Two more rebreather patches

[Paul Sargent]

On 27 Oct 2014, at 17:56, Rodrigo Severo <rodrigo at fabricadeideias.com> wrote:

>> For example: A one hour dive should be 20bar (60 litres) out of a standard 3
>> litre cylinder - just calculating 1 litre a minute. That's just way off. I
>> always use about ~50 bar on a ~40 metre dive, so I tend to make sure I've
>> got at least 100 bar when I jump in.
>> 
>> (or have a missed a different method of calculating things in the various
>> mails?)
> 
> The method of calculation is just that but, as I see things, it would
> happen the other way around:
> 
> After this dive Subsurface would show you that your oxygen SAC is 2,5
> l/min (50 bar of a 3 l cylinder used on an hour dive). So on your next
> planning you should use 2,5 l/min for oxygen and not the usual 1
> l/min.

The problem here is that going from a one hour to a two hour dive doesn’t double my O2 use. I might go from 50 bar to 70 bar (i.e. Same use for the ascent aspects, another 20 bar for the extra hour).

To me it sounds to me like we’d need a SAC component (litres/min) and an ascent component (litres/metre.bar). Diluent would just have a descent component (litres/metre.bar). In the example above reasonable figure might be:

    - O2 SAC: 1 litre/min
    - O2 Ascent: 
           30bar * 3l = 90l 
           90l / 40 metres = 2.25 litres/metre
           2.25l / 2.5 bar (Avg. Pressure during ascent) = 0.9l / metre.bar
    - Diluent Descent:
           50bar * 3l = 150l
           150l / 40 metres = 3.75l / metre
           3.75l / 2.5 bar (Avg. Pressure during descent) = 1.5l / metre.bar

Paul

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