Normal air has 21% oxygen and 79% nitrogen (and other trace gases). Nitrox is a gas with more
than 21% oxygen. The benefits are longer bottom times, shorter surface intervals, and more
energy after the dive (supposedly - that's empirical, not verifiable). Sounds like a good thing
all around. But there is a little more to it than just adding oxygen to your tank, so you have
to have training in it.
So about a week before the class was to be, we went in to sign up for it. I figured that was
plenty of time. Turns out the class was already full. :-( Actually it was overfull. The guy
we were talking to was the instructor, though, and he offered to start another class for us
(and possibly the overflow for the first class). So instead of Saturday we had class the
following Monday evening. Which worked out fine.
There were 7 people in the class on Monday night. So I guess it is pretty popular. We started
out in the room where they fill the tanks, he showed us the compressors and filters, where and
how to check the mix, and the general operation. Then we went up to the classroom for about two
hours and went over the hows and whys of nitrox. A short test at the end and we were done.
"Normal" air is 21% oxygen, and we're certified up to 40%, or EAN40. EAN32 (32% oxygen) is apparently
pretty common, you can actualy use it and still use normal air tables and be safer than with normal
air. 36% is also common, and 40% is the max we can get. You can get higher percentages, but then
you start getting into more serious side effects, so you need more training. I'm not sure how high
you can safely get, at some point it becomes tri-mix, which adds helium. I don't know much more about
that than that it exists.
Nitrox is, of course, a tad more expensive than air, but it's something like $8 or $9 instead of $6,
so it's not horrible. And if you really do have more energy when you finish a dive, it's worth it.