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Figure 4-17.MBU-14(V)1/P oxygen mask assembly
Figure 4-20.Cross-sectional view of valve during exhalation

Aircrew Survival Equipmentman 2 - Aviation theories and other practices
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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







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