Voltage Averaging Unit Assembly
Figure 5
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Crew Oxygen Flow Diagram
Figure 6
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B. A brush-like wire element, approximately 3 inches long inserted tightly into the coupling assembly, functions as a thermal compensator to prevent oxygen temperature extremes when the system is being used.
4. Pressure Reducer (Fig. 3)
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A. The pressure reducer attaches to the gaseous oxygen cylinders coupling assembly to decreas cylinder pressure to an intermediate value before it is further reduced by the pressure regulator. Due to extensive length of the distribution lines between the cylinder coupling assembly and the pressure regulator, use of the reducer provides safer system operating pressures.
B. The reducer decreases cylinder pressure over 600 psig to a pressure of 600 to 680 psig. If cylinder pressure should fall below 600 psig, the reducer outlet pressure would be approximately equal to the cylinder pressure. The pressure reducer will lock at 600 - 830 psig if the downstream flow is closed off. Normal operating pressure occurs when the downstream flow is open and there is sufficient flow.
5. Pressure Transducer (Fig. 3)
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A. High pressure indication is achieved through a pressure transducer which connects to the cylinder coupling assembly. The transducer senses cylinder pressure and converts it to a variable electrical signal which is fed to EICAS.
6. Overboard Discharge Port and Indicator (Fig. 1)
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A. The discharge indicator is a frangible plastic disc installed in the overboard discharge port located just aft of the forward cargo door. This disk will rupture if any of the oxygen cylinder safety relief valves open and vent thier excess pressure out the overboard discharge line. The purpose of the disk is to provide a visual indication from outside the aircraft, that an over-pressure condition in the high pressure system has occurred.
7. Voltage Averaging Unit (Fig. 5)
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A. The voltage averaging unit is installed in the forward cargo compartment RH ceiling beam, aft of the cargo door. The voltage outputs of the crew oxygen pressure transducers feed into the voltage averaging unit. These voltages are averaged togather to provide the input signal for EICAS.
B. A jumper wire between two pins of the averaging unit connector must be cut, capped and stowed when switching from a two-cylinder configuration to a single-cylinder configuration to prevent the oxygen pressure indicator from registering one half of the cylinder pressure. The jumper must be restored when reinstalling the second cylinder in the system. Refer to 35-11-00/201, Crew Oxygen System (Single/Dual Cylinder Wiring Change).
8. Pressure Regulator (Fig. 1)
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A. The pressure regulator reduces oxygen pressure from the pressure reducer to 60 to 85 psig. A pressure relief valve, discharging into the cabin, is included in the regulator to protect the system downstream of the regulator. The regulator is located in the aft right sidewall dripshield in the flight compartment.
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9. Mask Stowage Box (Fig. 4) 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：747-400 AMM 飞机维护手册 氧气 OXYGEN(11)
