TM 55-1510-220-10 Table 2-12.  Recommended Fluid Dilution Chart AMBIENT PERCENT PERCENT WATER FREEZING POINT OF TEMPERATURE DEFROSTING BY MIXTURE (°F) (°F) FLUID BY VOLUME VOLUME (APPROXIMATE) 30° and above 20 80 10° 20° 30 70 0° 10° 40 60 -15° 0° 45 55 -25° -10° 50 50 -35° -20° 55 45 -45° -30° 60 40 -55° Notes: 1.  Use anti-icing and deicing fluid (MIL-A-8243) or commercial fluids. 2.  Heat mixture to a temperature of 82°  to 93°C (180°  to 200°F). 2-100.  APPLICATION OF EXTERNAL POWER. CAUTION Before    connecting    the    power    cables from  the  external  power  source  to  the aircraft,   insure   that   the   GPU   is   not touching the aircraft at any point.  Due to  the  voltage  drop  in  the  cables,  the two  ground  systems  will  be  of  different potentials.  Should they come in contact while   the   GPU   is   operating,   arcing could   occur.      Turn   off   all   external power while connecting the power cable to,   or   removing   it   from   the   external power   supply   receptacle.      Be   certain that  the  polarity  of  the  external  power source is the same as that of the aircraft before  it  is  connected.    Minimum  GPU requirement  is  400  amperes  continuous and  1800  amperes  for  one  tenth  of  a second. An  external  power  source  is  often  needed  to  supply  the electric current required to properly ground service the aircraft electrical   equipment   and   to   facilitate   starting   the   aircraft’s engines.    An  external  DC  power  receptacle  is  installed  on  the (outboard  side  of'  the  right  engine  nacelle.    An  external  A(, power  receptacle  is  installed  on  the  outboard  side  of'  the  left engine nacelle. 2-101.  SERVICING OXYGEN SYSTEM. The   oxygen   system      furnishes   breathing   oxygen   to   the pilot, copilot and first aid position.  Oxygen cylinder location is shown in figure 2-20. a.  Oxygen System Safety Precautions. WARNING Keep  fire  and  heat  away  from  oxygen equipment.          Do     not     smoke     while working with or near oxygen equipment,     and     take     care     not     to generate sparks with carelessly handled tools    when    working    on    the    oxygen system. (1)    Keep oxygen regulators, cylinders, gages, valves, fittings, masks, and all other components of the oxygen system   free   of   oil,   grease,   gasoline,   and   all   other   readily combustible substances.  The utmost care shall be exercised in servicing, handling, and inspecting the oxygen system. (2)    Do   not   allow   foreign   matter   to   enter oxygen lines. (3)    Never allow electrical equipment to come in contact with the oxygen cylinder. (4)    Never    use    oxygen    from    a    cylinder without first reducing its pressure through a regulator. b. Replenishing Oxygen System. 1. Remove   oxygen   access   door   on   outside   of' aircraft (fig. 2-20. 2. Remove protective cap on oxygen system filler valve. 3. Attach oxygen hose from oxygen servicing unit to filler valve. 2-87