TM 1-1500-204-23-3 Figure 2-1.  Vaporization Characteristics Table 2-2.  AVGAS Identification Grade Color 80/87 Red 100/130 High Lead Green 100/130 Low Lead Blue 115/145 Purple 2-4.  Contamination of Fuels.  There are several forms of    contamination    in    aviation    fuels.        The    higher    the viscosity   of   the   fuel,   the   greater   is   its   ability   to   hold contaminants  in  suspension.    For  this  reason,  jet  fuels having     a     high     viscosity     are     more     likely     to     have contaminants.        Table    2-3    shows    visual    contaminant characteristics.  Water, solids, and microbial growths are the principal types of contamination. a. Water.  Either fresh or salt water may be present in  fuel,  and  either  may  be  present  as  dissolved  or  free water. (1), Dissolved  water.    Dissolved  water  is  water that has been absorbed by the fuel.  It cannot be seen and cannot  be  separated  out  of  the  fuel  by  either  filtration  or mechanical means.  The danger of dissolved water is that it  settles  out  as  free  water  when  the  fuel  is  cooled  to  a temperature lower than that at which the water dissolved. Such  a  cooling  of  fuel  is  likely  at  high  altitudes.    Once freed, all the dangers of free water are present. (2) Free  water.    Free  water  can  be  removed from fuel by adequate filtering.  It can be seen in the fuel as a cloud, an emulsion, droplets or, in large amounts, as water   on   the   bottom   of   a   tank,   sample   container,   or filter/separator.      Free   water,   either   fresh   or   salty,   can freeze   in   the   aircraft   fuel   system,   can   make   certain aircraft   instruments   malfunction,   and   can   corrode   the components  of  the  aircraft  fuel  system.    (Salt  water  is more  corrosive  than  fresh  water.)  Ice  in  an  aircraft  fuel system can make the engines fail. b. Solids.        Sediment    from    tanks,    pipes,    hoses, pumps, people, and the air contaminates fuel.  The most common elements of the sediment found in aviation fuels are   bits   of   rust,   paint,   metal,   rubber,   dust,   and   sand. Sediment is classified by particle size as shown in figure 2-2. (1) Coarse  sediment.      Particles   classified   as coarse are 10 microns in size or larger (25,400 microns = 1 inch).  Coarse sediment settles out of fuel easily, and it can  also  be  removed  by  adequate  filtering.    Particles  of coarse  sediment  clog  nozzle  screens,  other  fine  screens throughout the aircraft fuel system and, most dangerously, the fuel orifices of aircraft engines.  Particles of  this  size  also  get  wedged  in  sliding  valve  clearances and valve shoulders where they cause excessive wear in the fuel controls and fuel-metering equipment. (2) Fine  sediment.    Particles  classified  as  fine are   smaller   than   10   microns   in   size.      Removing   fine sediment   by   settling   or   filtering   is   effective   only   to   a limited degree; the particles can, however, be centrifuged out in a rotating chamber.  Fine sediment accumulates in fuel  controls  and  forms  a  dark  shellaclike  surface  on  the sliding valves.  It can also form a sludge-like material that makes fuel-metering equipment operate sluggishly. Particles of fine sediment are not visible to the naked eye, but  they  do  scatter  light.    This  light-scattering  property makes  them  show  up  as  point  flashes  of  light  or  as  a slight haze in the fuel. 2-3