


OK... I know I'm talking from a position of complete ignorance... but this is my thought process... and I suspect I'm greatly over simplifying things...