@trinsec
Sort of. IF both me and the other diver are at the same depth, then yes, incan stay there much longer. However if i go deeper then i will use my gas up much much quicker. So if the depth is different then it depends on the details.

@trinsec
SAC rate is basically just how fast i breathe air (the volume of air i breathe). So having a low sac rate just means i breathe very little air.

@freemo @trinsec

Wikipedia says, "After exhaling, adult human lungs still contain 2.5–3 L of air, their functional residual capacity or FRC. On inhalation, only about 350 mL of new, warm, moistened atmospheric air is brought in and is well mixed with the FRC."

But when I experimentally exhale into a bag I get what looks like about 2-3 L.

So, at 2.5 L per breath, that puts you at about 4.5 bpm (breaths per minute), which is low. (WP says adult normal range is 12-16, mine is usually <10 bpm at rest.

With different mixtures, I'm sure that throws the whole thing off, because more O2 would mean lower bpm for the same amount of CO2 produced, I think.

What does SAC mean?

@Pat
At depth despite still breathing 21% o2 in this case the increased pressure means it has a partial pressure of 1.4 which would be equivelant to breathing 140% O2 at surface. So yea. Also nitrogen narcosis slows down the system and likely helps me breath a bit slower.

SAC means "surface air consumption" its basically the rate you breathe at.
@trinsec

@Pat @trinsec
Also my guess as to your breath volume... You were probably trying to fully exhale when you were doing it. Wikipedia is probably talking about a typical breath youd make when relaxed, which is much shallower.

@freemo @trinsec

Yes, that's probably true. Also, it says, "..air is brought in and is well mixed with the FRC." Some air remains in the upper respiratory tract, so that amount probably wasn't in their figure.

Regarding intentional vs relaxed breathing, I noticed that you can't very effective measure your own breathing rate/volume because simply being aware of it effects the breathing and results.

It's kind of like trying to tickle yourself, it doesn't work.

@Pat
Yea when your aware of your breathing it does tend to change it. Thats one reason its so hard to get a good sac rate when you dive, because then you focus on your breathing and it changes.
@trinsec