Started doing the Multi - Filter component for dives.

Rodrigo Severo rodrigo at fabricadeideias.com
Mon Sep 29 07:57:45 PDT 2014


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.

> 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.


My 2cents,

Rodrigo


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