For American-style naming, it doesn't matter. An AL80 is named for its capacity at its service pressure. For Euro-style naming, LP and HP matter, as you need to know the service pressure to know the amount of gas in the given water-volume capacity.300bar diver:Is that LP or HP or does that not matter
Just use the Google query, [google]11.1 liters * ??? in cubic feet[/google], replacing the ??? with the service pressure in bar. (I'm not completely familiar with the customary service pressures of your cylinders, so I can't give complete answers.)
As for American, Imperial, Avoirdupois, metric, SI, and all those other systems, while I wouldn't say they are equally simple, they are all reasonably simple (and different problems lend themselves to different systems -- I bake cookies in metric units, but I mix beverages in English units ).