Fix up SAC calculations for ATM/bar confusion

We even documented that we did SAC in bar*l/min, but the "S" in SAC
stands for "Surface".  So we should normalize SAC rate to surface
pressure, not one bar.

It's a tiny 1% difference, and doesn't actually matter in practice, but
it's noticeable when you want to explicitly test for SAC-rate by
creating a test-dive that averages exactly 10m.  Suddenly you don't get
the round numbers you expect.

[ Side note: 10m is not _exactly_ one extra atmosphere according to our
  calculations, but it's darn close in sea water: the standard salinity
  of 1.03 kg/l together with the standard acceleration of 9.81m/s^2
  gives an additional pressure of 1.01 bar, which is within a fraction
  of a percent of one ATM.

  Of course, divers have likely chosen that value exactly for the math
  to come out that way, since the true average salinity of seawater is
  actually slightly lower ]

So here's a few test-dives, along with the SAC rate fixup to make them
look right.

(There's also a one-liner to dive.c that makes the duration come out
right if the last sample has a non-zero depth, and the previous sample
did not: one of my original test-dives did the "average 10m depth" by
starting at 0 and ending at 20m, and dive.c got a tiny bit confused
about that ;)

[ The rationale for me testing our SAC rate calculations in the first
  place was that on snorkkeli.net user "Poltsi" reported that our SAC rate
  calculations differ from the ones that Suunto DM4 reports. So I wanted
  to verify that we did things right.

  Note that Poltsi reported differences larger than the difference of
  BAR/ATM, so this is not the cause. I'll continue to look at this. ]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/divelist.c b/divelist.c
index 85fb535ed..143f4d43c 100644
--- a/divelist.c
+++ b/divelist.c
@@ -749,8 +749,8 @@ static int calculate_sac(struct dive *dive)
 			}
 		}
 	}
-	/* Mean pressure in bar (SAC calculations are in bar*l/min) */
-	pressure = depth_to_mbar(dc->meandepth.mm, dive) / 1000.0;
+	/* Mean pressure in ATM (SAC calculations are in atm*l/min) */
+	pressure = (double) depth_to_mbar(dc->meandepth.mm, dive) / SURFACE_PRESSURE;
 	sac = airuse / pressure * 60 / duration;
 
 	/* milliliters per minute.. */