slowly oxidized, the heat usually being carried away fast enough; however, when the heat can- not easily escape, the temperature may rise dangerously and a fire will break out. This is called spontaneous combustion. Oxygen does not bum, but does support combustion. Nitrogen neither burns nor supports combustion. Therefore, combustible materials bum more readily and more vigorously in oxygen than in air, since air is composed of about 78 percent nitrogen by volume and only about 21 percent oxygen. In addition to existing as a gas, oxygen can exist as a liquid and as a solid. Liquid oxygen is pale blue in color. It flows like water, and weighs 9.54 pounds per gallon. Liquid oxygen, commonly referred to as LOX, is normally obtained by a combined cooling and pressurization process. When the temperature of gaseous oxygen is lowered to – 182°F under about 750 psi pressure, it will begin to form into a liquid. When the temperature is lowered to – 297°F, it will remain a liquid under normal atmospheric pressure. Once converted into a liquid, oxygen will remain in its liquid state as long as the temperature is maintained below  – 297 ‘F. The liquid has an expansion ratio of 862 to 1, which means that one volume of liquid oxygen will expand 862 times when converted to a gas at atmospheric pressure. Thus, 1 liter of liquid oxygen produces 862 liters of gaseous oxygen. Until a few years ago, all oxygen carried in naval aircraft was in the gaseous state. As flight durations increased, however, it was found that the weight and space problems involved with carrying increasing amounts of gaseous oxygen were becoming intolerable. LOX has proven the answer to these problems. In its liquid state, oxygen can be “packed” into containers small and light enough to be carried even in fighter-type air- craft without weight and space penalty. In the aircraft, oxygen in the liquid state is carried in a container called a converter. This is a double-walled, vacuum-insulated container similar to the common Thermos bottle. The converter is equipped with the necessary valves and tubing for vaporizing the liquid and warm- ing the gas to cockpit temperatures. TYPES OF OXYGEN Aviator’s breathing oxygen (MIL-O-27210C) is supplied in two types (I and II). Type I is gaseous oxygen, and type 11 is LOX. Oxygen procured under the above specification is required to be 99.5 percent pure. The water vapor content must not be more than 0.02 milligram per liter when tested at 70°F and at sea level pressure. This is practically bone dry. Technical oxygen, both gaseous and liquid, is procured under specification BB-0-925. The moisture content of technical oxygen is not as rigidly controlled as that of breathing oxygen; therefore, the technical grade should never be used in aircraft oxygen systems. The extremely low moisture content required of breathing oxygen is not to avoid physical injury to the body, but to ensure proper opera- tion of the oxygen system. Air containing a high percentage of moisture can be breathed indefinitely without any serious ill effects. However, the moisture affects the aircraft oxygen system in the small orifices and passages in the regulator. Freezing temperatures can clog the system with ice and prevent oxygen from reaching the user. Therefore, extreme precautions must be taken to safeguard against the hazards of water vapor in oxygen systems. OXYGEN COMPONENT TEST STAND 1172AS100 Regulator test stands are designed for testing oxygen regulators for flow capacities, oxygen concentrations, pressure characteristics, and various leakage tests at different simulated altitudes. There are several models of test stands capable of testing the oxygen regulators, converters, etc. We will cover only the ones that most oxygen shops throughout the Navy use. If you happen to work in an oxygen shop that is using outdated equipment, ask the petty officer in charge of the work center to show you the literature that covers that equipment. In this rate training manual, we will discuss only the 1172AS100 test stand used for testing oxygen regulators. The Oxygen System Components Test Stand, Model 1172AS100, tests and evaluates miniature oxygen breathing regulators as well as panel and 11-3