ASTM D381 standard test method for gum content in fuels by jet evaporation
11. Procedure
11.1 Wash the beakers, including the tare, with the gum solvent until free of gum. Rinse thoroughly with water and immerse in a mildly alkaline or neutral pH laboratory detergent cleaning solution.

11.1.1 The type of detergent and conditions for its use need to be established in each laboratory. The criterion for satisfactory cleaning shall be a matching of the quality of that obtained with chromic acid cleaning solutions on used beakers (fresh chromic acid, 6-h soaking period, rinsing with distilled water and drying). For this comparison visual appearance and mass loss on heating the glassware under test conditions may be used. Detergent cleaning avoids the potential hazards and inconveniences related to handling corrosive chromic acid solutions. The latter remains as the reference cleaning practice and as such may function as an alternate to the preferred procedure-cleaning with detergent solutions.

11.1.2 Remove the beakers from the cleaning solution by means of stainless steel forceps or tongs (see 6.10) and handle only with forceps or tongs thereafter. Wash the beakers thoroughly, first with tap water and then with distilled water, and dry in an oven at 150°C for at least 1 h. Cool the beakers for at least 2 h in the cooling vessel placed in the vicinity of the balance.

11.2 Select the operating conditions, corresponding to the aviation motor gasoline or aircraft turbine fuel under test, from the data given in Table 1. Heat the bath to the prescribed operating temperature. Introduce air or steam to the apparatus and adjust the total flow to that established in 8.1 or 9.2. If an external preheater is used, regulate the temperature of the vaporizing medium to give the prescribed test well temperature.

11.3 Weigh the tare and test beakers to the nearest 0.1 mg. Record the masses.

11.4 If suspended or settled solid matter is present, mix or shake the contents of the sample container thoroughly using an appropriate method. At atmospheric pressure, immediately filter a quantity of the sample through a sintered-glass funnel of coarse porosity (see 13.3). Treat the filtrate as described in 11.5-11.7.

11.5 Measure a 50 +/- 0.5 mL test specimen in a graduated cylinder (see 6.9), and transfer it to a weighed beaker (see 6.2). (See Practice D4057 regarding sampling.) Use one beaker for each test specimen to be tested, and fill each beaker except the tare. Place the filled beakers and the tare in the evaporation bath, keeping the elapsed time between placing the first and last beaker in the bath to a minimum. When using air as the evaporation medium in an evaporation bath without a mechanical means to raise and lower the conical jets, use forceps or tongs (see 6.10) to replace the conical jet as each individual beaker is placed in the bath. When using steam, allow the beakers to heat for 3 to 4 min before using forceps or tongs to replace the conical jet (or lowering the conical jets by mechanical means), which shall be preheated either in the steam stream by placing on the steam outlet and positioning the jet between the beaker wells or on top of the hot evaporation bath prior to attaching to the outlets. Using forceps or tongs (if needed), center each conical jet above the surface of the liquid, and start the flow of air or steam, adjusting it to the specified rate. Maintain the temperature and rate of flow, and allow the test specimen to evaporate for 30 +/- 0.5 min.

NOTE 4 - When introducing the flow of air or steam, care should be taken to avoid splashing of the test specimen as this may lead to erroneous low results.

11.6 At the end of the heating period, remove the conical jets using forceps, tongs (see 6.10), or other suitable means, and transfer the beakers from the bath to the cooling vessel. Place the cooling vessel in the vicinity of the balance for at least 2 h. Weigh the beakers in accordance with 11.3. Record the masses.

11.7 Segregate the beakers containing the residues from motor gasolines for finishing as described in 11.8 through 11.12. The remaining beakers may be returned for cleaning and reuse.

11.7.1 Qualitative evidence of motor gasoline contamination can be obtained by weighing the residue at this point if retained samples of the original finished gasoline are available for reference testing. This reference testing is essential since motor gasoline may contain deliberately added materials that are nonvolatile. If evidence of contamination is obtained, further investigation is indicated.

11.8 For non-aviation fuels that have unwashed results that are <0.5 mg/100 mL (see 11.6, Section 12, and 13.2) it is not necessary to perform the washing steps identified in this section, as well as in those that follow (11.9 to 11.12) since the washed gum value will always be not more than the unwashed gum value. If the unwashed results are not <0.5 mg/100 mL, to each of the beakers containing the residues from non-aviation fuels, add approximately 25 mL of heptane and swirl gently for 30 s. Allow the mixture to stand for 10 +/- 1 min. Treat the tare beaker in the same manner.

11.9 Decant and discard the heptane solution, taking care to prevent the loss of any solid residue.

11.10 Repeat the extraction with a second portion of approximately 25 mL of heptane as described in 11.8 and 11.9. If, after the second extraction, the extract remains colored, repeat the extraction a third time. Do not perform more than three extractions (Note 5).

NOTE 5 - Further extractions (after the third extraction) are not to be performed, since portions of insoluble gum may be removed due to mechanical action, which could lead to lower solvent washed gum contents being determined.

11.11 Place the beakers, including the tare, in the evaporation bath maintained at 160 to 165°C and, without replacing the conical jets, allow the beakers to dry for 5 +/- 0.5 min.

11.12 At the end of the drying period, remove the beakers from the evaporation bath using forceps or tongs (see 6.10), place them in a cooling vessel, and allow them to cool in the vicinity of the balance for at least 2 h. Weigh the beakers in the same manner as described in 11.3. Record the masses.

12. Calculation
12.1 Calculate the existent gum content of aviation fuels as follows:
A = 2000(B - D + X - Y)

12.2 Calculate the solvent washed gum content of motor gasoline as follows:
S = 2000(C - D + X - Z)

12.3 Calculate the unwashed gum content of motor gasoline as follows:
U = 2000(B - D + X - Y)

where:
A = existent gum content, mg/100 mL,
S = solvent washed gum content, mg/100 mL,
U = unwashed gum content, mg/100 mL,
B = mass recorded in 11.6 for the sample beaker plus residue, g,
C = mass recorded in 11.12 for the sample beaker plus residue, g,
D = mass recorded in 11.3 for the empty sample beaker, g,
X = mass recorded in 11.3 for the tare beaker, g,
Y = mass recorded in 11.6 for the tare beaker, g, and
Z = mass recorded in 11.12 for the tare beaker, g.