Two more rebreather patches

Rodrigo Severo rodrigo at
Mon Oct 27 07:39:04 PDT 2014

On Mon, Oct 27, 2014 at 12:05 PM, Robert Helling <helling at> wrote:
> On 27.10.2014, at 12:10, Rodrigo Severo <rodrigo at> 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.

The method I'm proposing for diluent consumption calculation doesn't
need any extra parameter.

I'm talking about the situations when we have initial and end pressure
of gas (in this case diluent) and we want to calculate SAC for its

The current method to calculate gas consumption for OC dives on the
above situation would distribute the gas consumption on all samples
taking in consideration the depth so that when deeper, we suppose that
more gas is spent.

What I'm proposing for diluent is the same exact method as it's used
for OC but instead of distributing on all samples, only distribute it
on the samples that repreesent a descend.

We have to understand what exactly we will consider as descend as you
mentioned below. Let's hold this thought for a moment.

> If we track
> O2 consumption as well, we need the metabolism rate for that as well

Again I don't think we need any extra parameter.

Again I'm talking about the situations when we have initial and end
pressure of gas (in this case oxygen) and we want to calculate SAC for
its cylinder.

Here the calculation is the simpliest of all: just take the amount of
gas (oxygen) consumed during the dive (final pressure - initial
pressure) and divide it for the duration of the dive not taking in
consideration depth variation at all.

The result of this simple calculation would be the metabolism rate
itself. In fact it would be a little overestimated figure as there are
other factors contibuting to oxygen spend but the other factors are
much less relevant than the actual oxygen metabolized by the diver.

> 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.

Back to the topic of what actually represents a descend, I believe you
are entirely correct that we can't naivelly consider every change to a
deeper value as an actual descend.

But I believe this can be easily solved by only considering actual
descend when there is a minimum variation since the last considered
depth. I would proposed any >= 1 m variation as actual descend. We can
use a larger value if necessary.

One extra care would have to be taken here: we shouldn't compare each
sample depth with the previous sample depth. We should compare each
sample depth with the depth of the last sample we considered as an
actual descend.



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