Re: Subsurface mobile general question

[Paul-Erik Törrönen via subsurface]

On 2021-11-16 20:43, Robert Helling wrote:

thanks a lot for those files. My model only involved the total O2 and
diluent use, so I don’t need those pressures as a function of time.


A ok.


Here is what I found:
The data scatters quite a bit over the dives and that does not get
better using the correction. Plus there is a clear trend in this plot
vs depth: Dives deeper than 60m have an almost constant but pretty
much twice as high O2 consumption. Does that in any way correlate with
the workload?


I'd say it correlates more with the excitement. Many of the deeper dives 
are done with a scooter, so there should be less physical exertion.


I have only had my heart rate monitor on a couple of dives (shallower 
than 60m), where the rate was high in the beginning, but as the dive 
progressed, it went lower and lower (already at bottom time) towards the 
end of the dive which is where you anyway also have the deco-stops and 
periods of inactivity.


Other than that there should not be really any workload, I tried to pick 
dives which I recall being quite effortless, but this is obviously a 
subjective view.


One thing to keep in mind is that when you ascend from deeper dives, 
your unit is in pretty much constant lower-than-setpoint-state, which 
means that it pushes continuously O2 to the loop all the while you're 
also purging that gas. It is only when you ascend to a deco stop that 
the unit actually may get the loop gas to the setpoint. The deepest 
stops are usually 1-5min before you get to the 12-6m range where they 
become longer.


I looked at a couple of the dives and plotted the in-dive O2 
cylinder-pressure vs. depth and to me it looks like the only way to 
really distinguish at any level what is the true consumption from 
O2-loss from ascend purging would be to compare bottom time consumption. 
Which means that it can only be applied to \___/-shaped dives.


Maybe this is an exercise in futility and the only practical thing to do 
is to replace OC SAC with the O2-usage (l/min) for CCR-dives when that 
data is available? Ie. there is the in-dive O2-pressure data, and 
cylinder-volume available.


Poltsi
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Re: Subsurface mobile general question

[Robert Helling via subsurface]

Hi,

> On 16. Nov 2021, at 17:29, Paul-Erik Törrönen via subsurface 
>  wrote:
> 
> 
> These should have a simple down - bottom time - up-profile: 
> https://poltsi.fi/Sukeltaminen/ccr-logs.tar.bz2 
> 
> 
> A bit of explanation about the content. The archive contains both the 
> Subsurface-converted dives, one dive in each file, as well as the source 
> file, which is in the Sentinel native format.
> 
> The reason why I included also the latter is that it contains all sample 
> values for each interval whereas the converted xml-files only contain values 
> when they've changed. It might actually be easier to read the former and use 
> it, than the latter.

thanks a lot for those files. My model only involved the total O2 and diluent 
use, so I don’t need those pressures as a function of time.

Here is what I found:
The data scatters quite a bit over the dives and that does not get better using 
the correction. Plus there is a clear trend in this plot vs depth: Dives deeper 
than 60m have an almost constant but pretty much twice as high O2 consumption. 
Does that in any way correlate with the workload?

Best
Robert


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Re: Subsurface mobile general question

[Paul-Erik Törrönen via subsurface]

On 2021-11-15 22:43, Robert Helling wrote:

yes, let’s give it a try with some real dives. Would you mind sharing
some CCR logs with me that have depth information as well as initial
and final O2 as well as diluent pressures? Ideally some that don’t
have excessive jo-jo phases with excessive buoyancy adjustments
necessary but varying over a range of depths. Let’s see if my model
gives a more constant rate of O2 consumption than simply taking the
drop in O2 pressure divided by dive time.


These should have a simple down - bottom time - up-profile: 
https://poltsi.fi/Sukeltaminen/ccr-logs.tar.bz2


A bit of explanation about the content. The archive contains both the 
Subsurface-converted dives, one dive in each file, as well as the source 
file, which is in the Sentinel native format.


The reason why I included also the latter is that it contains all sample 
values for each interval whereas the converted xml-files only contain 
values when they've changed. It might actually be easier to read the 
former and use it, than the latter.


Each sample is taken at Recint-interval. The fields are:

R   Interval #
0017Depth (1/10m)
0001something
0040pO2 (1/100bar, used in display, calculated from F, G and H)
M1  something
T12 Temp (⁰C)
A449Filter (%)
B390Battery 1 (V)
C388Battery 2 (V)
D172Dil (bar)
E160O2 (bar)
F40 O2 Cell 1 (1/100bar)
G40 O2 Cell 2 (1/100bar)
H40 O2 Cell 3 (1/100bar)
I40 SP  (1/100bar)
J0  Ceiling (m)
S0  Temp 1 (scrubber)
T0  Temp 2 (scrubber)
U0  Temp 3 (scrubber)
V0  Temp 4 (scrubber)
W0  Temp 5 (scrubber)
X0  Temp 6 (scrubber)
Y0  Temp 7 (scrubber)
Z0  Temp 8 (scrubber)
x0  CO2 (mbar)

Poltsi
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Re: Subsurface mobile general question

[Robert Helling via subsurface]

Poltsi,

> On 15. Nov 2021, at 17:58, Paul-Erik Törrönen via subsurface 
>  wrote:
> 
> Well ugh, you went to log so I can't tell anymore (my math-skill is crap 
> nowadays), but since you essentially have a model here, then one way to test 
> this would be to run it with input data of depth and measured pO2 (instead of 
> the sp), as well as the actual fO2 of diluent for a number of simple 
> descend-ascend profiled CCR-dives, and see how closely the observed 
> O2-cylinder pressure matches the one from the model.
> 
> The volume of the loop would not be a factor if all the dives are done with 
> the same rebreather model.

yes, let’s give it a try with some real dives. Would you mind sharing some CCR 
logs with me that have depth information as well as initial and final O2 as 
well as diluent pressures? Ideally some that don’t have excessive jo-jo phases 
with excessive buoyancy adjustments necessary but varying over a range of 
depths. Let’s see if my model gives a more constant rate of O2 consumption than 
simply taking the drop in O2 pressure divided by dive time.

Best
Robert


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Re: Subsurface mobile general question

[Paul-Erik Törrönen via subsurface]

On 11/15/21 13:53, Robert Helling wrote:
So my guess would be that during the ascent you dropped f times the 
amount of diluent used during the dive of O2 that was coming from the O2 
cylinder. So, to find the metabolic O2 consumption, that should be 
subtracted from the gas use from the O2 cylinder.

Does that make sense?


Well ugh, you went to log so I can't tell anymore (my math-skill is crap 
nowadays), but since you essentially have a model here, then one way to 
test this would be to run it with input data of depth and measured pO2 
(instead of the sp), as well as the actual fO2 of diluent for a number 
of simple descend-ascend profiled CCR-dives, and see how closely the 
observed O2-cylinder pressure matches the one from the model.


The volume of the loop would not be a factor if all the dives are done 
with the same rebreather model.


Poltsi
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Re: Subsurface mobile general question

[Robert Helling via subsurface]

Hi Poltsi,

> On 15. Nov 2021, at 12:18, Paul-Erik Törrönen via subsurface 
>  wrote:
> 
> 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.

thanks a lot for that explanation. That makes a lot of sense. Would you agree 
with the following summary:

Summed over the whole dive, the total amount of gas dropped from the loop 
roughly equals the total amount of diluent used (you added diluent on the 
descent to maintain loop volume and dropped it on the way up). However, the gas 
that you drop is _not_ composed as the diluent but rather as the breathing gas 
(which is determined in terms of the set point). The latter has higher O2 
fraction, so you actually also drop O2 from the O2 cylinder. How much? You 
could compute that if you knew the set point and the depth at which you drop 
the gas (which we don’t since that is not recorded). But a first guess would be 
that happens in even amounts during the ascent.

Let’s do an example calculation. Let’s say, you are using air as diluent and 
constant set point sp.

Then at ambient pressure p, the gas you are breathing contains sp bar O2 and 
p-sp bar N2. The N2 comes from the air which broad with it 21/79 (p-sp), so the 
remaining sp - 21/79 (p-sp) = (1-21/79) sp - 21/79 p come from the O2 cylinder. 
The fraction of gas dropped that came from the O2 cylinder is thus (1-21/79) 
sp/p - 21/79.

Averaging that over the range pmax (deepest point) to psurface gives the 
fraction

f = (1-21/79) sp log(pmax/psurface)/(pmax - psurface)

So my guess would be that during the ascent you dropped f times the amount of 
diluent used during the dive of O2 that was coming from the O2 cylinder. So, to 
find the metabolic O2 consumption, that should be subtracted from the gas use 
from the O2 cylinder.

Does that make sense?

Best
Robert


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Re: Subsurface mobile general question

[Paul-Erik Törrönen via subsurface]

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):


scrubber='13.6 %' />
sensor3='1.22 bar' />



sensor3='1.11 bar' />
sensor3='1.11 bar' />





sensor2='1.15 bar' sensor3='1.18 bar' />

sensor3='1.19 bar' />
sensor3='1.15 bar' />
scrubber='27.9 %' />
scrubber='28.8 %' />
scrubber='29.9 %' />
scrubber='31.7 %' />
scrubber='32.2 %' />
batteryv2='3.8 V' />



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
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Re: Subsurface mobile general question

[Jan Mulder via subsurface]

Robert,

On 15-11-2021 10:33, Robert.Helling wrote:

Jan,

On 15. Nov 2021, at 09:11, Jan Mulder via subsurface 
 wrote:


The idea that there is a ratio between diluent use and O2 use is 
flawed. The main driver for O2 use is time and workload, and for 
diluent its the dive profile.





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. I guess you would want to know how much O2 your body 
consumes during the dive. The true number would be the sum of the O2 
from the O2 cylinder plus the O2 contained in the diluent minus the O2 
that was released from the loop to the water.  How bad is the 
approximation that takes into account only the O2 from the O2 
cylinder? If it’s a few percent, I would make sense to compute and 
display that number. If the error is bigger probably less so.


I think that the approximation to only take into account the O2 from the 
O2 cylinder is a good one with only small error.


I dive a mCCR (Kiss Classic) and adding diluent to the loop is a very 
explicit action by the diver. I only add diluent to the loop when 
descending (and than only when the loop volume is too low, and the pO2 
is as high as I want it to be). In other words. The diluent plays a very 
limited role in the pO2 management.


In the mCCR, the O2 from the O2 cylinder is added automatically using a 
bleed orifice that constantly puts the amount of O2 into the loop that 
is close to your metabolized O2. Obviously, the metabolized O2 also 
depends on workload, but heavy workload on any CCR is a bad idea (as the 
scrubber will not be able to get all the produced CO2 out of the mix). 
So, the mCCR diver sets the amount of O2 bleeded into the loop based on 
the "normal" workload. And a nice side effect of this, is that in a very 
relaxed deco phase of the dive with low workload, the pO2 rises.


And I agree with Anton that a model that can determine how much loop 
volume is metabolized and how much is wasted can be interesting, but 
primarily from academic point of view.


--jan
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Re: Subsurface mobile general question

[Anton Lundin via subsurface]

On 15 November, 2021 - Robert.Helling via subsurface wrote:

> Jan,
> 
> > On 15. Nov 2021, at 09:11, Jan Mulder via subsurface 
> >  wrote:
> > 
> > The idea that there is a ratio between diluent use and O2 use is flawed. 
> > The main driver for O2 use is time and workload, and for diluent its the 
> > dive profile. 
> > 
> > 
> 
> 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. I 
> guess you would want to know how much O2 your body consumes during the dive. 
> The true number would be the sum of the O2 from the O2 cylinder plus the O2 
> contained in the diluent minus the O2 that was released from the loop to the 
> water.  How bad is the approximation that takes into account only the O2 from 
> the O2 cylinder? If it’s a few percent, I would make sense to compute and 
> display that number. If the error is bigger probably less so.

It could be a interesting thing to model the loop volume and based on
cylinder pressure and depth "figure out" how much o2 was metabolized how
much was wasted. The same goes for dil, how optimally it was used. I'm
not sure anything of this is of any kind of practical value, but it
could be a fun exercise in aligning real data with models and track the
discrepancies.


I just know that my RB friends laugh at me when seeing how much dil i
wasted when I did some test dives on RB with them. Proper loop volume
management is quite tricky as a OC diver =)


//Anton


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Re: Subsurface mobile general question

[Robert.Helling via subsurface]

Jan,

> On 15. Nov 2021, at 09:11, Jan Mulder via subsurface 
>  wrote:
> 
> The idea that there is a ratio between diluent use and O2 use is flawed. The 
> main driver for O2 use is time and workload, and for diluent its the dive 
> profile. 
> 
> 

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. I 
guess you would want to know how much O2 your body consumes during the dive. 
The true number would be the sum of the O2 from the O2 cylinder plus the O2 
contained in the diluent minus the O2 that was released from the loop to the 
water.  How bad is the approximation that takes into account only the O2 from 
the O2 cylinder? If it’s a few percent, I would make sense to compute and 
display that number. If the error is bigger probably less so.

Best
Robert

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Re: Subsurface mobile general question

[Jan Mulder via subsurface]

On 15-11-2021 08:33, Robert Helling via subsurface wrote:


As a ball park approximation, many CCR dives (up to 65m) are done 
using air as a diluent, supplemented with oxygen from the O2 cylinder 
to make up for oxygen consumed as well as for higher O2 levels during 
shallower deco.





But what is the ratio of diluent use to O2 use? That test dive uses 
123 liters of O2 and 100 liters of air.


The idea that there is a ratio between diluent use and O2 use is flawed. 
The main driver for O2 use is time and workload, and for diluent its the 
dive profile.


--jan
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Re: Subsurface mobile general question

[Robert Helling via subsurface]

Hi Willem,

thank you for your comments.

> On 15. Nov 2021, at 08:18, Willem Ferguson via subsurface 
>  wrote:
> 
>> 
>> I meant more the other way around: If you diluent also contains O2, you 
>> underestimate the O2 consumption if you only use the pressure drop in the O2 
>> cylinder.
> The O2 consumption under CCR is not a dive characteristic because the diver 
> can bail out to OC. So O2-consumption would be a cylinder-level 
> characteristic, associated with cylinder 0 ?? (i.e. the diluent cylinder 
> because as far as I can remember the O2 cylinder does not explicitly exist in 
> the Subsurface CCR dive log). In pSCR bailout is often part of the normal 
> dive procedure during deco.
> 
Bailout is of course a special case. And yes, it should be handled dive segment 
by dive segment like an OC dive with several cylinders.

PSCR is again a completely different thing. I would argue it’s closer to OC as 
the absolute consumption is also depth dependent. Are there any significant 
number of divers still using those? All I knew personally moved on to CCR. I 
don’t really know about how Subsurface handles the two types of cylinders for a 
CCR. Amongst those dives we use for testing is test40.xml which is a CCR dive 
with both an air diluent as well as oxygen cylinder and gas is used from both. 
I don’t know if that is typical. Do we have magic cylinder numbers like 0 that 
indicate diluent? I wasn’t aware of that.

>> 
>> So here is a question I have: For a typical CCR dive, what is the ratio 
>> between used O2 and used diluent? I understand this depends on how often 
>> descend and how often you flush but what is a ballpark type figure?
>> 
> As a ball park approximation, many CCR dives (up to 65m) are done using air 
> as a diluent, supplemented with oxygen from the O2 cylinder to make up for 
> oxygen consumed as well as for higher O2 levels during shallower deco.
> 
> 

But what is the ratio of diluent use to O2 use? That test dive uses 123 liters 
of O2 and 100 liters of air.

Best
Robert


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Re: Subsurface mobile general question

[Willem Ferguson via subsurface]

On 2021/11/15 00:07, Robert Helling via subsurface wrote:

Hi Poltsi,

On 14. Nov 2021, at 22:44, Paul-Erik Törrönen via subsurface 
> wrote:


When CCR, you're mainly interested in the consumption of O2 as noted 
previously. The diluent is not really telling since it is mainly a 
function of your profile as you use diluent to equalize the pressure 
in the loop. A serrated profile (lot's of ups and down) == large 
volume of diluent used because of addition to loop when going down, 
then dumping it when ascending.


I can see that the amount of O2 used divided by total dive time could 
be of interest. But it’s definitely something else than SAC and thus 
should not be mixed with it. For example, in any statistics when you 
have both CCR and OC dives, it would make zero sense to combine the 
two. So even if we computed this number, it should not be displayed as 
SAC or stored as such. Let alone the number with an ambient pressure 
correction like the SAC makes zero sense for a CCR.




Of course this also happens to O2 also, but what would be interesting 
to see is how/when your consumption of O2 changes due to increased work.


I meant more the other way around: If you diluent also contains O2, 
you underestimate the O2 consumption if you only use the pressure drop 
in the O2 cylinder.


The O2 consumption under CCR is not a dive characteristic because the 
diver can bail out to OC. So O2-consumption would be a cylinder-level 
characteristic, associated with cylinder 0 ?? (i.e. the diluent cylinder 
because as far as I can remember the O2 cylinder does not explicitly 
exist in the Subsurface CCR dive log). In pSCR bailout is often part of 
the normal dive procedure during deco.




So here is a question I have: For a typical CCR dive, what is the 
ratio between used O2 and used diluent? I understand this depends on 
how often descend and how often you flush but what is a ballpark type 
figure?


As a ball park approximation, many CCR dives (up to 65m) are done using 
air as a diluent, supplemented with oxygen from the O2 cylinder to make 
up for oxygen consumed as well as for higher O2 levels during shallower 
deco.


Kind regards,

Willem



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Re: Subsurface mobile general question

[Robert Helling via subsurface]

Hi Poltsi,

> On 14. Nov 2021, at 22:44, Paul-Erik Törrönen via subsurface 
>  wrote:
> 
> When CCR, you're mainly interested in the consumption of O2 as noted 
> previously. The diluent is not really telling since it is mainly a function 
> of your profile as you use diluent to equalize the pressure in the loop. A 
> serrated profile (lot's of ups and down) == large volume of diluent used 
> because of addition to loop when going down, then dumping it when ascending.

I can see that the amount of O2 used divided by total dive time could be of 
interest. But it’s definitely something else than SAC and thus should not be 
mixed with it. For example, in any statistics when you have both CCR and OC 
dives, it would make zero sense to combine the two. So even if we computed this 
number, it should not be displayed as SAC or stored as such. Let alone the 
number with an ambient pressure correction like the SAC makes zero sense for a 
CCR.

> 
> Of course this also happens to O2 also, but what would be interesting to see 
> is how/when your consumption of O2 changes due to increased work.

I meant more the other way around: If you diluent also contains O2, you 
underestimate the O2 consumption if you only use the pressure drop in the O2 
cylinder.

So here is a question I have: For a typical CCR dive, what is the ratio between 
used O2 and used diluent? I understand this depends on how often descend and 
how often you flush but what is a ballpark type figure?

I vaguely recall, there was a discussion on this list a while ago is there is 
any hope of doing a decent prediction of how much gas is used in a CCR dive but 
if I remember correctly the consensus was no, nothing that would be of any use.

Best
Robert


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Re: Subsurface mobile general question

[Paul-Erik Törrönen via subsurface]

On 2021-11-14 05:01, Robert.Helling via subsurface wrote:

OK, that guess was wrong. Our SAC calculation does not notice it’s a
CCR dive. That is of course wrong. Created a PR that makes us refuse
to compute a SAC for CCR dives.


When CCR, you're mainly interested in the consumption of O2 as noted 
previously. The diluent is not really telling since it is mainly a 
function of your profile as you use diluent to equalize the pressure in 
the loop. A serrated profile (lot's of ups and down) == large volume of 
diluent used because of addition to loop when going down, then dumping 
it when ascending.


Of course this also happens to O2 also, but what would be interesting to 
see is how/when your consumption of O2 changes due to increased work.



It’s in the middle of the night and I could not test this since I
don’t have any CCR dive data.


I can send some CCR-dive logs your way if you still need them.

Poltsi
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Re: Subsurface mobile general question

[Robert.Helling via subsurface]

> On 14. Nov 2021, at 03:42, Robert.Helling via subsurface 
>  wrote:
> 
> my initial reaction is that there shouldn’t be a calculated SAC value for 
> rebreather dives. Or at least, if we computed something for O2, that should 
> be a completely different calculation as it would be independent of ambient 
> pressure. But I have to look at the source code to see what we do there 
> really.
> 

OK, that guess was wrong. Our SAC calculation does not notice it’s a CCR dive. 
That is of course wrong. Created a PR that makes us refuse to compute a SAC for 
CCR dives.

It’s in the middle of the night and I could not test this since I don’t have 
any CCR dive data.

Best
Robert___
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Re: Subsurface mobile general question

[Robert.Helling via subsurface]

Hi,

> On 14. Nov 2021, at 02:40, Dirk Hohndel via subsurface 
>  wrote:
> 
>> Based on my manual calculations (discounting compresability and assuming 
>> ideal gas laws), I calculated 1.13 l/min but, Subsurface is showing 3.3 
>> l/min.
>> 
>> I am normally around 0.65 L/min but, I did an O2 flush that skews this dive 
>> some.
>> 
>> Dive time: 35 mins
>> Average depth 34 feet (CCR ~ does not matter)
>> Oxygen consumed: 492 psi
>> Cylinder: 3L @ 3442 psi (237.3 bar)
>> 
>> * I was working out a new gear configuration or else I would have not been 
>> on the rebreather for such a short dive.
> 
> Maybe Robert has some comments? Or Linus?

my initial reaction is that there shouldn’t be a calculated SAC value for 
rebreather dives. Or at least, if we computed something for O2, that should be 
a completely different calculation as it would be independent of ambient 
pressure. But I have to look at the source code to see what we do there really.

Best
Robert___
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Re: Subsurface mobile general question

[Dirk Hohndel via subsurface]

Sending anything with the exception of cloud backend stuff to "only me" is 
really not helpful. I cannot possibly do support by myself for an app with at 
this point about twenty thousand active users...

> On Nov 13, 2021, at 2:16 PM, rick.warren4 wrote:
> 
> I have a general question on SAC and rebreathers. My assumption is that SAC 
> with a rebreather would be based on the oxygen consumption and not factor in 
> diluent.  Is that incorrect?

I have no idea. I have never used a rebreather, I have never thought through 
the logic of all that.

> Based on my manual calculations (discounting compresability and assuming 
> ideal gas laws), I calculated 1.13 l/min but, Subsurface is showing 3.3 l/min.
> 
> I am normally around 0.65 L/min but, I did an O2 flush that skews this dive 
> some.
> 
> Dive time: 35 mins
> Average depth 34 feet (CCR ~ does not matter)
> Oxygen consumed: 492 psi
> Cylinder: 3L @ 3442 psi (237.3 bar)
> 
> * I was working out a new gear configuration or else I would have not been on 
> the rebreather for such a short dive.

Maybe Robert has some comments? Or Linus?

/D

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