The best way to measure it is with an oxymeter.
94% feels near normal to me.
92% is okay, but I'm off my game.
90% is like being tired or fuzzy. Alertness is off but I don't feel unsafe. If I am, and I probably am, I can't tell.
88% is not good. I know I'm not really up to being PIC. No danger of nodding off, but I'm doing more sightseeing than being a thinking pilot.
It takes more O2 than one might think to keep the level up at say 16,500, but a nasal cannula can handle even 18k. Just crank the flow right on up to what's needed.
My aim point is to try hold 93%. Success with that will depend on my own condition that day as well as the flight's task load. Usually, after a longish flight, I'll keep the high altitude O2 flow going until I'm off the active on the ground. I'll be tired from the flight, there will be turbulence and traffic and other things to deal with, and this helps me fly well with all that.
This was discussed on another thread -
http://www.vansairforce.com/community/showthread.php?t=96381.
Without an oxymeter, I'd use .5 lpm at 10k, and increase it to 2.25 or so at 15k. I've got a more accurate table in my oxygen bag, though and this is just from memory. These days I use the measured blood oxygen instead of a look-up table. Having the table along in flight is a backup in case the oxymeter fails.
Dave
