SAC - RB vs OC

[Nick Kay]

OK, so help me out here - by brain hurts thinking about this

My personal SAC for diving OC:
- relaxed / no heavy exercise : around 12-14 lpm
- stressed / heavy exercise : around 16-18 lpm

On the mCCR I have the constant flow set to about 1 lpm
- relaxed / no heavy exercise : ppO2 will rise
- stressed / heavy exercise : ppO2 will fall

So, lets assume that an O2 flow rate of 1 lpm is "my rate"
- a body metabolises 4% of air breathed, so...
- with an O2 usage of 1 lmp, my SAC would be 25 lpm

So, although (believe it or not), I'm more relaxed on mCCR than OC, my "SAC" is between 1.5 and 2.0 times that of OC

The questions therefore are:
- are my calcs wrong (for SAC mCCR usage)
- am I missing something
- is it that I have more gas available on mCCR so I use it?

please help put me out of my misery

[daveb0000]

Originally Posted by Nick Kay

OK, so help me out here - by brain hurts thinking about this

My personal SAC for diving OC:
- relaxed / no heavy exercise : around 12-14 lpm
- stressed / heavy exercise : around 16-18 lpm

On the mCCR I have the constant flow set to about 1 lpm
- relaxed / no heavy exercise : ppO2 will rise
- stressed / heavy exercise : ppO2 will fall

So, lets assume that an O2 flow rate of 1 lpm is "my rate"
- a body metabolises 4% of air breathed, so...
- with an O2 usage of 1 lmp, my SAC would be 25 lpm

So, although (believe it or not), I'm more relaxed on mCCR than OC, my "SAC" is between 1.5 and 2.0 times that of OC

The questions therefore are:
- are my calcs wrong (for SAC mCCR usage)
- am I missing something
- is it that I have more gas available on mCCR so I use it?

please help put me out of my misery

when i did a sac test when i was doing a tdi deco course i think my stressed/heavy exercise was around 80lts a min that was hanging on to the platform and going like hell

how hard did you push yourself to get that result?

[Nick Kay]

Originally Posted by daveb0000

when i did a sac test when i was doing a tdi deco course i think my stressed/heavy exercise was around 80lts a min that was hanging on to the platform and going like hell

how hard did you push yourself to get that result?

Just using "average" figures - recorded after each dive

I know that as the mCCR is constant flow there will be "waste", i.e. I have to pre-breathe it and the O2 will still be pumping after I surface but not equivalent to 50%+ of the duration of the dive...

[daveb0000]

ahhh........ we did one at rest, motionless as if doing deco then one swimming around and then the go like hell one hanging on to the platform.

then we worked out dive plans using the figures we had.

[Rob Dobson]

On OC my SAC could be <8LPM but I used to use 12LPM for calcs.

Since switching to CCR that has gone out of the window and I think my SAC would be more like 25LPM now. You just get so used to breathing all the dive because on CCR it doesn't matter.

I think it's as simple as we lose our OC discipline...

[Nick Kay]

I just can't understand why my mCCR SAC is so much higher than my OC

OK, so say a 2L cylinder x (say) 150B available at a usage of 1 lpm = 5hrs available O2 (and the scrubber will only give me 3hrs) makes the whole thing irrelevant but why the difference????

[nigelH]

Originally Posted by Nick Kay

- a body metabolises 4% of air breathed, so...

STOP RIGHT THERE: That is the problem.

That is a number for air at surface pressure.
You can't use it for diving calculations.

You metabolise 1L of oxygen/minute, you just worked that out.
You can deliver that in 10/70 trimix at 100m or sitting on a mountain in Peru puffing and blowing thin air.

Once you have a rebreather SAC is now blindingly irrelevant until you start planning your bailout. I went to the Hebrides with the RB full, two 7L reasonably full 'bailout' tanks (O2 and 'mix) and a transfer whip. The boat filled my suit inflate tank with air every day and I cross filled the RB from the supposed bailout. I did 12 dives down to 40 meters and went home with about 90 bar in each. I can't be bothered to work out the 'SAC' because it's silly.

The reason we breathe is to move the CO2 out. The oxygen is just a lucky bonus because we can't sense oxygen. If there is no oxygen in the 'air' you are breathing you just go on happily until you fall over and die. It takes the same number of litres regardless of pressure to flush the CO2 out. That is why we use more gas on OC at depth. Go to 10 meters and we double the mass through-put for the same CO2 flushing litres but still only use that one surface litre of oxygen. Frankly we just throw more oxygen overboard. On an RB you carry on pumping the mix round and round the loop so you move those vital litres through your lungs to get the CO2 out and into the scrubber but the consumption of oxygen by your body stays the same. Hence the fixed inject on your mCCR and the regular tick-pissh on my eCCR.

[Nick Kay]

Originally Posted by Rob Dobson

On OC my SAC could be <8LPM but I used to use 12LPM for calcs.

Since switching to CCR that has gone out of the window and I think my SAC would be more like 25LPM now. You just get so used to breathing all the dive because on CCR it doesn't matter.

I think it's as simple as we lose our OC discipline...

I noticed on OC in Sharm last week
- dive 1 : 19 lpm
- dive 2 : 15 lpm
- dive 3 : 13 lpm
- dive 4 : 12 lpm

I remember thinking during/after dive 1 "I'm breathing a lot"!!!

and, dive 4 was into a current - in both directions!

[Rob Dobson]

Originally Posted by Nick Kay

I noticed on OC in Sharm last week
- dive 1 : 19 lpm
- dive 2 : 15 lpm
- dive 3 : 13 lpm
- dive 4 : 12 lpm

I remember thinking during/after dive 1 "I'm breathing a lot"!!!

and, dive 4 was into a current - in both directions!

Not very reassuring for OC bail scenarios is it?

[Nick Kay]

Originally Posted by nigelH

You metabolise 1L of oxygen/minute, you just worked that out.

But surely...

On OC, I metabolise 4% of (say) 12 lpm = 0.5 lpm of O2???


Accepted, "that its silly" to want to calculate SAC for the RB, but I'm sad enough to want to