Two more rebreather patches

[William Perry]

> On Oct 27, 2014, at 10:05 AM, Robert Helling <helling at atdotde.de> wrote:
> 
> 
> On 27.10.2014, at 12:10, Rodrigo Severo <rodrigo at fabricadeideias.com <mailto:rodrigo at fabricadeideias.com>> wrote:
> 
> Rodrigo,
> 
>> AFAICT the consensus about diluent consumption  is that it should be
>> calculated as if spend only during descends.
>> 
>> In other words, use the same method to calculate for OC, but consider
>> that during ascends and when maintaining depth the consumption is
>> zero. Distribute all consumption on descends taking in consideration
>> depths.
> 
> 
> what I meant was: I did not yet have time to code it. You are more than welcome to send a patch. Let me mention a few thoughts:
> 
> Not diving rebreathers myself, I can only guess how that would be computed. I would think that when descending from depth d1 to d2 with associated ambient pressures depth_tp_mbar(d1) and depth_to_mbar(d2) and the total loop volume (the actual rebreather loop plus your lung volume) is V than we need a total volume of
> 
> (depth_to_mbar(d2) / depth_to_mbar(d1) - 1) * V
> 
> of dillutant. This means that at least V is another parameter from the preferences (actually per dive) that the user needs to supply. If we track O2 consumption as well, we need the metabolism rate for that as well (but we have that for PSCR anyway). This needs UI and we should be a bit careful not having too many options/parameters.
> 
> What I am more worried about is that “dillutant is consumed on descent” only considers significant descents. So we should not just compare the current depth with that of the previous sample as otherwise we would pick up all the noise from the depth sensor. Just as an example: The depth resolution of my Vytec is one foot (or 30cm). This means that even when I think I am at constant depth the reading in the samples tends to oscillate by that amount due to sensor noise. That mean that on average on every other sample (i.e. every 20s) I am descending by 30cm. Over a one hour dive this sums up 54m of completely bogus descent (which if taken into account would completely ruin any dillutant consumption calculation as this is likely the order of magnitude of the total descent. To avoid this amplification of noise we have to take into account only “real” descends that are big enough in magnitude to make the CCR actually add dillutant which it does not when you hover around one depth and only oscillate slightly.

I just finished  by CCR training down in Bonaire last week so I don’t have a lot of experience, but I did find that minor depth changes did not impact the loop volume enough for me to feel the need to add diluent.  My inhale was cut very slightly short (I was diving with the auto-diluent-valve off after the initial big descent).  This would be equivalent to the occasional mask clear at depth.  If you are doing a ton of 10 meter ascents/descents in a cave system as it winds around then those will add up quickly, but 1-2 footers is noise.

Would tracking only the size of the counter lungs be sufficient?

-bill
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