George Patterson wrote:


In general, people can use a cranula or similar device to provide oxygen
up to about 20,000' (the FAA limits use of these to 18,000'). These
simply bleed oxygen into the air you breathe. Above that, you need a
low-pressure mask. These ensure that all you are breathing is oxygen and
are good up to about 25,000'.

My understanding is cannulas are still effective to at least 25,000',
and the FAA mask requirement is only to guard against pilots that might
inadvertently breathe through their mouth instead of their nose. The
masks I've used all allow ambient air into the mask; i.e., they are not
intended to supply only oxygen. They used staged valves for constant
flow systems, or just small holes in the sides of the mask for
pulse-delivery devices like Mountain High's EDS controller.



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Eric Greenwell
Washington State
USA