Supplemental Oxygen at high cabin alt

[Pelican]

Just wondering if somebody can tell me why flight deck oxygen masks deliver oxygen under positive pressure, yet passenger oxygen is just a free flowing system. The same applies to supplemental oxygen bottles that the cabin crew use. They are even encouraged to move around the cabin to check up on passengers and on some longhaul airlines the relief pilots are expected to make their way from the bunk to the flight deck breathing from an oxygen bottle.

If all this would happen once levelled off at 14.000 feet then I would understand but our SEP department makes no reference to that and indeed, they expect crew to start moving pretty much as soon as it all kicks off.

So, my question is, how long will you survive just when breathing oxygen that is not supplied under pressure. I thought that at high cabin altitudes you lungs cannot actually take up any oxygen unless under pressure. From what altitude can you expect to breath oxygen (from whatever source) that is not under pressure?

[NigelOnDraft]

You only need pressure breathing for sustained flight above ~40,000' with pure O2. For modern airliners this is not applicable, since this is effectively their Max Altitude, and they would descend PDQ anyway.

Positive pressure is used however for smoke clearing etc.