Handling of cylinder pressure data within a dive.

[Willem Ferguson]

I am looking at dive.c and the way the sample struct is used and I hope 
you can explain something to me. It has two variables pertinent to the 
sensor(s) of cylinder pressure: /sensor/ and /cylinderpressure/. The way 
I read it is that the /sensor/ variable identifies the pressure 
transmitter (a simple integer 1,2,...). This means that the 
/cylinderpressure/ value is qualified by the /sensor/ variable.

This means that, if, in open circuit diving, one has a second cylinder 
(stage, deco, etc) with a pressure sensor, then the appropriate pressure 
data are stored with each sample, and, in a sequential way, one can get 
the pressure data for each cylinder fitted with a pressure transducer.  
In open circuit, the pressure changes occur sequentially and Subsurface 
correctly plots the cylinder pressures in a sequential way because only 
changes in pressure is recorded in the dive log.

In a closed circuit situation, gas is being used from both the back 
cylinders simultaneously. Pressure change occurs continuously in both of 
the cylinders and, if one uses the variables /sensor/ and 
/cylinderpressure/, the pressures for the two cylinders are still being 
recorded accurately, even though both cylinders are discharging at the 
same time. This would mean that one does not need different variables in 
the dive structure to store the gas pressure in both the back cylinders.

Now, the CCR diver uses her baleout cylinder which, coincidentally, also 
has a pressure transducer. This cylinder would have /sensor/ value of 3, 
the 3rd cylinder. When the pressure changes take place in the baleout 
cylinder, these are also recorded. But they still have the appropriate 
time stamps.

If I understand it correctly, the existing /sample/ structure can 
therefore handle multiple cylinders, even if two of them discharge 
simultaneously.

There is one practical problem. What to do if a single sample contains 
/cylinderpressure/ data reflecting a simultaneous change in gas pressure 
in both cylinders, since there is only one index variable in the sample 
structure. One of the tasks with writing code for rebreathers is to 
solve this problem.

For non-instrumented cylinders, the procedure is simple, as long as gas 
changes can clearly be identified.

Is this understanding of the issue correct?
Kind regards,
willem







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