I guess I will give dive aholic a thanks and a likes for answering the ? before it goes to page 2.
Displaying the math right or wrong, 2000 psi for a single small tank is reasonable for a cave dive to be done.
The reason I say this is look at how many cave divers die who carry a whole lot of CF, yes even over the weekend, I would beg to say it might be safer to cave dive with a single tank as you can only go so far.
I'm perfectly happy diving a pair of hundreds and sucking down only a sixth of my gas.