Started doing the Multi - Filter component for dives.

[Anton Lundin]

On 29 September, 2014 - Rodrigo Severo wrote:

> On Fri, Sep 26, 2014 at 7:47 PM, Anton Lundin <glance at acc.umu.se> wrote:
> 
> I'm sorry to get on this discussion so late.
> 
> > O2 consumption _is_ depth dependent.
> 
> For CCR gas consumption calculations you can, and should, consider the
> O2 consumption independent of depth and dependent only on the divers
> metabolism.
> 

I don't agree. But if/when someone implements this it might be a good
starting point to have a reality check to see how off the calculated
values will be from the real values.

> > To achieve a ppO2 of 1.3 on 70m in a
> > XL "bladder" from a 10/50 Dil consumes more O2 than achieving ppO2 of
> > 1.3 in a Air Dil at 30m.
> 
> The differences you mention exists but are irrelevant as the
> calculations based on the example you gave show:
> 
>       O2 H2 Depth Diluent ppO2 at depth O2 volume to achieve ppO2 at depth
> 10/50 10 50    70                  0,80                               20,0
> Air   21  0    30                  0,84                                9,2
> 
> These calculations considered a target ppO2 of 1.3 ATA and a loop
> total volume of 5l for both dives.
> 
> If you consider a typical rebreather dive of at least 120 min for both
> cases you would have a typical consumption of 120l of O2 by the divers
> metabolism (considering the average metabolism of 1l/min of O2).
> 
> So for the 70m dive you would have a total O2 consumption of 140l and
> for the 30m dive a total O2 consumption of 129,2l.
> 
> Considering a 4l O2 cylinder that would mean 35 bar and 32,3 bar
> consumed respectively. That's less than 3 bar in an ideal situation.
> Who can track that? If you consider that CCR dives usually take longer
> than 120min and that you expend some extra O2 flushing the loop near
> the surface either for O2 sensor checking on the beginning of the dive
> and to increase ppO2 on deco from 6m upwards on the end of the dive,
> you will increase the consumption that's not dependent on the dives
> depth leaving the O2 consumption depending on depth issue even less
> relevant.
> 
> > The O2 metabolism will just be proportional to the workload, but to
> > calculate the O2 metabolism from the used O2 volume we would need a
> > counter-lung+loop+scrubber+head estimate.
> 
> Again looking at your example:
> 
> Simple calculation:
> O2 consumption per minute (70m dive) (total consumption/dive duration)
> = 1,17l/min
> O2 consumption per minute (30m dive) (total consumption/dive duration)
> = 1,08l/min
> 
> We arrived at these numbers when the real value for this example is
> exactly 1l/min.
> 
> I really don't think that including counter-lung+loop+scrubber+head
> estimate is actually necessary. It would be complicate to include
> counter-lung+loop+scrubber+head estimate as what exactly would we do
> with this? Use it to calculate the extra O2 used to achieve the
> desired ppO2 on descends only? Or would we also consider loop flushes
> with O2? How many? At which depths?
> 
> By the very nature of diving calculations in general and CCR in
> particular being conservative is The Right Thing (tm) to do. Using the
> simple calculation of volume expended/dive duration we get exactly
> that with a much simpler infrastratructure.
> 


To say that all ccr dives are 120min+ is just plan wrong, and
misleading in trying to show that the metabolic part will more
significant than the part to get the ppO2 up.

If you rather would look at a 60min dive the O2 consumption would
differentiate 15% instead of 8%. I would definitly be intrested in
having 15% better accuracy in my calculated metabolic rates.


Your metabolic rate varies quite allot with your workload. If you where
sleeping you might ventilate 6L/min thus consuming about .2L O2/min up
to a really heavy workload when you ventilating 60L/min and consuming
about 2L O2/min. I would say that normal diving ventilation of about 20
L/min (Hey, i always dive dry suite in cold waters) would get you about
.6-.7 L O2/min in metabolic rate.
I would definitely be interested in having ~15% better accuracy on those
calculated numbers, to correlate with your feeling about how the dive
went.


It would be really fun to correlate some of these numbers with some from
eg. Willem's rebreather with pressure loggers on both O2 and dil.


Anyhow, there is obviously some different opinions about this. The
simple way to solve this is to implement both models and let the user
decide. Anyhow the current depth-dependent fixed-rate interpolation used
for OC gas consumption doesn't fit for any of the interpolation needed
to be done for CCR, so new models for how diluent and O2 consumption is
interpolated, needs to be implemented.
A first step would be to not apply any OC-style-interpolation at all on
CCR dives.


//Anton


-- 
Anton Lundin	+46702-161604