ASTM D2878 Test Method for Estimating Apparent Vapor Pressures and Molecular Weights of Lubricating Oils
8. Procedure
8.1 Weigh the clean test specimen cup and hood to the nearest 1 mg. Transfer, by means of a pipet, 10.00 +/- 0.05 g of test specimen to the cup. Assemble the cup and hood, being careful not to splash oil on the underside of the hood. Weigh the assembly and record the net test specimen weight to the nearest 1 mg.

8.2 With cover in place, but without the hood and test specimen cup attached, allow the evaporation cell to acquire the temperature of the bath (controlled to +/-0.5K (+/-1°F)) at which the test is to be made by immersing the cell in it, as shown in Fig. 1. Allow the cell to remain in the bath at least 1/2 h before beginning the test. During this period, allow clean air (Warning - See Note 2) to flow through the cell at the prescribed rate, 2.583 +/- 0.02 g/min (2 L/min at standard temperature and pressure), as indicated by the rotameter. Then remove the cover, thread and weighed hood and sample cup into place, and replace the cover. Tighten the three knurled cover-tightening screws securely to prevent air leakage under the cover. Pass clean air through the cell for the required period.

NOTE 5 - Warning: Do not perform this test with air at temperatures in exces of the autoignition temperature of the test specimen as determined by Test Method E 659 or Test Method D2883, or both.

8.3 At the end of the test period, remove the assembled test specimen cup and hood from the cell, and allow to cool to room temperature. Determine the net weight of the sample to the nearest 1 mg.

9. Determination of Molecular Weight and Apparent Vapor Pressure
9.1 If a value of M is already available from Test Method D2503 or equivalent, 9.2-9.4 and 10.1 may be omitted, even though this value is for the whole lubricant instead of the part vaporized, as the calculation is not very sensitive to M error.

9.2 Conduct a test on the sample in accordance with the procedure in Section 7, at 477K (400°F). The proper test time to evaporate 5 % (0.500 g) may be estimated from the flash point of the lubricant as measured by Test Method D92, from Table 1.

NOTE 6 - The need for a run at 477K (400°F) is, created by lack of exact values for the first two constants in Eq 3, Eq 4, and Eq 5 for other temperatures.

9.3 For synthetic and redistilled petroleum oils, the variation of W/t with W is not great, and the 5 % point shall be approximated by linear interpolation of two points taken at different W values. For single-distilled petroleum or unknown oils, three points shall be plotted, representing the estimated time and also half and twice that time. These readings may all be obtained on one sample by stop and start operation of the apparatus.

9.4 When a single data point that does not fall within the 5 +/- 1 % evaporated range is used (as is often justifiable on synthetic oils) or the evaporation is measured at some other level of W, this fact shall be reported in Section 11.

9.5 The test for apparent vapor pressure is conducted in accordance with Section 8 for the estimated time at the selected temperature. If the 5 +/- 1 % criterion is not met, proceed as in 9.3.

10. Calculations
10.1 Calculation of Molecular Weight:
10.1.1 Use the evaporation time, t, (in seconds) obtained in 9.3 to evaporate 5 +/- 1 %.

10.1.2 Calculate the molecular weights of lubricants in general as follows:
logM = 3.028 - 0.164 log(10 335 PW/t)

10.1.3 For lubricants of known composition, slightly greater accuracy is obtained with special equations:
10.1.3.1 For polyol esters:
logM = 3.181 - 0.207 log(10 335 PW/t)

10.1.3.2 For dibasic esters:
logM = 3.089 - 0.190 log(10 335 PW/t)

10.1.3.3 For mineral oils:
logM = 2.848 - 0.106 log(10 335 PW/t)

10.1.4 The molecular weight equations all contain the standard value of k at 477K (400°F) from Table 2. If a change greater than +/-3 % in this value is caused by the calibration in Section 7, adjustments shall be made in the constant 10 335 by multiplying it by the factor (k/k').

10.2 Calculation of Apparent Vapor Pressure:
10.2.1 Use the molecular weight, M, as calculated in 10.1 or predetermined in 9.1 to calculate the vapor pressure as follows:
p = 672 PW/tkM
where k is obtained from Table 2. Use Eq 2 to extend this table. If a special equation was required in 7.1, use it rather than Table 2 or Eq 2.

10.2.2 For the special case of lubricants run at 477K (400°F) for 6.5 h as required in several military aircraft engine oil specifications, with P 5 760 torr:
logp = 1.164 log(10W) - 1.255
where 10 W = percent evaporated from a 10-g sample.

10.2.3 These results may be converted to SI units by the equations
p' = 133.32p and P' = 133.32P
where:
p' = apparent vapor pressure, Pa
P' = ambient atmospheric pressure, Pa