The diluter-demand function of the A14, from what I have read, occurs below a certain altitude, 30,000' if I recall correctly. In this mode the mixing chamber mixes cabin air with O2 and the amount of O2 increases with altitude. Above this altitude it supplies 100% O2. The pressure breathing function is a separate function that has different dial settings for increasing altitudes. Here the pressure of the O2 supplied not the mixture to the mask increases. The markings on the A14 dial suggest 5 different pressures for settings of Safety, 41,000', 43,000', 45,000', & above 45,000'. Not sure why the 1st dial setting is marked Safety but it is the 1st position when the dial is turned off normal.

I've read the same about blood boiling. I think, as you have pointed out, they are referring to an uncontained and uncirculating liquid and meant to make a point about how low the pressure is. Articles I've read also use statements like "seems to boil" instead of boils. I guess the body can contain the vapor pressure sufficiently so it doesn't reach atmospheric pressure and really boil. Apparently the skin will expand and early experiments with dogs in space or a vacuum chamber discovered this. Saliva boiling on the tongue makes sense.

I do plan to get experience in a hypobaric chamber and am hoping they will let me use my own mask, regulator, and bottle however, I suspect this might be considered a liability for then to allow. I still plan to ask. What better way to test the performance of the system I intend to use at FL350-400!