FED-STD-791D
6.6 Determination of non-combustible contamination or ash.
6.6.1 Clean crucibles with hot detergent, wash and rinse with tap and distilled waters and
dry. Place crucibles in a muffle furnace maintained at 810C (1500F) for 1/2 h. Remove and
place crucibles in a desiccator. Allow closed desiccator and contents to cool and come to
equilibrium with the atmosphere in the vicinity of the balance for 2 h. Handle crucibles with
clean tongs. Weigh crucibles to the nearest 0.1 mg. Repeat this procedure until constant weight
is obtained.
6.6.2 Handling the membrane filter only with clean forceps, lay it, contaminant side
upwards, in the center of the crucible. Wet it with filtered isopropyl alcohol and push it gently
into the crucible.
6.6.3 Cover the crucible containing the filter membrane with a porcelain lid and place it
in a cold muffle furnace. Start the furnace and allow to come to 810C (1500F). Hold at this
temperature for 1/2 hour. Remove and place crucibles in a desiccator. Allow closed desiccator
and contents to cool and come to equilibrium with the atmosphere in the vicinity of the balance
for 2 h. Handle crucibles with clean tongs. Weigh crucibles to the nearest 0.1 mg. Repeat this
procedure until constant weight is obtained. Increase in weight is inorganic material or ash.
NOTE 4:
An alternate method is to place the crucible in a triangle and put a porcelain cover
over it. Place a small flame under the crucible to one side. When dry, char the
filter membrane, being careful not to ignite. Increase the burner flame until all
organic matter is distilled from the filter. Place the crucible in a preheated muffle
furnace 810C (1500F) for 1/2 h. Weigh as in 6.6.3.
7. CALCULATIONS
A- B
Contaminant, mg per L of oil =
C
where:
A = final weight of the membrane filter with the material collected in filtration, mg.
B = initial weight of the test membrane, mg.
C = volume of sample in L filtered through the test membrane.
D-B
Ash, mg per L of oil =
C
where:
D = final weight of the crucible and noncombustible material, ash, mg.
B = initial weight of the crucible, mg.
C = volume of sample in L filtered through the test membrane.
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