Originally Posted by
Capn Bloggs
Not numerically. I do know that when our masks are on Emergency, there is significant overpressure, to the point where it is difficult to talk normally. The Emerg setting is for smoke/fumes use, not for a "normal" depressurisation.
Looking at 14.5 psi at 0 vs 3-5 psia at altitude. To make a significant amount of change is 3-8 psia. A mask will cover a triangle 6 inches tall and 6 inches wide. The area of that is 18 square inches, so you would need between 54 pounds and 144 pounds of strap tension to hold the mask on the person's face; not including the preload to keep from leaking out past the rubbery flesh of the face.
I expect the pressure delivered is on the order of .1 to .4 psig, which is a slight positive pressure that works because the actual need is to provide a partial pressure of oxygen equivalent to 10,000 ft and the 3-5 psi at altitude on pure oxygen accomplishes that; there's also a need to exclude smoke/toxic gases. Yes, even that low gauge pressure makes it tough to talk. Considering that lungs have a section area close to 100 square inches, even a 0.1 psi load is like having a 10 pound weight on your chest to resist.
From the scuba people:
"The important number isn't the total pressure in the lung but rather the
difference in perssure between inside the lung and outside the lung. If the pressure in the lung is 95-110 cm H2O higher than the pressure outside the lung, the lung can rupture.
(100cmH2O= 0.097ATA= 0.098bar= 1.42PSI= 73.55mmHg approx.)"
Search scubaboard with the above text for more, sometimes grisly, details. Mind that this is the rupture pressure; it will be very uncomfortable before reaching this level.