Rick Murchison:
Thanks. It's hard to express these concepts as clearly as you have.
Guys, Respiratory Minute Volume is a concept that was developed outside of diving. It's a medical/physiological term and you don't get to redefine it arbitrarily to meet your wishes. It's a useful term, but it means what it means: volume of gas breathed over a minute. That's it. You can use different units (SI, Imperial, furlong/firkin/fortnight, whatever), but if you're not expressing volume over time (and nothing else), you're not talking about RMV.
SAC/SCR, on the other hand, appears not to have any broadly accepted definition. I see everything from pressure drop over time to RMV to other things used as its basis. Since we're calling it SURFACE consumption (the "S" in SAC or SCR), I'm going to use it to mean my RMV at the surface. As I descend, my RMV remains fairly constant (lets assume I'm just sitting on a dropping platform), but the mass of gas I'm sucking goes up. At 2 ATA, I'm going to use up the contents of my cylinder in half the time compared with at the surface. That fact doesn't change my RMV *or* my SAC. It DOES change the rate at which pressure in the cylinder drops. In other words, psi/min is NOT a constant, but RMV/SAC are (or could be).
So what would the difference be between RMV and SAC? Not a whole heck of a lot, really. If I breathe 0.63 cf/min on top of Mount Everest, I'll breathe 0.63 cf/min at the bottom of the Marianas Trench (assume constant gas mixture and coma, if you like). The number of gas molecules breathed per minute in each case, though, will be VERY different. I don't have good direct ways of measuring numbers of gas molecules in my cylinder, but I can easily know my depth (or ambient pressure) and the pressure in the cylinder and how it's dropping. This is why some people like to think of SAC as involving PSI over time. It turns out that humans consume gas, not pressure. Our lungs tend to work at the same volumes irrespective of pressure. Knowing your "consumption" in PSI only helps if you know the rate for that particular cylinder size and your particular depth. As noted above, it is NOT a constant.
Knowing a SAC (or RMV) expressed in volume over time makes estimating remaining gas time easy. If I know, say, that I have an 80 cf 3000 psi tank that started at 3000 psi and is currently at 1500 psi and my SAC (or RMV) is 0.5 cf/min and that I'm currently at 30 fsw, it's quick and easy to figure out when I'll run dry: at the surface, the tank would now have 40 cf of gas. Since I'm at 2 ATA it currently "contains" the equivalent of 20 cf. Since I breathe 0.5 cf/min (regardless of depth) I've got 40 minutes left. (Obviously I picked numbers that made the math easy, but you get the point).
The whole point of SAC, as I see it, is actually the REVERSE calculation. If I know my depths and times enough to have an accurate average depth and know the volume and working pressure of the tank as well as the pressure change over the dive, I can compute my RMV as if I'd been on the surface the whole time. Therefore, Surface Air Consumption *is* Respiratory Minute Volume, computed from data available in real-world diving conditions, including average depth, cylinder size and working pressure, and pressure drop over the course of the dive.
) My girlfriend thinks I'm really weird but....
