SAC = Surface Air Consumption. RMV = Respiratory Minute Volume. Both are volumetric calculations of gas consumption. Imperial versus metric, the units don't matter, so long as you are calculating them correctly. RMV consumption will directly reflect your depth as it increases with increasing depth (e.g. gas density). As the name implies, SAC is your consumption at the surface and allows you to calculate your gas consumption at any depth, just multiply the atm of pressure by your surface consumption.