ASTM D6468 Test Method for High Temperature Stability of Distillate Fuels
6. Apparatus
6.1 Aging Tubes, 25 x 200 mm, heavy wall test tubes made of borosilicate glass.

6.2 Heating Bath, with liquid heating medium, thermostatically controlled to maintain the sample in the aging tube within 1.5°C of 150°C. It must be large enough to hold aging tubes immersed in the heating liquid to a depth above the level of samples in the tubes. The bath and its location shall be such to enable shielding of the samples from direct light during aging. The volume of bath and its heat recovery rate shall be such that the temperature of the medium does not drop more than 5°C when the maximum number of aging tubes are inserted, and recovery to 150°C shall not require more than 15 min. (Warning - The flash point of the liquid heating medium must be at least 180°C. Bath vapors and oil sample vapors shall be properly vented. Exposed hot surfaces on the apparatus and hot heating medium can cause severe burns.)

6.3 Thermometer, either glass or digital, whose accuracy in the 140 to 160°C range is certified or traceable to a certified thermometer. Use to monitor the temperature of the heating bath in 6.2.

6.4 Membrane Filter Holder, to fit 47-mm membrane filters, fitted to a heavy-walled 500-mL or 1-L vacuum flask.

NOTE 1 - Several types of membrane filter holders are available. To reduce electrostatic hazards, an all metal filter holder equipped with grounding cables is recommended. Such an apparatus and correct grounding practices are described in Test Method D5452. A fritted glass filter holder is less preferred because of a tendency to become partially clogged during use so that filter pads that do not have uniform deposits are obtained. Glass filter holders that use a 75-µm (200-mesh) screen to support the filter are available; however, since the screen can be an unbonded electrostatic charge collector, these are not recommended for use with flammable liquids.

6.5 Vacuum Source, that limits the maximum vacuum to 27 kPa (200 mm Hg) below atmospheric pressure. The vacuum should rise to 27 kPa within 10 to 15 s after the sample is added to the filtration funnel.

NOTE 2 - Use of reduced vacuum improves retention of particulate on the relatively porous filter media.

6.6 Reflection meter, Photovolt Model 577 Digital Reflection Meter, complete with search unit Y with a green filter and polished black glass standard.

NOTE 3 - Other reflection meters or search units, or both, can be used, but they are likely to provide only similar (not identical) results. For example, Photovolt Model 577 digital reflection meter equipped with search unit W usually gives somewhat lower percent reflectance values. Correlation of these values is discussed in Appendix X1.

NOTE 4 - Older reflection meters, including but not limited to Photovolt Model 670 analog meter, are satisfactory.

7. Reagents and Materials
7.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. 8 Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.

7.2 Acetone, reagent grade. (Warning - Extremely flammable.)

7.3 Adherent Insolubles Solvent (Trisolvent or TAM), a mixture of equal parts by volume of reagent grade toluene (Warning - Flammable. Vapor harmful.), acetone (Warning - see 7.2), and methanol (Warning - Flammable. Vapor Harmful. May be fatal or cause blindness if swallowed or inhaled. Cannot be made nonpoisonous.).

7.4 Hydrocarbon Solvent, 2,2,4-trimethylpentane (isooctane), 99.75 % purity minimum (Warning - see 7.2).

NOTE 5 - Heptane is a satisfactory alternative hydrocarbon solvent. However, small differences may be seen due to slightly different solubility characteristics. Iso-octane is specified to be in agreement with the hydrocarbon solvent used in other middle distillate stability test methods such as Test Methods D2274 and D4625.

7.5 Filter Paper (Filter Pad), Whatman No. 1, 47-mm diameter, or equivalent.

NOTE 6 - Filter papers of 42.5 or 55-mm diameter are technically satisfactory. Filters with a diameter of 47 mm permit a small unused margin for identifying the sample and fit all filtration apparatuses.