[Subsurface-divelog/subsurface] Plot OC-pO2 graph for SCR dives (#1166)

[Willem Ferguson]

Sorry, I missed this.
From a coding perspective: This looks good to me.
Could you explain, in which situation this "OC equivalent" becomes
relevant, why would one look at this line?
Further, why don't you directly base this onto your branch which makes
the divemode time depenent (which is in my opinion close to merging)?
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Hi Robert,

Your first question. When diving SCR rebreather, an important factor is
the oxygen drop over the mouthpiece, i.e. the difference between OC-pO2
and actually-measured pO2 in the loop of the rebreather; i.e. the
vertical difference between the red and the green graphs in my initial
screenshot. The larger the pO2 drop, the more decompression is added to
the end of the dive because less oxygen usually means more nitrogen.
However, the smaller the pO2 drop, the less efficient the rebreather is
working and the SAC rate increases. On active SCR this is controlled by
setting the rate of continuous gas addition. On pSCR this is controlled
by opening or closing the gas release valve on the counterlung. So the
vertical distance between the two graphs should ideally not be too large
or too small, or at least managed within the objectives of the dive. In
the case of a sticky or free-flowing injector, the red and green graphs
start to converge, i.e. the closer one moves to an OC situation with
little re-circulation of gas. Therefore sufficient distance between the
red and green graphs is also an indication of well-working equipment. As
indicated before, it is a situation very analogous to the setpoint of
CCR equipment which provides a reference of how efficiently the gas
addition management system works.

Your second question. This code is entirely independent of the
divemode-dependent code, since pSCR dive logs is already part of the
software.  As you know, the divemode-dependent branch primarily deals
with bailout, which is an entirely different issue. Anyway, that is how
I rationalised this to myself a month ago.

I hope I make some sense.

Kind regards,

willem