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|  MIL-F-27656C(USAF)
4.6.6.2 Degree of filtration test. The hydraulic fluid and air used in the degree of filtration test shall
be clean and filtered prior to the test. Figure 6 shows a degree of filtration test setup with a cleanup
device attached.
4.6.6.2.1 Test procedure for determination of oil cleanliness. The test procedure for determination of
oil cleanness shall be as follows.
a.
A cleanup apparatus consisting of needle valves, a Millipore fixture, Millipore Type HA
disc (1-inch diameter) manometer, flowmeter, and pump with adjustable internal bypass shall be
installed on the test stand similar to the setup shown on figure 6.
b.
A constant flow shall be established and maintained for 30 minutes, with at least
periodic agitation of the contaminant mixing chamber, at a pressure differential of 50 to 60 psi across
the Millipore filter assembly.
With the flow held constant, the pressure drop across the Millipore fixture shall be recorded
c.
at the beginning and end of the flow period.
variation exceeds 1-1/2 percent, the system shall be recleaned until the 1-1/2 percent maximum
variation is achieved.
e.
Two thousand milliliters of the clean fluid shall be collected in a 5,000-ml beaker from
the sampling valve downstream of the test filter housing.
f.
A membrane filter 47-mm diameter disk, absolute 1.0 micron (type AA Millipore, or
equal), shall be washed with 200 ml of petroleum ether and dried to constant weight at 125 F. The
weight shall be accurate to 0.1 mg. The disk shall be heated at 125F for 30 minutes, then cooled 30
minutes in a desiccator.
The weighed filter membrane shall be assembled into the filter holder assembly (see
g.
figure 7) and fitted to a 5,000-ml vacuum flask connected to a vacuum pump ( minimum 25 inches
HG vacuum).
h. The 2,000-ml fluid sample shall be filtered through the membrane disk The beaker shall be
washed with 800 ml of petroleum naptha and 800 ml of petroleum either (boiling point 95 0to 130 F).
The wash fluid shall be filtered through the membrane disk. One hundred milliliters of petroleum naphtha
and one hundred milliliters of petroleum ether shall then be used to wash down the funnel, and then this
wash shall be filtered.
17
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