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 6.  Bleed Relief Valve (Fig. 1)
__________________
 A.  The bleed relief valves are found in the low pressure distribution system. The purpose of the bleed relief valve is to vent any trapped air from the low pressure system during the initial surge pressurization of the system.
 B.  The valve will relieve pressure that exceeds 31 (| 1.8) psig. The pressure will exceed 27 (| 1.5) psig during the surge period when the system is actuated. When the manifold pressure drops to 28 psig, the valve will close.
 7.  Unitized Valve Assembly (Fig. 1)
_______________________
 A.  The unitized valve assembly has an inlet port assembly, a mainfold, mask shutoff valves, mask outlets, and a door latch mechanism.
 B.  The unitized valve assembly is connected to the low pressure distribution line by a flexible hose at the inlet port. When pressure in the low pressure distribution line rises between 16 and 29 psig, the door latch actuates and opens the oxygen box door. The door latch mechanism can be operated manually for maintenance access.
 C.  The mask shutoff valves controls the flow of oxygen to the mask outlets after pintel is removed. The mask shutoff valve is closed when the flap-type lever is in the up position and opened when the flap-type lever is in the down (normal) position.
 (1)  
A pintel assembly is inserted into the mask shutoff valve and is connected to the mask hose by a short chord. The pintel assembly (when inserted) is used to hold the mask shutoff valve closed when the valve is in the down (normal) position. Pulling the mask to the face withdraws the pintel, opening the mask shutoff valve and allowing oxygen to flow to the mask outlets.

 (2)  
During normal flight conditions, the mask shutoff valves are in the down position with the pintel assembly inserted and the oxygen box door is held closed by the door latch mechanism.

 8.  Oxygen Mask (Fig. 4)
___________
 A.  The oxygen mask assembly is an oronasal cup-shaped continuous flow type unit and is connected to the outlet valve nozzle with flexible plastic tubing. A 1 liter reservoir bag is attached to the facepiece with a check valve incorporated at the mask. The mask is designed to conform with facial contours and may be held in position with an elastic head strap.
 9.  Cylinder Coupling Assembly__________________________(Fig. 3)
 A.  A cylinder coupling assembly is installed on each oxygen cylinder outlet fitting. It supplies close coupled mounting connections for the pressure transducer and pressure reducer.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL

 B.  A brush-like wire element approximately 3 inches long, inserted tightly into the coupling assembly, functions as a thermal compensator to aid in heat dissipation.
 10. Pressure Reducer (Fig. 3)
________________
 A.  The pressure reducer attaches to the cylinder coupling assembly and decreases cylinder pressure of 600 to 1850 psig to a pressure of 600 to 680 psig. When pressure at the inlet port falls below 600 psig, outlet pressure is equal to inlet pressure. Due to the length of the distribution lines between the cylnder coupling assemblies and the continuous flow control units, utilization of the reducer provides safer system operating pressures. Differential pressures on the inlet and outlet sides of the valve modulate the valve piston to provide the desired pressure output.
 11. Oxygen Pressure Transducer (Fig. 3)
__________________________
 A.  Cylinder pressure indication is achieved through pressure transducers which are components of each cylinder coupling assembly. The transducer senses cylinder pressure and converts it to a variable electrical signal which is fed to the voltage averaging unit. The output of the voltage averaging unit is then fed to the EICAS. The transducer also has a voltage regulating zener diode which maintains regulated dc output to the voltage averaging unit. This makes sure that the transducer output signal is proportional to the pressure input to the transducer regardless of fluctuations in supply voltage.
 12. Voltage Averaging Unit (Fig. 5)
______________________
 A.  The voltage averaging unit is installed in the forward cargo compartment RH ceiling beam, aft of the cargo door.
 B.  The voltage output of the passenger oxygen cylinder pressure transducers feed into the voltage averaging unit. These voltages are averaged within the unit to supply the input signal to the EICAS.
　
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