Here's my take on it.
5ft tall, 2150 psi, it's probably an A bottle (see
Standard Cylinder Sizes). Or a K bottle, or a 1A, or a B. The name depends on who made it. Any way you look at it, its around 44 liters, which at 2015 PSI makes it about 215 cubic feet of O2.
To fill the AL80 to 30%, you don't fill 9% with O2 and top with air, as would seem intuitive. That gets you only 28%. Here's the math:
0.09*100% + 0.91*21% = 28%.
Though it would be foolish to do it this way, think of it as filling 70% with nitrogen, then top with O2. Since your source of nitrogen is only 79% pure (contaminated with 21% oxygen), you need
70%/0.79 = 88.6% of the tank filled with air, the remainder (11.4%) with O2.
Doing the math forward (as the first equation), you end up with
0.114*100% + 0.886*21% = 30%.
So in practice you add 11.4% * 3000psi = 342psi of O2. You can do this from any starting pressure of air in the tank, so long as your O2 tank has sufficient pressure to do the job.
Now, how many times can the O2 tank do this? Lets assume that on the last fill you're going to get, you will bleed your AL80 to 100psi before starting. Thus at the end of the O2 portion of the fill, you have 442psi in both tanks. That means you waste 442/2015*215= 47 cubic feet of O2, because you can't get it out of the tank. What you can get out is 215-47=168 cu ft.
Each fill requires 11.4%*80 cu ft, or 9.12 cu ft, so the number of fills is 168/9.12=18.4.
In practice, you may get MORE than this, because after the first fill, you are starting with 30% in the tank, and if you don't suck it dry you don't require the full 9.12 cu ft per fill. Of course, knowing how much to put in a tank already partially filled with EAN requires a good understanding of the math behind it. That, or a good understanding of the fancy app on your iPOD that does the math for you. You also MUST test the mix both before and after filling.
Using a booster would allow you to recover part of the 47 cu ft you would otherwise waste (all boosters require some input pressure). Using a blending stick would allow you to use every last cubic foot, but that's another thread...
D