ASTM D2715 Test Method for Volatilization Rates of Lubricants in Vacuum
7. Specimen Preparation
7.1 Remove dissolved gases from the bulk lot prior to test using a separate vacuum chamber. Break the vacuum in the chamber with helium. A large enough sample of material should be degassed in this pretreatment so that it will suffice for all anticipated test runs. A mass spectrometer can be used to verify complete degassing.
7.2 If required as evidenced by creepage of lubricant in first run, coat the container and the thermocouple with the anti-wetting agent (6.1). Silicones are especially likely to require this precaution.
7.3 Add to the container the required amount of sample, 75 +/- 5 mg/cm2 of area exposed for evaporation. Press solids and semisolids into the container with sufficient pressure to assure the apparent surface area approximates the real surface area. If a coherent surface cannot be achieved, note this fact in the report.
8. System Calibration
8.1 Calibrate the system in the vacuum, using one of the calibration materials, over the temperature range to be used, following the procedure shown in 9.1-9.8.
8.2 The rates obtained are compared with those predicted by the Langmuir equation:
G = 7.77p√M/T
where:
G = evaporation rate, g/cm2•s,
p = vapor pressure, Pa,
M = molecular weight, and
T = temperature, K.
8.2.1 If the measured rates differ by more than +/-20 % from those calculated, take all possible corrective steps to locate the source of the discrepancy. Use of a calibration factor is not encouraged, but may be tolerated in some cases if so reported. A factor greater than 2 or less than 0.5 casts such doubt on the results as to practically invalidate them and require corrective action.
