Subsurfaces SAC calculation

[Ridalo]

Hi all,

I am using Subsurface 4.5.3

Wndows 10

First, thanks to the developers for a great program.

I am trying to understand the SAC figure that Subsurface calculates, and shows on the 'Info' tab.

It seems to be the volume of gas (at surface pressure) used during the dive divided by the dive time.

Am I correct?

The number I would like to see would be the volume of gas used divided by time corrected for the average depth of the dive.

Is this possible?

Something like (in metric)

SAC=((Bar used x Cyl vol)  / ((Av depth+10) / 10) / Divetime )

So, if you were using a 10 litre cylinder, and used 50 bar in 10 mins, at an average depth of 15m, your SAC would be 20 litres/min

I appreciate that the numbers calculated by subsurface will be better than mine, since the non linearity in the compressiveness of gases is not taken into account in my calculation.

Or am I missing something?

[Stuart Vernon]

“So, if you were using a 10 litre cylinder, and used 50 bar in 10 mins, at an average depth of 15m, your SAC would be 20 litres/min”

 

That is what it does.

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[Ridalo]

Vernon,

Thank you for your reply.

Mine does not seem to.

If I give an example from an actual dive

Start pressure 206 bar - end pressure 44 bar, so 162 bar used.

Cylinder was an S80, so 11.1 litres

Dive time 55 min

Average depth 15.7 m

From these numbers, I calculate an SAC of 12.7 litres/min

Subsurface reports 30.1 /min.

There is something going on that I do not undersand

[Robert C. Helling]

Would you mind attaching the .xml file of this dive? Then I could have a look at it.

Thanks

Robert 

[Ridalo]

Robert,

Thanks

Here is an xml of that dive.

[Robert C. Helling]

That indeed explains the problem: In your file it says somewhere:

<water salinity="35 g/l"/>

This salinity in subsurface is really a misnomer, we use it to actually indicate the density of water, i.e. the mass of the salt plus that of the water per liter, while your number is clearly just the salt. Thus it translates the depth to a pressure that is almost the surface pressure and hence your impression. The thing is: In subsurface, the salinity is only displayed, there is no UI element to set it (outside the planner). Thus this value must have come from your dive computer. So we need to change how we interpret the salinity value that we get from the dive computer.

Thanks for reporting this. For the time being, you can just delete the salinity line from your xml file and get the correct value (for some standard density of water.

Best

Robert 

[Robert C. Helling]

Am Freitag, 26. Februar 2016 13:39:28 UTC+1 schrieb Robert C. Helling:

Am Freitag, 26. Februar 2016 10:51:24 UTC+1 schrieb Ridalo:

Robert,

Thanks

Here is an xml of that dive.

That indeed explains the problem: In your file it says somewhere:

<water salinity="35 g/l"/>

This salinity in subsurface is really a misnomer, we use it to actually indicate the density of water, i.e. the mass of the salt plus that of the water per liter, while your number is clearly just the salt. Thus it translates the depth to a pressure that is almost the surface pressure and hence your impression. The thing is: In subsurface, the salinity is only displayed, there is no UI element to set it (outside the planner). Thus this value must have come from your dive computer. So we need to change how we interpret the salinity value that we get from the dive computer.

I checked back with Jef Driessen, the author of libdivecomputer (which subsurface uses to talk to the dive computers). According to him the Oceanic Oci 279 does give any information about salinity. Do you have any idea how that value of 35g/l got into subsurface? Did you import the dive from some other software?

Best

Robert 

[Ridalo]

Robert,

Thank you for your prompt reply, and explanation.

The dive computer was downloaded to DiveMate, and DiveMates data was then imported to Subsurface.

So, Divemate

I only take a tablet on dive trips, and on that trip I expected to do more dives than the OCi can store, so needed to have the download facility in DiveMate

In the interest of pedantry, (since I am going to have to edit many dives), would it be sensible to edit the 35 to 1035 rather than deleting the line completely?

Richard

[Robert C. Helling]

Of course you can. I just wrote a patch that should fix this problem automatically in the future.

Out of curiosity, since I am not too familiar with Divemate: What file format do you use for the transfer from Divemate to Subsurface? 

Best

Robert

[Ridalo]

Robert

One of the choices in Divemates export menu is "Export to a Subsurface logbook"   It is a .xml file.  So I used that.

Richard