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 13. Thermal Compensator (Fig. 3)
___________________
 A.  Thermal compensators are installed at the inlet of each of the cylinder coupling assemblies. The thermal compensator prevents oxygen temperature buildup in the cylinder coupling assembly when the system is being serviced. The compensator consists of a brush-like wire element approximately 5 inches long inserted into a stainless steel tube and the union that attaches to the cylinder coupling assembly. The wire element fits tightly into the tube and the union.
 B.  Whenever the union is rotated with respect to the compensator, the wire bristles may interfere with sealing surfaces and cause leaks. The tube, union, and B-nut must be handled as a single unit. The upstream end of the compensator assembly has a standard flareless tube connection.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL

 14. Operation (Fig. 6 and 7)
_________
 A. Functional Description
 (1)  
The oxygen system stores oxygen under high pressure and supplies oxygen at low pressure to each seat and lavatory. The system is actuated automatically by an aneroid in each flow control unit when cabin pressure reaches or exceeds an equivalent of 13,250 to 14,500 foot altitude. The system can also be activated by operating a switch on the P5 Pilot's Overhead Panel in the event the system fails to activate automatically or at the discretion of the flight crew.

 (2)  
When the system is actuated, high pressure oxygen flows from the cylinder to the pressure reducer. Medium pressure oxygen flows from the pressure reducer to the flow control unit and low pressure oxygen flows from the control unit to the oxygen boxes. During the first few seconds, the flow control unit releases a pressure surge that opens the oxygen box door, and lets the masks drop within easy reach of the personnel.

 (3)  
When the mask is pulled to the persons face, a short cord attached to the mask hose pulls the pintel assembly from the mask shutoff valves in the unitized valve assembly, and lets the valve to open and oxygen to flow to the mask.

 (4)  
The user of the mask inhales pure oxygen until the reservoir bag on the mask is empty, then inhales air during the remainder of the inhalation cycle. The quantity of oxygen supplied to the mask is controlled by the altitude compensation mechanism in the flow control units which senses cabin pressure.

 (5)  
Actuation of the system closes a pressure switch on the low pressure side in the flow control unit (Fig. 8). This switch supplies a ground for the R36 decompression relay on the P414 Main Power Distribution Panel and signals the EICAS to show the advisory message, SUPRNMRY OXYGEN ON.

 B. Controls
 (1)  Normal Control
 (a)  With 28-volt dc power available at the battery bus, close the OXYGEN VALVE & IND circuit breaker on the P7 Overhead Circuit Breaker Panel and do a check of the EICAS for oxygen system pressure. If system is correctly serviced, the aneroid controls in the flow control unit automatically start oxygen flow to the oxygen box when cabin altitude increases to 13,250 to 14,500 feet.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (2)  Alternate Control
 (a)  To operate the system, put the SUPNNMRY OXYGEN switch, on the P5 Pilot's Overhead Panel, in the ON position. This causes the control unit solenoid to override the aneroid control and open the actuation valves in the electropneumatic control unit, regardless of cabin altitude.
 NOTE: When the system is pressurized by putting the switch
____ in the ON position, the masks will drop, regardless of cabin altitude.
 (3)  Flow Control Unit Resetting
 (a)  The flow control unit must be reset after actuation or the system is inoperable. 1) Reset is accomplished by a solenoid actuated by momentarily
 moving the SUPRNMRY OXYGEN switch on P5 Pilot's Overhead Panel to RESET position.
　
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