Has to do with the way the tanks are measured. In North America tank volume is dependant on being filled to the rated pressure (an AL80 is 77 cuft only IF filled to 3000 psi). Whereas, if you use metric your tank volume is the internal volume of the tank (if you fill it with water) and internal volume x bar = total volume. So basically your SAC is the same as your RMV (I calculate SAC as litres/minute - same with RMV) but SAC has problems with the cuft tanks because the volume (cuft) depends on the rated pressure fill (xxxx psi) which is not standard, ie. you don't know how much the total volume of your tank is unless you know its rated pressure. An 80cuft tank is 10.3 lt so if you use that instead and use your SAC as lt/min (28.3 lt/cuft) you cut out a conversion step.