TM 1-1510-218-10 2-79 aft toilet compartment outlets.  Adjacent to the system control   knob   is   another   push-pull   knob,   placarded CABIN / PULL ON / PASS READY.  This actuates the on-off    valve    supplying    oxygen    to    the    eight    cabin outlets.  The cabin and aft toilet compartment outlets are  located  on  individual  overhead  service  panels  at each seat station.  A placard on the overhead oxygen control panel indicates that all masks must be plugged in  and  immediately  available  for  flights  above  25,000 feet.      An   exception   to   this   is   the   pilot   and   copilot masks,  which  shall  be  plugged  in  and  hung  on  the cockpit     sidewall     hooks     at     all     times,     ready     for immediate  use.    The  filler  fitting  is  located  behind  a panel  externally  accessible  on  the  right  side  of  the empennage. Due  to  the  possibility  of  the  oxygen  valve control   cable   freezing,   the   crew   oxygen valve shall be kept open during flight. NOTE INPH must be selected on individual Audio Control Panels when using oxygen mask to provide intercommunications between pilots through the headsets.  b.  Pilot   and   Copilot   Masks.       The   pilot   and copilot    oxygen    masks    are    diluter-demand    100% regulator  masks,  which  provide  the  proper  dilution  of oxygen  with  cabin  air  to  conserve  oxygen  at  lower cabin  pressure  altitudes.    Placing  the  diluter  control lever on the mask regulator in the   NORMAL position permits    the    regulator    to    automatically    schedule    a proportional increase in oxygen as the cabin pressure altitude increases.  When not in use the masks should be stowed with the lever in the 100% position.  While in use at altitudes below 20,000 feet, the lever may (at the   crew’s   discretion)   be   placed   in   the   NORMAL position   to   conserve   oxygen.      Each   diluter-demand mask  has  a  pressure  detector  in  the  oxygen  supply line  to  provide  a  visual  indication  of  oxygen  pressure.   A   red   signal   viewed   in   the   window   of   the   detector indicates  low  pressure  and  a  green  signal  indicates adequate pressure. c.   Passenger  Masks.     The  passenger  masks are kept in sidewall and seat back pockets in the cabin and  in  the  sidewall  pocket  in  the  toilet  compartment.   All  masks  are  easily  connected  by  pushing  the  plug firmly into the outlet and turning clockwise approximately     one-quarter     turn.          Unplugging     is accomplished by reversing the motion.  When stowing the  mask,  coil  the  breathing  line  around  the  mask  to avoid any sharp bends in the line. d.  Oxygen    Duration.        The    oxygen    duration depends upon the amount of oxygen available and the demand.    The  amount  of  useable  volume  available  is 1,222  Liters  (L),  as  measured  at  21  °C,  760  mm  of pressure  and  no  water  vapor.    These  conditions  are referred to as "Normal Temperature, Pressure, Dry" or NTPD.    The  rated or 100% capacity of the cylinder is available at 1,850 psig and 21 °C.  A percentage of the capacity at other stabilized cylinder temperatures and pressure   may   be   obtained   from   Figure   2-24.    The demand  upon  this  amount  depends  upon  the  number of crew and  passenger  masks  in  use  and  the  flow  of oxygen from the masks.  Planning flow rates may be found in Table 2-5.  The duration or amount divided by demand,  of  various  combinations  of  passengers  and altitude, is found in Table 2-6.  For other conditions or configurations, the duration may be readily calculated with the data provided. e.   Emergency Operation. A control is provided on the masks for the pilot and copilot.  The control may be set at 100% OXY when required. 2-70.  OXYGEN SYSTEM  D T . Do not smoke while oxygen is in use. a.   Description.   The oxygen system is provided primarily  as  an  emergency  use  system;  however,  the system may be used to provide supplemental (first aid) oxygen.      A   49-cubic   foot   1,222   (L)   usable   oxygen supply cylinder charged with aviators breathing oxygen is  installed  in  the  unpressurized  portion  of  the  aircraft behind    the    aft    pressure    bulkhead.        The    oxygen pressure   gauge   on   the   copilot’s   subpanel   gives   a direct   reading   of   cylinder   pressure.      The   pressure regulator and control valve are located adjacent to the supply cylinder and are actuated by a push-pull control knob,   placarded    PULL    ON    SYS    READY,   located immediately  aft  of  the  overhead  circuit  breaker  panel.   The  control  knob  operates  a  cable,  which  opens  and closes    the    shutoff    valve    on    the    supply    cylinder, Figure 2-25.    Opening  the  shutoff  valve  charges  the primary   oxygen   supply   line,   which   in   turn   delivers oxygen   to   the   pilot,   copilot,   first   aid   oxygen   mask outlets,  and  to  the  passenger  oxygen  system  control valve.   Adjacent to the system  control knob is another push-pull   knob,   placarded   PASSENGER  MANUAL O'RIDE.    This  activates  the  passenger  masks  when the  PULL  ON  SYS  READY  knob  is  pulled  on.    Any time    the    primary    oxygen    supply    line    is    charged, oxygen will be available from the first aid oxygen outlet by    manually    opening    the    overhead    access    door, placarded  FIRST  AID  OXYGEN-PULL,  and  opening the  ON  /  OFF   valve  inside.    The  first  aid  oxygen  is located in the aft portion of the aircraft containing the WARNING WARNING