[PATCH 2/2] Correctly calculate SAC rate in the presense of surface events

[Dirk Hohndel]

Lutz Vieweg <lvml at 5t9.de> writes:

> On 02/24/2013 07:59 PM, Linus Torvalds wrote:
>> This assumes that you are not breathing your cylinders while at the
>> surface, which may or may not be correct, but is usually the right
>> thing.
>
> Hmm. I would rather not speculate whether someone was breathing from
> his cylinder while being at the surface.
>
> To me it sounds more plausible to completely disregard all tank-pressure
> and time measurements that were taken at the surface, and compute
> SAC only from measurement periods that are known to have been spend
> under the surface.
>
> Otherwise, all kinds of funny stuff - such as testing or unintentional
> free-flows, or some heavy breathing done while climbing a boat ladder
> may or may not become part of the SAC calculation, depending also
> on the dive computers idea of "what is surface time, how long are
> samples still recorded, etc.".

Well - we are only dealing with the parts here that show depth = 0. So
if the dive computer stopped recording, we don't see that, anyway.

But you make a good point here in that we could use the tank pressure
data (if we have it) to improve our guess. We should remove air that was
used while on the surface from our calculations.

>> Regardless, we're better off giving a conservative (higher) SAC
>
> In comparison to the huge SAC influencing factors that neither
> Subsurface nor the dive computer nor the user can predict for future
> dives, I think it would be wrong to claim that Subsurface could
> actually make a "conservative" statement.
> Therefore, I think reproduceability and avoidance of odd side effects
> (like surface free flows) for the SAC calculation are the best that
> Subsurface could offer.

One of my favorite oddities is the influence of temperature. Especially
if there is a huge gradient in temperature, and suddenly pressure in the
tank goes up - that's especially visible if you do multi tank dives and
have a sensor connected to a tank that you are not breathing from...

/D