Subsurface mobile general question
poltsi at poltsi.fi
Mon Nov 15 03:18:09 PST 2021
On 2021-11-15 11:33, Robert.Helling via subsurface wrote:
> this is clear. I was asking to get an understanding how bad the error
> is to ignore that O2 is not only contained in the O2 bottle but also
> in diluent.
Well the thing is that you really do not consume any of the O2 in the
The reason is that the diluent should always have a lower ppO2 at
maximum dive depth than your setpoint, which means that you are always
enriching the loop gas with your O2 cylinder.
Consider this case:
You plan a dive to 47m and your rule is to have a max. 1.0 ppO2 for
diluent => you use air (21%O2, let's not consider trimix now)
Your CCR is set to maintain a setpoint of 1.2, incrementally from a base
of 0.7 at surface to 1.2 at 18m depth.
So already at surface the CCR will be adding O2 into the loop in order
to maintain the surface ppO2 and since you're breathing in the case,
adds it continually. That added O2 is what you're actually consuming.
Even at 47m depth, you're consuming the added O2 from the O2-tank as the
CCR is maintaining the 0.2 difference between the setpoint of the CCR
and the diluent ppO2.
The only situation where you would defacto consume the diluent O2 is if
the ppO2 of the diluent is higher than the max depth of the dive. This
would be a really, *really* badly planned dive.
Of course if you really want to be pedantic, then yes, you're consuming
the O2-molecules from the diluent, but that's not what we want to track
Ascending from the dive, you do not use any diluent. What happens is the
gas volume in the loop expands, which you purge. In addition the ppO2 in
the loop goes down, so the CCR pumps in more O2 from the tank to
maintain the setpoint (which usually at this point is held at the 1.2
until at least the 6m depth. And because this happens already at depth,
it will happen continuously.
To give an indication on how much O2 is lost during ascend, here are
some samples with the O2-pressure data from an ascend from 45m to 6m
<sample time='25:45 min' depth='43.97 m' o2pressure='117 bar'
scrubber='13.6 %' />
<sample time='27:20 min' depth='43.88 m' po2='1.19 bar' o2pressure='116
bar' batteryv2='3.82 V' sensor1='1.16 bar' sensor2='1.2 bar'
sensor3='1.22 bar' />
<sample time='30:30 min' depth='42.94 m' po2='1.17 bar' o2pressure='115
bar' batteryv2='3.81 V' sensor2='1.18 bar' sensor3='1.19 bar' />
<sample time='31:20 min' depth='38.72 m' po2='1.2 bar' o2pressure='114
bar' sensor1='1.16 bar' sensor2='1.21 bar' sensor3='1.23 bar' />
<sample time='31:50 min' depth='35.53 m' po2='1.11 bar' o2pressure='113
bar' scrubber='16.6 %' sensor1='1.09 bar' sensor2='1.11 bar' />
<sample time='32:35 min' depth='30.56 m' po2='1.08 bar' o2pressure='112
bar' scrubber='17.7 %' sensor1='1.06 bar' sensor2='1.09 bar'
sensor3='1.11 bar' />
<sample time='33:20 min' depth='28.22 m' po2='1.09 bar' o2pressure='111
bar' pco2='0.0027 bar' batteryv1='3.83 V' sensor1='1.07 bar'
sensor3='1.11 bar' />
<sample time='33:35 min' depth='27.09 m' po2='1.21 bar' o2pressure='110
bar' sensor1='1.18 bar' sensor2='1.22 bar' sensor3='1.25 bar' />
<sample time='34:10 min' depth='23.81 m' po2='1.09 bar' o2pressure='109
bar' pco2='0.0024 bar' batteryv2='3.8 V' sensor3='1.11 bar' />
<sample time='34:55 min' depth='20.25 m' po2='1.09 bar' o2pressure='108
bar' scrubber='21.5 %' batteryv2='3.8 V' sensor2='1.1 bar' sensor3='1.12
<sample time='35:05 min' depth='19.5 m' o2pressure='107 bar' />
<sample time='36:10 min' depth='16.88 m' po2='1.15 bar' o2pressure='106
bar' scrubber='23.6 %' batteryv2='3.8 V' sensor1='1.12 bar'
sensor2='1.15 bar' sensor3='1.18 bar' />
<sample time='37:10 min' depth='15.47 m' po2='1.17 bar' o2pressure='105
bar' sensor1='1.15 bar' sensor2='1.17 bar' sensor3='1.19 bar' />
<sample time='37:40 min' depth='14.81 m' o2pressure='104 bar'
sensor3='1.19 bar' />
<sample time='38:10 min' depth='13.13 m' po2='1.12 bar' o2pressure='103
bar' scrubber='26.3 %' sensor1='1.1 bar' sensor2='1.13 bar'
sensor3='1.15 bar' />
<sample time='39:05 min' depth='12.38 m' o2pressure='102 bar'
scrubber='27.9 %' />
<sample time='40:10 min' depth='12.19 m' o2pressure='101 bar'
scrubber='28.8 %' />
<sample time='41:15 min' depth='11.16 m' o2pressure='100 bar'
scrubber='29.9 %' />
<sample time='41:55 min' depth='9.38 m' o2pressure='99 bar'
scrubber='31.7 %' />
<sample time='42:25 min' depth='8.91 m' o2pressure='98 bar'
scrubber='32.2 %' />
<sample time='43:30 min' depth='7.59 m' o2pressure='96 bar'
batteryv2='3.8 V' />
<sample time='44:50 min' depth='6.84 m' po2='1.16 bar' o2pressure='95
bar' scrubber='35.9 %' sensor2='1.17 bar' sensor3='1.18 bar' />
<sample time='45:50 min' depth='6.38 m' po2='1.17 bar' o2pressure='94
bar' batteryv2='3.8 V' sensor1='1.15 bar' sensor2='1.18 bar' />
<sample time='46:20 min' depth='6.47 m' po2='1.18 bar' o2pressure='93
bar' scrubber='38 %' sensor1='1.16 bar' sensor2='1.18 bar' sensor3='1.2
<sample time='47:10 min' depth='5.91 m' o2pressure='92 bar'
scrubber='39.3 %' />
So ~21min 30s during which the O2 cyl (2l) pressure dropped 25bar (50l).
My metabolic rate for O2 consumption during the bottom time was
~0.6l/min so that means that I vented out about 46l of O2 during the
ascend. More realistically, the metabolic rate would be something like
1l/min during ascend due to all the excitement, so ~30l of O2 vented
out/lost from the O2-cylinder.
Does that help you?
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