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The oxygen flow and supply pressure are independent of the cabin altitude.
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The oxygen is supplied to passengers for a specific period of time, which can either be 15 or 22 minutes.
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A maximum flight profile is predetermined for such a system
1 A new oxygen system called OBOGS (On Board Oxygen Generation System) is under development. This system will provide oxygen continuously.
3.1.1.2. Gaseous Systems
A gaseous system has certain advantages, over the chemical system:
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It is customizable by selecting the number of high pressure oxygen bottles (up to 14 cylinders on the A340).
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The oxygen flow and supply pressure depend on the altitude. The flow rate is controlled by an altimetric flow regulation device in each mask container. It enables passenger oxygen consumption to be optimized: The lower the altitude, the lower the oxygen flow.
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The oxygen supply time depends on the flight profile, and on the number of cylinders installed.
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There is no oxygen flow below a cabin pressure altitude of 10,000 feet.
3.1.2. Passenger Oxygen Requirement
To help operators determine their needs in terms of supplementary oxygen, regulations provide the minimum required oxygen quantity versus the flight altitude. This information is given for flight crewmembers, cabin crewmembers, as well as for passengers. Nevertheless, oxygen reserves for crewmembers are always much more significant than for passengers and, consequently, the descent profile is always more limited by the passenger oxygen system than by the crew oxygen systems.
“FAR 121.329 (c)(1) For flights at cabin pressure altitudes above 10,000 feet, up to and including 14,000 feet, there must be enough oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration, for 10% of the passengers.
(c)(2) For flights at cabin pressure altitudes above 14,000 feet, up to and including 15,000 feet, enough oxygen for that part of the flight at those altitudes for 30 % of the passengers. (c)(3) For flights at cabin pressure altitudes above 15,000 feet, enough oxygen for each passenger carried during the entire flight at those altitudes.”
“FAR 121.333 (e)(2) […] there must be not less than a 10 minute supply for the passenger cabin occupants.” (e)(3) [...] For first-aid treatment of occupants […], a supply of oxygen must be provided for two percent of the occupants for the entire flight after cabin depressurization at cabin altitudes above 8,000 ft, but in no case to less than one person.”
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