TM 55-1510-222-10 is  in  use,  a  check  of  the  supply  pressure  should  be made   at   intervals   during   flight,   to   note   the   quantity available  and  to  approximate  the  supply  duration.   The outside temperature is reduced as an aircraft ascends to higher   altitudes.      Oxygen   cylinders   thus   cooled   by temperature  change  will  show  a  pressure  drop.    This type of drop in pressure will raise again upon return to a lower or warmer altitude. b.   Oxygen masks.  Oxygen masks for the pilot and copilot   are   provided.      To   connect   a   mask   into   the oxygen system, the individual connects the line attached to the mask to the flexible hose which is attached to the cockpit sidewall.  The microphone in the oxygen mask is provided with a cord for connecting with the microphone jack.    To  test  mask  and  hose  integrity,  the  individual places  the  supply  control  lever  on  the  regulator  control panel to the ON position, puts on and adjusts his mask, selects TEST MASK position, and checks for leaks. WARNING Pure oxygen will support combustion. Do not smoke while oxygen is in use. If  any  symptoms  occur  suggestive  of the  onset  of  hypoxia,  immediately  set the  emergency  pressure  control  lever to     the     EMERGENCY     position     and descend below 10,000 feet.  Whenever carbon monoxide or other noxious gas is present or suspected, set the dilutor control   lever   to   100%   OXYGEN   and continue  breathing  undiluted  oxygen until the danger is past. c.   Normal Operation. Oxygen pressure is maintained at all times to the regulator control panels if the   cylinder   shut-off   valves   are   open   and   if   there   is pressure  in  the  cylinders.    Each  individual  places  the supply lever (green) on his regulator control panel to the ON    position,    and    the    diluter    lever    (white)    to    the NORMAL OXYGEN position. d.   Emergency Operation. For emergency operation,    the    affected    crew    member    selects    the EMERGENCY    position    of    the    emergency    pressure control  lever  (red)  on  his  regulator  control  panel.    This selection provides 100% oxygen at a positive pressure, regardless of the position of the diluter control lever on his panel. e.   First Aid Operation.  A first aid oxygen mask is installed   in   the   aft   cabin   area   as   a   supplemental   or emergency   source   of   oxygen.      The   mask   is   stowed behind    an    overhead    cover,    placarded    FIRST    AID OXYGEN PULL.  Removing the cover allows the mask to drop out of the container, exposing a manual control valve, which releases oxygen to the mask when placed in  the  ON    position.    After  using  the  mask,  the  manual valve   in   the   container   must   be   turned   OFF   before stowing the mask and replacing the cover. f. Oxygen Duration Example Problem. WANTED Duration in minutes of oxygen at 100% capacity. KNOWN Two man crew plus one passenger, cabin pressure altitude   =   15,000   feet,   crew   masks,   normal,   100% capacity. METHOD Find "two man crew plus one pass" line, move right then down to 15,000 - "normal" read 232.1 minutes. WANTED Duration  of  oxygen  for  previous  example  data  at 84% of capacity. KNOWN 232.1   minutes   duration   at   100%,   84%   capacity, total aircraft flow = 13.9 LPM. METHOD Multiply 232.1 X 0.84 = 194.9 minutes.  or Multiply 3,226  X  0.84  =  2709.8,  divide  by  13.9  LPM  =  194.9 minutes. WANTED Duration  of  oxygen  for  complement  at  other  cabin pressure altitude, at less than 100% capacity. KNOWN Cylinder at 84% capacity, 100% capacity = 3,226 L, cabin pressure altitude = 21,000 feet. 1  crew  mask  =  LPM  (100%),  1  passenger  mask  = LPM 2-54