Air consuption, difference with a manuel calcul

[Christophe P]

Hello all,

Perhaps you have noticed this. For each dive, we set the block used (and therefore the volume) as well as the starting and ending pressure. With it, we can calculate the volume of air used in liters ( Pressure consumed x Volume). However, subsurface does not give the same result. Strange! Example: I consume 175 bars in a S80 (11.1 liters). Total consumption = 175 x11.1 = 1942.50 liters. But Subsurface indicates 1720.45 liters. Can you tell me if this is a bug or if I have incorrectly configured the software?

I'm using Win 10 and surface 4.9.10 (the last version).

Thank a lot. 

Best Regards

Christophe

[Dirk Hohndel]

Likely you are running into the issue that at pressures relevant for scuba cylinders gasses aren't ideally compressible anymore.

Subsurface actually takes gas compressibility into account when calculating SAC rates.

Our resident physics professor has written a nice post about this - and there are a few threads on this here in the forum as well

https://thetheoreticaldiver.org/wordpress/index.php/2017/11/02/when-real-gas-corrections-matter/

I hope this answers your question

/D

[Adric Norris]

Linus' comments in this thread went into a decent amount of detail on the topic.

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[Christophe P]

Thanks a lot Dirk. It's perfect and the article is very clear.

Have a great evening.

BR

Christophe