Figure 4-18.—Cross-sectional view of a combination inhalation/exhalation valve.
The MBU-12/P oxygen mask subassembly is
a lightweight, low profile, pressure-demand type
of oxygen mask. The mask features an integral
facepiece/hard shell. The facepiece is of pliable
silicone and the hard shell is of polysulphonate.
The mask also features a combination in-
halation/exhalation valve, a flexible, soft silicone
hose, and an MS27796 connector. The mask is
fitted with an antistretch cord, which is se-
cured at the upper end to the valve and at the
lower end to the MS27796 connector. The hard
shell is outfitted with a microphone receptacle
assembly on the outside and a microphone bracket
on the inside for positioning the noise canceling
microphone.
Combination Inhalation/Exhalation Valve
The valve offers the advantage of miniature
size, but it does exhibit slightly higher exhalation
resistance. The valve is sensitive to both dust and
contamination. As a PR, you are responsible for
cleaning the mask.
Operation of the valve is very simple.
The valve is installed in the mask hard
shell in such a way that one side of the
valve “sees”
mask pressure and the other
side “sees” hose pressure (fig. 4-18). Upon
inhalation, the pressure within the mask be-
comes less than the pressure within the oxygen
hose; the flapper of the valve opens and
oxygen from the hose enters the oxygen mask
(fig. 4-19). Upon exhalation, the pressure within
the mask becomes greater than the pressure within
the hose; the flapper closes; the spring/diaphragm
collapses; and the exhalation is exhausted (fig.
4-20).
Sizing
To operate properly, the MBU-12/P oxygen
mask must be the correct size for the aircrew
member. The concept of sizing as used here
refers to the basic methods to be followed by
the Aircrew Survival Equipmentman for re-
quisitioning the proper size oxygen mask as-
sembly from supply. Once the basic oxygen mask
size has been determined and requisitioned, the
MBU-12/P oxygen mask is ready for buildup to
the ultimate configuration desired. To select the
proper size, use the caliper shown in figure
4-26
