What type of air is converted by OBOGS for crew oxygen supply?

OBOGS, or On-Board Oxygen Generation System, is specifically designed to provide crew members with a supply of oxygen for breathing during flight by utilizing the aircraft's environmental systems. The system works by extracting oxygen from partially conditioned air, which is air that has been previously processed for comfort and safety in the cockpit but is not yet suitable for breathing due to its composition.

Using partially conditioned air allows OBOGS to maintain efficiency while optimizing the extraction process for oxygen. The system separates oxygen from nitrogen and other gases in the available air, providing high-purity oxygen directly into the crew's breathing apparatus. This method is advantageous because it does not require large tanks of high-pressure oxygen, thus enhancing the overall weight savings and reducing the complexity of air supply arrangements in the aircraft.

In contrast, fresh air is unprocessed ambient air, high-pressure air refers to air at increased pressure typically stored in tanks, and recycled air suggests the reuse of air already present in the aircraft environment. These options do not meet the needs of the OBOGS, which relies on the chemistry of the partially conditioned air to produce breathable oxygen effectively.