Time of useful consciousness at FL250 is 10-15 seconds without
supplemental O2.

wrote:
[snip]
But these systems are not designed to deliver anything close to pure O2 to
the user. The maximum O2 concentration they CAN deliver will provide
sufficient O2 PP at about 18000 ft for cannulas and about 26000 ft (if I
recall correctly) for masks.
[snip]