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