referred  to  as  particulate  contamination.  Con- tamination  is  always  present  to  some  degree,  even in  new,  unused  fluid,  but  must  be  kept  below  a level that will adversely affect system operation. Hydraulic   contamination   control   consists   of requirements,  techniques,  and  practices  necessary to  minimize  and  control  fluid  contamination. CLASSIFICATION There are many types of contaminants which are  harmful  to  hydraulic  systems  and  liquids. These  contaminants  may  be  divided  into  two different  classes—particulate  and  fluid. Particulate Contamination This  class  of  contaminants  includes  organic, metallic solid, and inorganic solid contaminants. These  contaminants  are  discussed  in  the  following paragraphs. ORGANIC  CONTAMINATION.—  Organic solids  or  semisolids  found  in  hydraulic  systems are  produced  by  wear,  oxidation,  or  polymeriza- tion.  Minute  particles  of  O-rings,  seals,  gaskets, and  hoses  are  present,  due  to  wear  or  chemical reactions. Synthetic products, such as neoprene, silicones,    and   hypalon,   though   resistant   to chemical reaction with hydraulic fluids, produce small  wear  particles.  Oxidation  of  hydraulic  fluids increases with pressure and temperature, although antioxidants are blended into hydraulic fluids to minimize   such   oxidation.   The   ability   of   a hydraulic   fluid   to   resist   oxidation   or   poly- merization  in  service  is  defined  as  its  oxidation stability.  Oxidation  products  appear  as  organic acids,   asphaltics,   gums,   and   varnishes.   These products combine with particles in the hydraulic fluid to form sludge. Some oxidation products are oil  soluble  and  cause  the  hydraulic  fluid  to increase in viscosity; other oxidation products are not  oil  soluble  and  form  sediment. METALLIC  SOLID  CONTAMINATION.— Metallic contaminants are almost always present in a hydraulic system and will range in size from microscopic  particles  to  particles  readily  visible to the naked eye. These particles are the result of wearing  and  scoring  of  bare  metal  parts  and plating  materials,  such  as  silver  and  chromium. Although  practically  all  metals  commonly  used for  parts  fabrication  and  plating  may  be  found in hydraulic fluids, the major metallic materials found   are   ferrous,   aluminum,   and   chromium particles. Because of their continuous high-speed internal   movement,   hydraulic   pumps   usually contribute   most   of   the   metallic   particulate contamination  present  in  hydraulic  systems.  Metal particles  are  also  produced  by  other  hydraulic system  components,  such  as  valves  and  actuators, due to body wear and the chipping and wearing away of small pieces of metal plating materials. INORGANIC   SOLID   CONTAMINA- TION.— This contaminant group includes dust, paint particles, dirt, and silicates. Glass particles from  glass  bead  peening  and  blasting  may  also be found as contaminants. Glass particles are very undesirable  contaminants  due  to  their  abrasive effect on synthetic rubber seals and the very fine surfaces  of  critical  moving  parts.  Atmospheric dust,  dirt,  paint  particles,  and  other  materials  are often drawn into hydraulic systems from external sources.  For  example,  the  wet  piston  shaft  of  a hydraulic  actuator  may  draw  some  of  these foreign materials into the cylinder past the wiper and dynamic seals, and the contaminant materials are  then  dispersed  in  the  hydraulic  fluid. Contaminants may also enter the hydraulic fluid during maintenance when tubing, hoses, fittings, and components are disconnected or replaced. It is  therefore  important  that  all  exposed  fluid  ports be  sealed  with  approved  protective  closures  to minimize  such  contamination. Fluid Contamination Air,  water,  solvent,  and  other  foreign  fluids are  in  the  class  of  fluid  contaminants. AIR CONTAMINATION.— Hydraulic fluids are adversely affected by dissolved, entrained, or free air. Air may be introduced through improper maintenance or as a result of system design. Any maintenance  operation  that  involves  breaking  into the  hydraulic  system,  such  as  disconnecting  or removing  a  line  or  component  will  invariably result   in   some   air   being   introduced   into   the system.  This  source  of  air  can  and  must  be minimized by prebilling replacement components with new filtered fluid prior to their installation. Failing to prefill a filter element bowl with fluid is  a  good  example  of  how  air  can  be  introduced into  the  system.  Although  prebilling  will  minimize introduction of air, it is still important to vent the system where venting is possible. Most hydraulic systems have built-in sources of air. Leaky seals in gas-pressurized accumulators and reservoirs can feed gas into a system faster 3-7