drrich2
Contributor
It doesn't enable me to stay down longer or anything, right?
It's been quite some years since I took my nitrox course, and I don't know how the educational material may've changed. I've learned some things on Scuba Board that have helped me gain some perspective, and from elsewhere. Maybe a bit of that might help you see why more oxygen doesn't make your tank last longer.
1.) Many people believe respiration (inhale/exhale cyclically) is about renewing the oxygen content in our lungs.
2.) But for most people, that's not what drives respiration. It's not marked oxygen depletion that makes you feel the need to take another breath. You inhale 21% oxygen & exhale ~ 16-17% oxygen (I think I've seen both figures).
3.) It's CO2 buildup that drives your respiration. CO2 is not just some 'waste gas' plants use to make their food. It's important in regulating the acidity of our blood. Hyperventilating, which can drive down CO2 levels in the blood, can make you pass out. Skip breathing, which can raise CO2 levels in your blood, can give intense head aches.
4.) Much of the oxygen carried in our blood is carried by hemoglobin, a pigment contained in red blood cells. It has the interesting property of binding to oxygen at high oxygen partial pressures (e.g.: your lungs) and letting go of it at low oxygen PP's (e.g.: in your tissues where it's being consumed). So your red blood cells are key in moving oxygen from your lungs to your tissues.
5.) In normal, healthy people, the blood hemoglobin has a saturation rate in the high 90's on room air on the surface (with a partial pressure O2 of 21% inhaled). So nitrox providing a higher oxygen mix (e.g.: EAN 32%) is not going to meaningfully increase that.
6.) But be mindful oxygen dissolved into the liquid component of your blood, and elsewhere, still matters. Oxygen toxicity is serious, potentially fatal (e.g.: underwater seizure) and respecting maximum operating depths for a given mix well advised.
So, at least in theory, if you 'slipped' a nitrox tank onto a diver who thought he was diving air (don't!), and he didn't exceed the MOD, that tank should in theory last him the same time (assuming he ends his dive due to running low on gas) an air tank would have.
Richard.