ASTM D6468 Test Method for High Temperature Stability of Distillate Fuels
11. Procedure
11.1 Adjust the heating bath to a temperature of 150 +/- 1.5°C (see 6.3).
11.2 Assemble the filter apparatus with a new filter paper. Filter at least 100 mL of the fuel sample through the paper. Use the inline vacuum regulator so that the maximum vacuum is 27 kPa (200 mm Hg). Measure two 50 +/- 2-mL volumes of filtered fuel and decant into each of two aging tubes (see Appendix X3).
NOTE 9 - Where practical, filter a fuel sample larger than 100 mL. This will reduce the possibility that the paper may absorb trace materials that affect stability.
11.3 Place the uncapped sample tubes in the heating bath for 90 +/- 3 min, or 180 +/- 5 min. Place the tubes in the bath in the same order in which they are to be removed. (Warning - Fuels will be heated above their flash points. There is a possibility that certain light fuels such as kerosine will boil under conditions of the test. Both situations raise the risk of fire when an ignition source is present. Fuel samples that may contain gasoline or other volatile components should not be tested.)
NOTE 10 - The severity of the test is increased by aging for longer times at 150°C. The selection of the aging time depends on application and should be established by correlation with other tests or with application requirements.
11.4 Remove the samples from the heating bath and allow to cool gradually in air to 20 to 25°C over a period of 90 min to 4 h. Cool in the dark to prevent photochemical reactions. Do not accelerate cooling by immersing in a cooling bath as this can result in small particle size and a lower pad rating. If samples are allowed to stand more than 4 h before filtering, insoluble gum may adhere to the aging tubes, resulting in erratic data. (Warning - The hot samples can cause severe burns. Use protective equipment.)
11.5 Prepare a filtration assembly with a new filter paper, and attach all grounding clips to ground. Filter one of the two aged fuel samples.
11.6 While maintaining vacuum, wash the aging tube with three small portions, about 15 mL each, of iso-octane and filter through the filter paper. Wash the inside of the filter assembly with iso-octane, and remove the funnel portion of the assembly. While still maintaining vacuum, use a gentle stream of isooctane from a squeeze bottle to wash the filter clean of any traces of fuel oil, and allow to dry under vacuum for 1 or 2 min (see Note 11). Shut off vacuum, and remove the filter from the assembly with forceps. When multiple tests are carried out, the margin of the pad may be used for identification after drying. If the deposit on the filter pad is not evenly distributed, reject the pad and rerun the test after cleaning the membrane filter holder (see 9.2).
NOTE 11 - Some laboratories may contain sufficient airborne contamination to change results if the air drying period is prolonged.
11.7 Place the test filter on the stack of at least ten unused filter pads, center the search unit on the filter, and record the meter value as percent reflectance filter pad rating to one decimal place.
11.8 Repeat 11.5-11.7, using the second of two aged fuel samples.
