FED-STD-791D
6.3 All equipment used for handling samples must be cleaned as described in Section
6.1. Precaution must be taken to minimize apparatus contamination from all sources including
airborne particles.
6.3.1 Duplicate tests should be performed whenever more than 1 L (1 qt) of lubricant
from the same lot or batch is available.
6.4 Preparation of test filters.
6.4.1 Identify each filter by marking the rim with a ballpoint pen, or by marking the petri
dishes that will contain them. Place each filter separately in petri dish and dry it for 1/2 h in an
oven regulated to 90C (194F). The petri dishes are then placed in a desiccator and moved to
the balance room in the vicinity of a balance on which weighings are to be made. Allow 1 h for
the closed desiccator and contents to come to equilibrium with ambient temperature. Use
forceps to place the test membrane on the balance pan by handling the edge only.
6.4.2 An air ionizer must be kept permanently on the base of the balance adjacent to the
pan to eliminate weighing errors due to electrostatic charge effects. Make all weighings to the
nearest 0.1 mg.
6.5 Laboratory filtration and contamination determination.
6.5.1 Thoroughly clean outside of sample container in the region to be opened by
washing with detergent and water and rinsing with tap water and filtered isopropyl alcohol.
Shake the 1 L (1 qt) sample of lubricant for 15 min. Cautiously open can so as to prevent the
introduction of metal chips into the sample.
6.5.2 To keep airborne contaminants on the membrane surface to a minimum, pour some
of the sample into the filter funnel before applying vacuum. Filter the entire contents of the 1 L
(1 qt) sample and thoroughly wash down the inside of the sample can with filtered petroleum
ether, passing the washings through the filter.
6.5.3 Wash down the inside of the filter funnel. This will complete the transfer of the
contaminant to the filter membrane. With the vacuum applied, carefully remove the funnel.
Wash the periphery of the membrane by directing a gentle stream of petroleum ether from the
edge toward the center, taking care not to wash any contaminant from the surface of the
membrane.
6.5.4 Carefully remove the test membrane from the filter base, using clean flat forceps,
and place it in a covered petri dish. Dry and reweigh as described in Section 6.4, taking care not
to wash any contaminant from the surface of the membrane.
NOTE 3:
For greater accuracy, control membranes may be used as described in ASTM D
2276
(Particulate Contaminant in Aviation Turbine Fuels).
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