Poseidon CCR dives: Nitrogen loading

[Willem Ferguson]

On 06/11/2014 11:33, Robert Helling wrote:
> after getting a good night of sleep, I am reading this again (several 
> times indeed) and I am not sure I understand what you are trying to 
> tell me.
>
Let me see if I can improve. It was late in the evening for me as well.

> I understand that for a CCR segment there are two cylinders in use 
> simultaneously, diluent and O2. For the deco calculation to work, it 
> has to know that the diluent is (and figure the actual breathing gas 
> from that and ambient pressure and set point).
Correct
>
> The other question is what to plot as a pressure (as in cylinder 
> pressure, not partial pressure) graph. Ideally both, but at the moment 
> we don’t do that. So, at least for the .txt import it seems we are 
> plotting diluent pressure. Is that what you are complaining about?
I was not intending to complain at all :-) .   No, we plot the Oxygen 
cylinder pressure because that is the default cylinder (cylinderindex 
=0) that is plotted by Subsurface, as is done for all OC dives that do 
not have cylinder changes. The diluent cylinder pressure still needs to 
be added.
>
> MY question is: Where does this information (and also the information 
> about the O2 cylinder pressure) as a function of time come from?
These are pressure reading taken directly by the Poseidon computer. In 
the xml contains samples with attributes of pdiluent and of pressure. 
Look, you will find them in the xml. The attributes are a bit sparse 
compared to the other attribute assignments, perhaps recorded at 
pressure drops of roughly around 8 bar. Remember the ball park pressure 
drop is around 40 bar per dive.
> Is that in the .txt/.csv as as far as I can see, it is not in the .xml.

See above.
> Or do we only have start and and pressures for the cylinders (again 
> they seem to be missing for the xml and I don’t know about the 
> others). In that case, for OC, cylinder pressures to be plotted get 
> interpolated assuming a constant SAC. But for CCR that (as was 
> discussed at length before) does not make sense. For O2 one might try 
> a depth independent consumption (leading to a linear plot of O2 
> cylinder pressure) but for diluent it is ascent/descent dependent in a 
> complicated way that we have not yet figured out how to model, so at 
> the moment, any interpolation would be bogus.
>
> So, what to you expect to see and what are you seeing instead?
>
At the moment a graph of the O2 cylinder pressure, based on the info below.
I hope that Tomaz will be able to add a second graph of diluent cylinder 
pressures.
> Best
> Robert
<sample time="05:08" depth="10.0 m" pressure="176.0 bar"/>
<sample time="12:19" depth="16.5 m" pressure="171.0 bar"/>
<sample time="22:30" depth="34.0 m" pressure="167.0 bar"/>
<sample time="32:03" depth="26.0 m" pressure="161.0 bar"/>
<sample time="45:21" depth="18.5 m" pressure="151.0 bar"/>
<sample time="46:10" depth="18.0 m" pressure="150.0 bar"/>
<sample time="57:14" depth="00.0 m" pressure="143.0 bar"/>
<sample time="59:43" depth="00.0 m" pressure="141.0 bar"/>

<sample time="03:36" depth="09.5 m" pdiluent="172.0 bar"/>
<sample time="06:45" depth="13.5 m" pdiluent="168.0 bar"/>
<sample time="11:20" depth="19.5 m" pdiluent="164.0 bar"/>
<sample time="13:14" depth="18.5 m" pdiluent="163.0 bar"/>
<sample time="19:31" depth="33.5 m" pdiluent="151.0 bar"/>
<sample time="20:11" depth="34.5 m" pdiluent="151.0 bar"/>
<sample time="36:15" depth="28.0 m" pdiluent="145.0 bar"/>

The start pressures are in the .txt control file.and in the notes node 
of the xml.
The end pressures are calculated by Subsurface by interpolation.

With Poseidon CCR dives, cylinder 0 is assumed to be the oxygen cylinder and cylinder 1 is
assumed to hold the diluent gas. If baleout takes place, it would be to
cylinder 2 and onwards. The oxygen pressures (mbar) are found in
plot_data.pressure[2], defined in profile.h. pressure[0] has the raw
sensor pressure data while pressure[1] contains the interpolated values
between measured data points. Similarly, the pressure of the diluent gas
is held in plot_data.diluentpressure[2] (raw measurements in
diluentpressure[0] and interpolations in diluentpressure[1].

The debug function that I referred to yesterday prints all the calculated values
of pressure to a file. I attach the pressures debug output for the current dive. It is in
a format that I can easily access with R.

Is this a bit better, I hope??
Kind regards,
willem
  


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.subsurface-divelog.org/pipermail/subsurface/attachments/20141106/f72a9ae9/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: debug_print_profiledata.dat
Type: application/ms-tnef
Size: 366704 bytes
Desc: not available
URL: <http://lists.subsurface-divelog.org/pipermail/subsurface/attachments/20141106/f72a9ae9/attachment-0001.bin>