“I wonder how much free oxygen is available in a cave..”
Depends on the cave. I’m not a cave diver because I want to die in bed, but I do know several. I doubt any cave divers would trust this thing. heck they take 3 of everything.
My bother's an experienced cave diver. After a bad experience a few years back he came to the same conclusion you came to... decided he's rather die in bed.