FED-STD-791D
in an oven at 96C (205F) for 1 h, cooled at least 1 h in a desiccator, and
reweighed. The resulting deposit weight should be added to that obtained from
the filter screen, and the total obtained reported as sludge.
6.21 Check the coking tendency of the oil as follows:
a. Being extremely careful not to dislodge any deposits, remove the
coking tube from the coking head.
b. Allow the tube to cool for at least 1 h.
c. Immerse the tube up to its flange in n-hexane, and allow it to soak for
10 min.
CAUTION:Handle carefully to avoid dislodging deposit.
d. Repeat the soaking for 10 min in clean n-hexane.
e. Swab out the thermocouple wells with pipe cleaners to remove any oil.
Then install a clean thermocouple in one of the wells.
f. Connect the heater to the rig power source, and heat tube to 204C
(400F) maximum.
g. Remove the thermocouple, allow the tube to cool in air at room
temperature for at least 1 h, weigh to nearest 10 mg, and record.
6.22 Disassemble and clean the deposition test rig. (Refer to 6.3, 6.4, and 6.5.)
7. CALCULATIONS
7.1 Calculate the following:
a. Sludge is expressed as the difference between the final and initial
weights of the filter screen.
b. Coke is expressed as the difference between the final and initial
weights of the coking tube.
c. Deposit rating is calculated as follows:
Deposit rating = Sludge + 10 * coke
8. REPORTING
8.1 Report the weight, in g, of the sludge and the coke separately, and the deposit
rating of the oil.
8.2 Report the following operating data:
a. Elapsed time (to within 1/2 h) to pump-pressure increase of 34.5 gage kPa (5
psig).
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