Additional  information  on  the  various  types of  viscometers  and  their  operation  can  be  found in the Physical  Measurements  Training  Manual, NAVAIR   17-35QAL-2. Viscosity Index The viscosity index (V.I.) of an oil is a number that indicates the effect of temperature changes on  the  viscosity  of  the  oil.  A  low  V.I.  signifies a  relatively  large  change  of  viscosity  with  changes of  temperature.  In  other  words,  the  oil  becomes extremely thin at high temperatures and extremely thick at low temperatures. On the other hand, a high  V.I.  signifies  relatively  little  change  in viscosity  over  a  wide  temperature  range. An   ideal   oil   for   most   purposes   is   one that  maintains  a  constant  viscosity  throughout temperature changes. The importance of the V.I. can  be  shown  easily  by  considering  automotive lubricants.  An  oil  having  a  high  V.I.  resists excessive thickening when the engine is cold and, consequently,  promotes  rapid  starting  and  prompt circulation; it resists excessive thinning when the motor is hot and thus provides full lubrication and prevents  excessive  oil  consumption. Another example of the importance of the V.I. is the need for a high V.I. hydraulic oil for military aircraft,  since  hydraulic  control  systems  may  be exposed   to   temperatures   ranging   from   below –65°F  at  high  altitudes  to  over  100°F  on  the ground. For the proper operation of the hydraulic control  system,  the  hydraulic  fluid  must  have  a sufficiently  high  V.I.  to  perform  its  functions  at the extremes of the expected temperature range. Liquids  with  a  high  viscosity  have  a  greater resistance  to  heat  than  low  viscosity  liquids  which have  been  derived  from  the  same  source.  The average   hydraulic   liquid   has   a   relatively   low viscosity.  Fortunately,  there  is  a  wide  choice  of liquids  available  for  use  in  the  viscosity  range required  of  hydraulic  liquids. The  V.I.  of  an  oil  may  be  determined  if  its viscosity   at   any   two   temperatures   is   known. Tables,  based  on  a  large  number  of  tests,  are issued  by  the  American  Society  for  Testing and  Materials  (ASTM).  These  tables  permit calculation  of  the  V.I.  from  known  viscosities. LUBRICATING   POWER If  motion  takes  place  between  surfaces  in contact,   friction   tends   to   oppose   the   motion. When  pressure  forces  the  liquid  of  a  hydraulic system between the surfaces of moving parts, the liquid spreads out into a thin film which enables the parts to move more freely. Different liquids, including   oils,   vary   greatly   not   only   in   their lubricating ability but also in film strength. Film strength is the capability of a liquid to resist being wiped or squeezed out from between the surfaces when  spread  out  in  an  extremely  thin  layer.  A liquid will no longer lubricate if the film breaks down, since the motion of part against part wipes the  metal  clean  of  liquid. Lubricating  power  varies  with  temperature changes;   therefore,   the   climatic   and   working conditions  must  enter  into  the  determination  of the  lubricating  qualities  of  a  liquid.  Unlike viscosity,   which   is   a   physical   property,   the lubricating  power  and  film  strength  of  a  liquid is   directly   related   to   its   chemical   nature. Lubricating  qualities  and  film  strength  can  be improved  by  the  addition  of  certain  chemical agents. CHEMICAL   STABILITY Chemical stability is another property which is  exceedingly  important  in  the  selection  of  a hydraulic liquid. It is defined as the liquid’s ability to   resist   oxidation   and   deterioration   for   long periods. All liquids tend to undergo unfavorable changes under severe operating conditions. This is the case, for example, when a system operates for   a   considerable   period   of   time   at   high temperatures. Excessive temperatures, especially extremely high temperatures, have a great effect on the life of a liquid. The temperature of the liquid in the reservoir  of  an  operating  hydraulic  system  does not   always   indicate   the   operating   conditions throughout the system. Localized hot spots occur on bearings, gear teeth, or at other points where the liquid under pressure is forced through small orifices.  Continuous  passage  of  the  liquid  through these  points  may  produce  local  temperatures  high enough  to  carbonize  the  liquid  or  turn  it  into sludge,  yet  the  liquid  in  the  reservoir  may  not indicate an excessively high temperature. Liquids  may  break  down  if  exposed  to  air, water, salt, or other impurities, especially if they are in constant motion or subjected to heat. Some metals,  such  as  zinc,  lead,  brass,  and  copper,  have undesirable   chemical   reactions   with   certain liquids. These chemical reactions result in the forma- tion  of  sludge,  gums,  carbon,  or  other  deposits which clog openings, cause valves and pistons to stick or leak, and give poor lubrication to moving 3-3