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 becomes 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 requisitioning the proper size oxygen mask assembly 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

Figure 4-19.-Cross-sectiona1 view of valve during inhalation.

Figure 4-20.-Cross-sectional view of valve during exhalation.

Figure 4-21.-Measuring for proper size of O2 mask.

4-21; measure the distance from the tip of the bottom surface of the chin to the point of maximum depression of the nasal root (smallest part of the upper end of the nose). Once the basic size has been determined, requisition a new mask through normal supply channels.

ATTACHMENT OF BAYONET AND RECEIVER MECHANISM

Before you fit the mask to the helmet, you should check to see if the MBU-12/P has been configured to the correct assembly. For complete information on configuration buildup, refer to chapter 13 of NAVAIR 13-1-6.7. Once you have a complete assembly of the right size, you're ready to fit the mask to the helmet.

The mask has four mask retaining straps. You should thread the straps through the slots in the offset bayonet fittings, as shown in figure 4-22. Then insert each offset bayonet fitting into a receiver mechanism to the second locking position. When the bayonet is inserted in the receiver, the first click is caused by the entry of the bayonet into the entrance of the receiver housing, but it is not a locking position. There are normally three clicks when inserting the bayonet to the second locking position of the receiver. When the bayonet is in the second locking position, the end of the bayonet will be approximately even with the outer

Figure 4-22.-Mask retaining straps.

edge of the exit slot on the back of the receiver. This proper positioning of the bayonet end can be checked either visually or by passing a finger over the exit slot on the back of the receiver.

Have the aircrew member place the helmet on his head and hold the oxygen mask in proper position on his face.

Inspect each receiver mechanism assembly to be sure that the rotating feature of the device is locked in its central position. If the rotating feature is found not to be centered, loosen the two locking screws on the nameplate of the receiver. Adjust the receiver until the bayonet is at right angles to the receiver. Retighten the locking screws (fig. 4-23).

The receiver has a rotating feature that allows a 15-degree angle of freedom so that the receiver can be adjusted slightly in either direction if this becomes necessary after attachment to the helmet. Place the receiver on the helmet so that the upper and lower straps have equal tension. The receiver should be positioned as close to the edgeroll as possible to minimize bayonet/edgeroll interference.

With a grease pencil, trace around each receiver to mark its outline on the helmet (fig. 4-24). You should use this outline to align the receiver on the helmet. Now remove the helmet from the wearer and using the back plate of the receiver as a template, mark the locations for the four receiver mounting holes on the helmet (fig. 4-25). Remove the earcup assemblies and drill four mounting holes in the helmet. Do this on each side by using a No. 25 (0.1495-inch diameter) drill. It

Figure 4-23.-Adjusting receiver assembly.

Figure 4-24.-Marking helmet.

Figure 4-25.-Marking holes.

is necessary to peel back the pile fastener tape inside the helmet to allow the attachment of the bayonet receivers. Attach the receiver mechanism assemblies to the helmet with the backup plate inside the helmet. Note that the forward receiver holes nearest the edge of the helmet need longer screws. At this time you can replace the pile fastener tape and replace the earcup assemblies. It maybe necessary to use adhesive when replacing the pile tape.