Liquid Oxygen Forms of Oxygen and Characteristics Gaseous Oxygen 16-4 Figure 16-4.  “ Aviator’s breathing oxygen ”  is marked on all oxygen  cylinders designed for this purpose. Pure gaseous oxygen, or nearly pure gaseous oxygen, is  stored and transported in high-pressure cylinders that are  typically painted green. Technicians should be cautious to  keep pure oxygen away from fuel, oil, and grease to prevent  unwanted combustion. Not all oxygen in containers is the  same. Aviator’s breathing oxygen is tested for the presence  of water. This is done to avoid the possibility of it freezing  in the small passage ways of valves and regulators. Ice could  prevent delivery of the oxygen when needed. Aircraft often  operate in subzero temperatures, increasing the possibility  of icing. The water level should be a maximum of .02ml per  Oxygen is a colorless, odorless, and tasteless gas at normal  atmospheric temperatures and pressures. It transforms into  a liquid at –183 °C (its boiling point). Oxygen combines  readily with most elements and numerous compounds. This  combining is called oxidation. Typically, oxidation produces  heat. When something burns, it is actually rapidly combining  with oxygen. Oxygen itself does not burn because it does not  combine with itself, except to form oxygen or ozone. But,  pure oxygen combines violently with petroleum products  creating a significant hazard when handling these materials  in close proximity to each other. Nevertheless, oxygen  and various petroleum fuels combine to create the energy  produced in internal combustion engines.  Liquid oxygen (LOX) is a pale blue, transparent liquid.  Oxygen can be made liquid by lowering the temperature to  Oxygen systems that increase the quantity of oxygen in  breathing air are most commonly used as primary systems  in small and medium size aircraft designed without cabin  pressurization. Pressurized aircraft utilize oxygen systems as  a means of redundancy should pressurization fail. Portable  oxygen equipment may also be aboard for first aid purposes. Use of portable pulse oximeters has become more common  in aviation. These devices measure the oxygen saturation  level of the blood. With this information, adjustments to  the oxygen flow rates of onboard oxygen equipment can be  made to prevent hypoxia.  Figure 16-5  shows an oximeter into  which a finger is inserted to measure oxygen saturation of the  blood in percentage. Heart rate is also displayed. By increasing the quantity of oxygen available in the lungs,  less pressure is required to saturate the blood. This is the basic  function of an aircraft oxygen system. Increasing the level  of oxygen above the 21 percent found in the atmosphere can  offset the reduced pressure encountered as altitude increases.  Oxygen may be regulated into the air that is breathed so as  to maintain a sufficient amount for blood saturation. Normal  mental and physical activity can be maintained at indicated  altitudes of up to about 40,000 feet with the sole use of  supplemental oxygen. Production of gaseous oxygen for commercial or aircraft  cylinders is often through a process of liquefying air. By  controlling temperature and pressure, the nitrogen in the  air can be allowed to boil off leaving mostly pure oxygen.  Oxygen may also be produced by the electrolysis of water.  Passing electric current through water separates the oxygen  from the hydrogen. One further method of producing gaseous  oxygen is by separating the nitrogen and oxygen in the air  through the use of a molecular sieve. This membrane filters  out nitrogen and some of the other gases in air, leaving nearly  pure oxygen for use. Onboard oxygen sieves, or oxygen  concentrators as they are sometimes called, are used on some  military aircraft. Their use in civil aviation is expected.  liter of oxygen. The words “Aviator’s Breathing Oxygen”  should be marked clearly on any cylinders containing oxygen  for this purpose.  [Figure 16-4] Large transport-category and high-performance passenger  aircraft pressurize the air in the cabin. This serves to push  more of the normal 21 percent oxygen found in the air into  the blood for saturation. Techniques for pressurization are  discussed later in this chapter. When utilized, the percentage  of oxygen available for breathing remains the same; only the  pressure is increased. 