Conch hit it on the head. You have to know your dive tables. Best way to do that? Take the class, it’s worth it. The different mixes the different lengths (and depths) you can go down to. For example with EAN 32 you can do 100 fsw for 30 min while with EAN 36 you can do 40 min at the same depth. You have to make sure you use the right dive table with the mix you are running. Personally I like Nitrox becuase I don’t dive deep and I can GENERALLY stay down a little longer with EAN.