The valve is equipped with a fusible metal safety  plug  and  a  safety  disc  to  release  the contents of the cylinder if the pressure becomes excessive because of high temperatures. The safety plug is filled with a fusible metal designed to melt at  temperatures  ranging  from  208°  to  220°F (97.8° to 104.5°C). The cylinder and valve assembly is connected to  the  oxygen  tubing  by  silver  soldering  the tubing to a coupling nose and securing the nose to the valve outlet with a coupling nut. The self-opening (automatic) oxygen cylinder valve is automatically opened when it is connected to the oxygen line. The use of this type of valve permits remote location of the oxygen cylinder to places less vulnerable during combat and more readily  accessible  for  servicing. Regulators The success or failure of high-altitude flight depends primarily on the proper functioning of the oxygen breathing regulator. Acting as a meter- ing  device,  the  regulator  is  the  heart  of  the oxygen system. To perform successfully in an air- craft  system,  a  regulator  must  deliver  the  life- supporting oxygen in the quantities demanded throughout  its  entire  range  of  operation. Although  personnel  of  the  PR  rating  are primarily   responsible   for   maintenance   of regulators,   the   AME   is   responsible   for   per- forming operational checks in the aircraft and for removal  and  installation.  In  other  words,  the AME removes a malfunctioning regulator from the aircraft and delivers it to the shop where the PR   determines   the   trouble   and   makes   the necessary  repairs.  When  the  trouble  is  corrected, the AME reinstalls the regulator in the aircraft. Tubing Two  types  of  tubing  are  used  in  aircraft oxygen systems. Low-pressure aluminum alloy tubing is used in lines carrying pressures up to 450 psi. High-pressure copper tubing is used in lines carrying pressure above 450 psi. NOTE: Some of the newer naval aircraft are equipped with high-pressure oxygen lines made of aluminum alloy. Lines running from the filler valve to each of the   cylinders   are   called   filler   lines.   Those running from the cylinders to the regulators are called distribution or supply lines. Oxygen lines, like all other lines in the aircraft, are identified by strips of colored tape. The strips of tape are wrapped around each line near each fitting  and  at  least  once  in  each  compartment through which the line runs. The color code for oxygen lines is green and white with the words Breathing Oxygen  printed in the green portion, while black outlines of rectangles appear in the white portion. Resistance to fatigue failure is an important factor  in  oxygen  line  design  because  the  line pressure in a high-pressure system will at times exceed 1,800 psi, and at other times be as low as 300 psi. Because of these varying pressures and temperatures, expansion and contraction occur all the   time.   These   fluctuations   cause   “metal fatigue,” which must be guarded against in both the  design  and  the  construction  specifications  for tubing.  Steps  are  taken  during  installation  to prevent fatigue failure of the tubing. Tubing is bent in smooth coils wherever it is connected to an inflexible object, like a cylinder or a regulator. Every precaution is taken to prevent the accidental discharge of compressed oxygen because of faulty tubing or installation. Although simple in con- struction  and  purpose,  tubing  is  the  primary means  by  which  oxygen  is  routed  from  the cylinders to the regulator stations. High-pressure   tubing   is   usually   seamless copper tubing, and is manufactured in accordance with   strict   specifications.   It   has   an   outside diameter  of  3/16  inch  and  a  wall  thickness  of 0.035   inch.   For   application   in   high-pressure oxygen installations, copper tubing is type N (soft annealed), and is pressure tested at not less than 3,000  psi. High-pressure  tubing  is  used  between  the oxygen cylinder valve and the filler connection in all systems, between the cylinder valve and the regulator  inlet  in  high-pressure  systems,  and between  the  cylinder  valve  and  pressure  reducer in reduced high-pressure systems. To   connect   high-pressure   copper   tubing, adapters  and  fittings  are  silver  soldered  to  the tubing  ends.  Due  to  the  high  pressures  involved, the security (leak tightness) of all high-pressure lines  relies  primarily  on  a  metal-to-metal  contact of   all   its   fittings   and   connections.   A   fitting properly silver soldered to the end of a length of copper tubing will not come loose or leak. Some of the later models of naval aircraft use aluminum  alloy  or  stainless  steel  tubing  in  high- -pressure oxygen system installations. Replacement tubing  should  be  manufactured  of  the  same  type 4-6