FED-STD-791D
6.5 Filter the sample as follows:
6.5.1 Measure 100 mL of the sample into the funnel and cover the funnel.
6.5.2 If filtration time only is being determined, proceed to 6.5.2.1 and stop. If particle
count is to be measured, proceed to 6.5.2.2 and continue.
6.5.2.1 Within one minute after pouring the sample into the funnel, apply suction of 36
to 43 cm (14 to 17 in) of mercury to the filtration apparatus and start the timer. When the first
dry spot appears on the membrane filter, stop the timer, and disconnect the suction source.
6.5.2.2 Within one minute after pouring the sample into the funnel, start timer and apply
suction of 36 to 43 cm (14 to 17 in) of mercury to the filtration apparatus. Disconnect the
suction source and stop timer when the fluid level is 0.5 cm above the filter membrane, then add
the rinse from 6.5.3 before the last of the hydraulic fluid has passed through the filter.
NOTE 2:
If the time of filtration is more than 8% of the filtration time requirement, repeat
the test using the filtration procedure in 6.5.2.1.
6.5.3 Wash the graduated cylinder with 50 mL of filtered petroleum ether. Remove the
cover from the funnel, pour the washings into the funnel, replace the cover, and apply suction.
Perform this washing operation once again, disconnecting the suction source between washings.
6.5.4 Allow suction to continue until the filter is dry.
6.6 Prepare the test filter for microscopic counting as follows:
6.6.1 Using clean forceps, remove the filter from the filter holder, and sandwich it
between the cleaned glass slides (see 6.3).
6.6.2 Using transparent tape, seal the remaining unsealed edges of the slides.
6.7 Prepare a blank for analysis, using a fresh test filter and substituting filtered
petroleum ether for the sample, as specified in 6.4, 6.5, and 6.6.
6.8 Place the slide under the microscope (45X), and scan the entire filter to ascertain that
the particle distribution on the filter paper is uniform. If the distribution is not uniform, discard
the filter and repeat the test.
6.9 Estimate the number of particles present in the filter grid area (100 squares), listing
the numbers of particles by sizes according to the six size ranges given in Table I.
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