FED-STD-791D
6.3 Preparation of centrifuge tubes. Clean one centrifuge tube for each compatibility test
to be made in the following manner:
a.
Rinse tubes with naphtha.
b.
Clean tubes with cleaning solution.
c.
Rinse tubes with distilled water.
Rinse tubes with denatured ethyl alcohol.
d.
6.4 Support the centrifuge tubes in an inverted position in an oven maintained at 121 "
1C (250 " 2F) for at least one-half hour.
6.5 Remove the tubes from the oven, place them in a desiccator, and permit them to cool
to room temperature.
6.6 Number each tube, weigh to the nearest mg, and replace tubes in the desiccator until
they are to be used.
6.7 Thoroughly shake sample and reference oils prior to sampling.
6.8 Place 110 " 1 mL of a reference oil and sample oil in a 400-mL beaker using the
same graduated cylinder each time the sample is measured, but a clean graduated cylinder for
each of the reference oils.
6.9 Heat the beaker containing the reference oils and test oil mixture in an oven at 121
" 1C (250 " 2F) for 20 + 1 min. Remove the beaker from the oven and stir (with a
mechanical stirrer) the contents while still hot for 5 min.
6.10 Transfer 100 mL of the contents to the weighed centrifuge tubes.
6.11 Cork the centrifuge tubes and store them in an upright position in a darkened area
such as a drawer or cupboard at room temperature for a period of 30 " 1 days.
6.12 At the conclusion of the storage period, remove the centrifuge tubes from the
storage area, place it in the centrifuge, and operate the centrifuge at 1500 " 25 rpm for a period
of 30 " 1 min.
NOTE 2:
In transferring the centrifuge tubes from the storage area to the centrifuge, care
must be taken not to disturb any material which may have separated from the oil.
6.13 Remove the centrifuge tubes from the centrifuge and decant and discard the
supernatant oil. Permit the centrifuge tubes to drain in an inverted position at room temperature
for a period of 2 h. Discard the drainings. If the residue is a solid, wash it with naphtha a
sufficient number of times to insure that it is free of oil. decanting is possible
NOTE 3: If the separated material is not sufficiently compacted by the centrifuging to
permit decanting the supernatant oil, continue the centrifuging for 15-minute intervals until
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