Should we set oxygen masks to Normal or 100%

[Genghis the Engineer]

I imagine that it's very deliberate that they haven't quoted a specific time, because it's very much along the lines of "how long is a piece of string".

Gas will be supplied by the system at pressure only 2-3mb above that in the cabin, so the greater the cabin pressure the greater the gas flow, and the shorter life of the oxygen in the system.

So, on 100% oxygen, you'll get far longer out of it at FL250 than you would at, say, FL150 where the pressure and thus supply density is greater. On "normal" it's more complex, since at higher altitude the total gas supply rate will be less, but the proportion of oxygen will be greater.

In general, I'd expect any system to supply you with a partial pressure of oxygen which is equivalent to somewhere around 8,000-10,000 ft. But, a certain amount of supplementary oxygen can be supplied at lower density, and thus flowrate, at greater altitude. So, you'd need to work out from the precise mixing rules of a system what time of use you'd get on "normal" at various altitudes, it wouldn't be all that straightforward.

So, the best assurance that one can really offer, is that the certification requirements will mandate that you have enough oxygen to keep the flight crew conscious, and the cabin crew and pax alive and undamaged (but not necessarily conscious) in a max rate descent from the service ceiling to FL120 (where the worst effect of hypoxia should be a loss of night vision and a temporarily reduced level of judgement a bit like having had a couple of drinks). This does mean that if you aren't flight crew, I'd make sure you're sat down and strapped in once you've got your mask on, just in case the descent is slightly protracted and you run into the "temporary loss of consciousness" bracket. But, I'd be very surprised if the captain initially levels off anywhere above FL100 - bu66er if you're over the Himalayas I suppose, and he may do a limited climb subsequently if, say, mid-Atlantic in a 4-engine jet since you might not have enough fuel to make a diversion at low level.

For best time, I would always select normal unless you have cabin fumes, in which case you obviously have to use 100%, I can't see any reason to wait for level-off since normal will give you enough 02 below FL250, and above FL250 it may not give you enough on 100% to remain conscious indefinitely anyway.

So far as positive pressure is concerned, it has two uses. One is an extra margin of safety if there are cabin fumes. The other is prolonged flight above FL250 cabin altitude. These are both extreme cases, the first clearly you'll know about, the second seems unlikely in an airliner operation (okay, maybe over the Himalayas again). There is a known side-effect to this which is very bruised chest muscles and possibly a few burst minor blood vessels, as you reverse the breathing process becoming push-out, relax to breath in. All temporary, but unpleasant.

Obviously, if your operations manual gives something else, for goodness sakes do that and not anything I'm suggesting.

G