SAC calculation

Alexander Bösch

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Jul 31, 2020, 2:45:17 PM7/31/20

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Hi I have one Question

When I make a dive and calculate the SAC manually I get a other result then subsurface.

In the attachment a picture of a dive
https://ibb.co/s1HCYwC

Average Depth = 14.1m (2.141bar)
Duration = 100min
Tank = 12L 300bar
Start bar = 320bar
End bar = 110 bar
Air consumption = 210 bar

SAC = 210bar * 12L / 100min / 2.141bar = 11.77l/min

In the app I get the SAC 8.8l/min
https://ibb.co/s1HCYwC

What's the difference? Or what is in my calc wrong?

Stuart Vernon

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Jul 31, 2020, 2:46:59 PM7/31/20

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14.1m is 2.41 bar, not 2.141.

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Stuart Vernon

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Jul 31, 2020, 2:49:03 PM7/31/20

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Also, your calculation is based on Ideal gas behavior. Subsurface corrects for non-Ideal gas behavior.

320 bar is well beyond the threshold where that matters.

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Dirk Hohndel

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Jul 31, 2020, 2:49:35 PM7/31/20

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Also, please note that the ideal gas law is basically wrong - Subsurface takes compressibility of gases into account when calculating your SAC rate. At 320bar, your 12L cylinder does NOT contain 12*320 = 3840 liters of air

/D

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Alexander Bösch

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Jul 31, 2020, 2:50:00 PM7/31/20

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True but I still get 10.45

12*210/100/2.41= 10.456bar and not 8.8 :-)

Alexander Bösch

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Jul 31, 2020, 2:50:46 PM7/31/20

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OK thx

Alexander Bösch

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Jul 31, 2020, 2:53:48 PM7/31/20

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OK but in 200 bar I have still SAC difference from 1L/min?
That's correct?

Dirk Hohndel

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Jul 31, 2020, 2:54:12 PM7/31/20

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You can see the calculations in Subsurface.
In the equipment tab, hover above the pressure values. Assuming air (different gases compress differently), you'll see that at 320bar there's only about 3355l of air in your cylinder, at 110bar you have 1307l left, so you used 2048l of air instead of the 2520l that a naive calculation would assume.

/D

> On Jul 31, 2020, at 11:50 AM, Alexander Bösch <boes...@gmail.com> wrote:
>
> True but I still get 10.45
>
> 12*210/100/2.41= 10.456bar and not 8.8 :-)
>

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Alexander Bösch

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Jul 31, 2020, 2:57:34 PM7/31/20

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OK I will do it when I am back on the laptop

Thx for fast response

Robert C. Helling

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Jul 31, 2020, 3:01:45 PM7/31/20

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You might think the difference between real gas and ideal gas is small, but it can have quite an effect just in the pressure region relevant for diving cylinders. I write about it here: https://thetheoreticaldiver.org/wordpress/index.php/2017/11/02/when-real-gas-corrections-matter/

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