Liquid Oxygen

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Aviators breathing oxygen (ABO) comes in both gaseous (type 1) and liquid (type 11) states. Liquid oxygen (LOX) is converted to a gas before its delivered to the aircrew. LOX requires frequent monitoring to prevent contamination and to ensure safe use. A surveillance program is the primary method of ensuring that each operation in the LOX supply system is carried out in strict compliance with established procedures. Surveillance begins with procurement or generation of LOX and continues throughout storage, handling, transfer, and servicing of aircraft.

The best assurance of personnel safety lies in the safety education of the people themselves. The safety of personnel can be assured only when there is thorough understanding of potential hazards, the correct procedures and equipment are used, and the equipment is in good working condition. Knowledge of a job situation and appropriate safety equipment is vital to successful completion of a job. Follow established safety procedures in NAVAIR 06-30-501.

Description and Properties of Liquid Oxygen

Oxygen can exist as a solid or gas, depending upon the temperature and pressure under which it is stored. At atmospheric pressure, oxygen exists as a solid at temperatures below its melting point, - 361F (-281C). Solid oxygen turns into a

liquid at its melting point and remains in this state until the temperature rises to its boiling point, -297F (-183C).

At this latter temperature, LOX vaporizes into the gaseous state. Gaseous oxygen will turn into liquid at atmospheric pressure by cooling to a temperature below -297F. By increasing the pressure, gaseous oxygen can be liquified at higher temperatures, up to its critical temperature,

- 182F ( -119C). Oxygen will not condense to a liquid at temperatures above its critical temperature regardless of the pressure applied. The pressure required to liquify oxygen at its critical temperature is known as its critical pressure, 736.5 psig. The application of high pressure and ultra-low temperatures to convert gases to their liquid state is known as the science and technology of cryogenics. LOX is a cryogenic fluid.

Physical Properties of Liquid Oxygen

Gaseous oxygen is colorless odorless, tasteless, and about 1.1 times as heavy as air. LOX is an extremely cold, pale blue fluid that flows like water. One gallon of LOX weighs 9.519 pounds, which is 1.14 times heavier than the weight of 1 gallon of water. LOX is stored and handled at atmospheric pressure in well-insulated containers that maintain the liquid at its boiling point ( -297F). Therefore, LOX is boiling as it slowly turns into gaseous oxygen. As the expanding gas from the boiling liquid increases in amount, it builds up pressure within the container. Therefore, the expanding gas must be vented to the atmosphere. Confinement of liquid oxygen can be dangerous to personnel, causing severe injury and death.

This section contains procedures and requirements for the quality control of LOX that is stored, transferred, handled, and used for breathing purposes by aircrews. This section applies to AME supervisors who must ensure all safety procedures and equipment are used during LOX servicing and handling by qualified personnel.


Personnel selected to perform operations in the LOX supply system should be trained and have a thorough knowledge of the characteristics of LOX, the significance of contamination, and the dangers involved. Only those personnel who demonstrate understanding of safety and who maintain reliable performance should be assigned the duties and responsibilities of handling LOX.

LOX Contamination

During the handling and transfer of LOX, environmental contaminants must be prevented from entering the system. LOX strongly attracts and absorbs atmospheric gases. Contaminants make the ABO unusable. Conscientious attention to correct procedures during handling and transfer operations will prevent contamination and ensure safety.

The aircraft LOX converter system should be sampled and tested for contamination as follows:

Test for odor as soon as possible after a report of in-flight odors by the pilot or aircrew. Any abnormal psychological or physiological effects to an aircrew during or after flight should be cause to suspect possible oxygen contamination. Possible oxygen contamination should also be considered in any aircraft mishap when the circumstances of the mishap are vague or unknown. A sample should be taken and sent to a test site for analysis with supporting details of the incident, including history of the supply source of LOX. Appropriate reports must be submitted in accordance with OPNAVINST 3750.6. An information copy should be provided to the Naval Air Engineering Center program manager.

Applicable squadrons selected by area commands must, during each calender month, take a LOX sample from at least one filled converter and residual LOX from one converter (taken from an aircraft after a flight mission), and forward both to a test site for contamination checks.

Aircraft oxygen and LOX systems, and LOX converters, must be purged in accordance with the applicable maintenance instructions manual (MIM) and/or NAVAIR 13-1-6.4, Oxygen Equipment Manual. Purging is done when the system or the converter is left open to the atmosphere, when empty, or whenever contamination is suspected.


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