Subsurface mobile general question

[Paul-Erik Törrönen]

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 
diluent.

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 
here.

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 
(with stops):

<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 
bar' />
<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 
bar' />
<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?

Poltsi