Subsurface not working out SAC on sidemount dives

[Michael Andreen]

Hi,

Actually, I thought I had seen the same behavior as Andrew, so I did a simple test. Taking a 
single tank dive and just adding a 2nd tank with same size and similar pressure change 
results in double SAC for the dive, even if the new tank is not marked as used.

Same behaviour with 4.8.6 that I happened to have an AppImage of.

/Michael


On Saturday 5 November 2022 13:55:05 CET Willem Ferguson via subsurface wrote:
> Hi Andrew,
> 
> I also dive sidemount, so I have had to work through these issues. You 
> need to consider that the SAC rate that Subsurface calculated for you 
> specifically are not representative of your dive. No problem with the 
> Subsurface software at all: it might be a case of garbage in, garbage 
> out. Let me explain. For Subsurface to calculate SAC correctly, it needs 
> to know when you are using each of the two cylinders. Without that 
> information it cannot do the SAC calculation. Since you are not 
> recording the change from using one cylinder to the other cylinder, it 
> assumes you are diving using the first cylinder for the whole dive. So 
> it records your total gas volume used from that cylinder, taking into 
> account the whole duration of the dive. Obviously this is not accurate. 
> The fact that the second cylinder in your cylinder table is not striked 
> out with a red line means that Subsurface did not record you using that 
> cylinder and considers it as an unused cylinder. Therefore the SAC 
> calculated is only based on the one cylinder you used and is not 
> accurate at all. It is one of the prices to pay when diving with more 
> than one cylinder: if you do not provide the appropriate information 
> Subsurface cannot calculate the gas usage or SAC. Many dive computers 
> allow recording cylinder changes during a dive. On my Galileo it means 
> pressing one of the buttons twice for each change, a rather trivial 
> activity once you get used to diving with a dive computer. The changes 
> are loaded as part of the upload from the dive computer to Subsurface. 
> For quite a few years I did this recording by writing the times of 
> changes on a wrist slate and logging the changes by hand. If you record 
> cylinder changes then the SAC calculation will be accurate. So your 
> choice is to take no extra trouble and understanding that the SAC rate 
> value is totally inaccurate, or to take the trouble and record the 
> cylinder changes.  Once you progress to using a stage cylinder or a deco 
> cylinder you will be faced with the same problem. So this is not unique 
> to sidemount at all. Enjoy your dives.
> 
> Kind regards,
> willem
> 
> 
> 
> On 2022/11/04 21:50, Andrew Trevor-Jones wrote:
> > Sorry, my point was that subsurface is ALREADY calculating SAC for my 
> > dives with two tanks.  I wasn't making a suggestion for how it should 
> > do it.
> >
> > I'm not diving with stages.  I am effectively diving sidemount but the 
> > tanks are on my back.  Both tanks have the same mixture.
> >
> > Here are the cylinders:
> >
> > Here is the log showing the calculated SAC:
> >
> > And yes, I fully understand that Subsurface takes the compressibility 
> > of the gases into consideration in the calculation and wrote it that 
> > way: "total gas used" takes this into account.
> >
> > Or are you saying it is not already calculating this way and I have 
> > some kind if weird version of Subsurface?
> >
> > Andrew.
> >
> > On 4 Nov 2022, at 10:11 pm, 'Robert C. Helling' via Subsurface Divelog 
> > <subsurface-divelog at googlegroups.com> wrote:
> >
> > What you are missing is that Subsurface does not know you are side 
> > mount diving. When diving with stages which contain different gases it 
> > does not make much sense to add everything together. Rather you want 
> > to compute the SAC rate per gas. To this end, Subsurface needs to know 
> > during which part of the dive which gas was used and this is the point 
> > of the gas switches.
> >
> > You would at least need to tell Subsurface somehow it should consider 
> > the two independent cylinders as one combined reservoir of gas. One 
> > might argue that it makes sense as a default to compute SAC and 
> > gas-use per gas rather than per cylinders, so to add all cylinders 
> > containing the same (or sufficiently similar) gas together.
> >
> > BTW, to compute the SAC, Subsurface uses a slightly more sophisticated 
> > formula than the one you suggest (one that takes gas compressibility, 
> > actual surface pressure and water density into account).
> >
> > Best
> > Robert
> >
> > andrew.tr <http://andrew.tr>... at gmail.com <http://gmail.com> schrieb 
> > am Freitag, 4. November 2022 um 06:47:19 UTC+1:
> >
> >     I often use 2 independent tanks on a dive (back mount but no
> >     manifold so two independent regulators).
> >
> >     Subsurface always calculates to overall SAC/RMV for the dive
> >     correctly.
> >
> >     It should notneed to know about gas switches or anything like
> >     that.  All it needs to know is the starting/ending pressures and
> >     size of each tank.  From there it calculates the total gas used
> >     and divides it by (average depth in metres/10)+1
> >
> >     Or am I missing something?
> >
> >     Andrew.
> >
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