ASTM D2715 Test Method for Volatilization Rates of Lubricants in Vacuum
9. Procedure A
9.1 Immerse the thermocouple in the sample, and bring the furnace to approximate operating temperature.

9.2 Suspend the sample and the container in position over the furnace, and tare to near the upper limit of the range.

9.3 Assemble the vacuum apparatus and pump the system to give a chamber pressure of 10(-6) to 10(-5) Pa (10(-8) to 10(-7) torr).

9.4 Start the liquid nitrogen flowing and cool the cold plate to 143 K (-200°F) or lower. Stabilize the furnace temperature.

9.5 Measure the pressure near the furnace position with the nude ionization gage.

9.6 Move the furnace into operating position surrounding the specimen. Start the recorder, and mark the recorder chart start of heat.

9.7 Hold the temperature constant at the required level for sufficient length of time to measure the rate of weight change and determine constancy of this rate.

NOTE 1 - A time derivative computer may be used to report rate of weight change directly.

9.8 Monitor pressure changes manually or by the second pen on the recorder when available. When the test temperature is reached, and a steady weight loss condition attained, establish the sample weight and measure the evaporation rate for this percentage point of the original weight. If the material has uniform molecular weight throughout, the rate will not change with progressing evaporation. If the rate changes, continue measurement until the time for a single rate determination exceeds 3 h.

9.9 Determine rates for several temperatures, using a fresh sample for each determination. Temperature intervals of 25 K, which approximate a ten-fold increase in rate, are usually suitable.

NOTE 2 - If the sample is known to be an essentially pure compound, repetitive measurements are permissible. If such purity is merely suspected, judgment may be made on the basis that a sample is not to be reused after a determination in the course of which the rate has changed more than 25 % at a single temperature. However, if the supply is limited, it is possible to obtain some meaningful data on a spot basis, as indicated below.

9.10 After primary data have been obtained at increasing temperature levels on a sample which meets the above criterion of less than 25 % change during any single measurement, make spot measurements at decreasing temperature levels to detect any changes in the specimen.

10. Procedure B
10.1 Immerse the thermocouple, suspend the sample, and position the furnace as described in 9.1-9.3.

10.2 Assemble the vacuum apparatus and pump the system to give a chamber pressure of 10(-3) to 10(-2) Pa (10(-5) to 10(-4) torr).

10.3 Conduct the rest of the test as described in 9.4-9.10.

11. Calculations
11.1 When the evaporation rate proves to be constant within the limit of a 25 % decrease during a determination, or 25 %/h if the determination takes less than 1 h, the evaporation rate for each temperature is as follows:
R = (w0 - w1)/(t1 - t0)
where:
R = evaporation rate, g/s,
w1 = weight of sample at the end of the test, g,
w0 = initial weight of the sample, g,
t1 = time at the end of the test, s, and
t0 = initial time of the test, s.

11.2 If the sample has a changing rate, this rate is calculated for each of the standard degrees of evaporation required in 12.2 as follows:
11.2.1 The weight required at each evaporation level is:
wr = (100w0 - Ew0)/100
where:
wr = weight at specified evaporation loss, g,
w0 = initial weight of sample, g, and
E = evaporation loss, %.

11.2.2 Draw a line tangent to the curve on the recorder chart at each weight corresponding to the evaporation loss from 11.2.1 and calculate the evaporation rate as follows:
R = (wa - wb)/(tb - ta)
where:
R = evaporation rate, g/s,
wa = weight at one point on the tangent line, g,
wb = weight at a second point on the tangent line, g,
ta = time at a point on the tangent line corresponding to wa, s, and
tb = time at a point on the tangent line corresponding to wb, s,

11.3 The evaporation rate per unit area is:
E = (R/A)C
where:
E = evaporation rate per unit area, g/cm2•s,
R = evaporation rate from 11.1 or 11.2, g/s,
A = surface area of sample exposed for evaporation, cm2, and
C = calibration factor from 8.2, if applicable.

11.4 If the molecular weight of the sample is known, the rates may be converted to vapor pressures by the equation given in 8.2. As the molecular weight enters as square root, the allowable error is twice that for the vapor pressure.