Originally Posted by
Peter H
Flight Alloy: It is true that limited pressure breathing is possible, but this is limited to just the slight muscular 'overcapacity' the diaphragm muscles are capable of, in the range of fractions of an atmosphere.
Capn Bloggs: Flight Alloy, what are you on about. You have obviously never worn a flight crew mask with Emergency set on the dial.
Trying to get a ball-park figure I found a 2003 Lancet article on the "Physiological hazards of flight at high altitude"
https://www.thelancet.com/journals/l...059-3/fulltext
"Which mentioned "positive pressure breathing oxygen masks that deliver up to 70 mm Hg", that's just under 10% of an atmosphere (760mm Hg).
Any idea what pressure modern flight crew masks actually deliver?
Peter, thanks for putting some numbers to our claims, it makes determining fact from fiction objective as it should be. Some documentation I found, Hypoxia an Invisible
'Enemy Cabin depressurization effects on human physiology' from 2006, states this:
Another additional means for hypoxia protection is positive pressure breathing, which is usually found in modern crew oxygen masks and means the delivery of pure oxygen under pressure into the respiratory tract. For civil applications positive pressure breathing is able to increase additionally the oxygen partial pressure by around 20 to 30mbar provided that the overpressure condition is limited to some minutes only.
At sea level, oxygen has a partial pressure of about 200 mbar, at 40,000 ft, ambient pressure is reduced to around 180mbar, so with 100% O2 setting providing 180mbar of oxygen and a further 30 mbar of pressure breathing, one is back up to 210mbar of breathing oxygen, about the sea as at sea level. In this configuration, the pressure breathing element only provides 14% of the total oxygen delivered.
As one can plainly see, the overpressure provided is minimal, compared to sea level, the overpressure is 3% of atmospheric, at altitude, it constitutes only 15% over ambient.
So as I had stated before, yes, limited pressure breathing is possible, but it only constitutes an overpressure of 3% of an MSL atmosphere, aka a very minimal overpressure fraction, again due to the fact that our lung muscles have very limited overcapacity to process that pressure and not suffer any rupture injuries. I am sure it may 'feel' significant to the pilots who have experienced it, but numbers show that the overpressure component is a minimal, almost negligible component of the breathing system at altitude in all aircraft without the provision of pressure suits.