The precipitation
number, as determined by ASTM D
91-52 (Federal Test Method 3101), is
the amount of solid
sediment in an oil. This
is defined as
the number of
milliliters of precipitate formed when 10 ml
of the oil are mixed
with 90 ml of ASTM precipitation naphtha
and centrifuged. Unless the base
oil is a cylinder stock or a black oil, in which case some
asphaltic material may
precipitate, the sediment
should be nil in
unused oils.
Used gear oils, whether straight
mineral oil or one
containing additives, may have
high precipitation numbers. These may
consist of metal particles, water, sludge, or material
arising from degradation of
oil or
additives.
