FED-STD-791D
6.4 Preparation of test filters.
6.4.1 Place a 3-m membrane filter in a petri dish and dry for 30 min in an oven
regulated to 90C with petri dish lid ajar. Remove petri dish and with lid closed, place in
vicinity of balance on which weighings are to be made. Allow filters to cool for 30 min and
weigh to the nearest 0.1 mg. Use forceps to handle the test membranes. An air ionizer must be
utilized during all weighings to eliminate errors due to electrostatic charge effects.
6.5 Filtering of test sample.
6.5.1 Inspect visually the top of the sample container for dust and dirt. If present, wash
top of container using laboratory wash bottle containing filtered petroleum ether.
6.5.2 Shake sample mechanically for 5 min prior to test. Puncture small opening in top
of container (vent in case of screw cap containers) and heat to a maximum of 60C to facilitate
filtering. Lubricant formulations which filter readily (30 min or less per L [qt]) need not be
heated.
6.5.3 Cautiously open containers so as to prevent the introduction of metal particles into
the sample. Pour approximately 50 mL of sample into the assembled filtering apparatus
containing the pre-weighed filter and apply vacuum. Filter entire contents of the sample
container and record filtering time in min. All of the sample must be filtered through one filter.
Thoroughly wash down the inside of the sample container with filtered petroleum ether, passing
the washings through the filter.
6.5.4 Wash down the inside of the filter funnel using laboratory wash bottle containing
filtered petroleum ether. With vacuum still applied, carefully remove the funnel. Wash the
periphery of the membrane by directing a gentle stream of filtered petroleum ether from the edge
toward the center, taking care not to wash any contaminant from the surface of the membrane.
6.5.5 Carefully remove the test membrane from the filter base and place it in a petri dish.
Dry and reweigh as described in 6.4.1.
7. CALCULATIONS
7.1 Calculate particulate contaminant per liter of oil from the initial and final weight of
the test filter and the volume of sample filtered.
A- B
Particulate contaminant, mg/liter of test =
C
where:
A = final weight of test filter in mg.
B = initial weight of test membrane in mg.
C = volume of sample measure in liters passed through the filter.
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