ASTM D5483 Test Method for Oxidation Induction Time of Lubricating Greases
ASTM D5483 Standard Test Method for Oxidation Induction Time of Lubricating Greases by Pressure Differential Scanning Calorimetry
9. Procedure
9.1 Before starting a test, the control thermocouple calibration shall be conducted at the test temperature (either 210, 180, or 155°C) according to 8.2.1 and 8.2.2. When the test temperature is not known, the calibration should be conducted at 210°C.
9.2 Weigh 2.0 ± 0.1 mg of grease into a sample pan. Spread the sample evenly upon the flat portion. Do not spill any of the sample into the trough portion of the pan (See Fig. 3).
NOTE 5 - Examples of suitable and poor sample on pan patterns are shown in Fig. 3.
9.3 Place the uncovered pan containing the sample onto the platform of the cell according to the PDSC manufacturer's instructions for placing the sample pan. Place an empty pan of the same configuration onto the cell platform according to the PDSC manufacturer's instructions for placing the reference pan. Close the cell and the pressure release valve.
9.4 Beginning at ambient temperature (approximately 22°C), program the sample temperature to increase at a rate of 100°C/min to the test temperature.
9.5 Allow the sample to equilibrate at the test temperature for 2 min.
NOTE 6 - This step did not appear in the test method which was used in the round robin to generate the precision statement. The round robin test method used the software of a PDSC manufacturer to determine when equilibration at test temperatures occurred. Step 8.5 removes this dependence on one PDSC manufacturer and is not expected to significantly affect the measured OIT since this step precedes the pressurization of the test cell with oxygen.
9.6 Open the oxygen valve and slowly pressurize the cell to 3.5 ± 0.2 MPa (500 ± 25 psig). This should require approximately 2 min to reach maximum pressure. The oxidation induction time is measured from the time when the oxygen valve is opened.
9.7 As soon as the pressure has equilibrated, check the cell purge rate and adjust to 100 ± 10 mL/min with the outlet valve.
9.8 After a duration of 120 min from the time when the oxygen valve was opened, close the oxygen valve and slowly release the cell pressure by opening the cell pressure release valve. In the case of a sample for which the approximate oxidation induction time is known, the test can be stopped after the oxidation exotherm has occurred.
9.9 Plot the thermal curve and measure the extrapolated onset time for the oxidation exotherm. Report this time, to the nearest tenth of one minute, as the oxidation induction time for the sample. If more than one oxidation exotherm is observed, record the oxidation induction time for the largest exotherm (See Fig. 4).
NOTE 7 - A typical thermal curve is shown in Fig. 4.
9.10 If the induction time is less than 10 min, rerun the test at the next lower temperature, starting at 9.2. Allow the cell to cool to ambient temperature before running the test at the next lower temperature.
9.11 After the oxidation induction time requirement specified in 8.10 is satisfied, perform a duplicate test.
9.12 If the difference between the two results is greater than the 95% determinability limit stated in the Precision and Bias section of this test method (Section 12), then reject the results and determine two more oxidation induction times for the grease by returning to 9.2. Otherwise, average the oxidation induction times of both runs.
10. Calculation of Induction Times for Other Temperatures
10.1 After an oxidation induction time has been determined for a particular grease, a value can be estimated for other temperatures using the following equation:
t = A exp (17 500/T)
where:
t = estimated oxidation induction time, min,
A = oxidation coefficient of the grease, and
T = temperature, K (for desired temperature).
The oxidation coefficient (A) is constant for a given grease and is calculated by (Eq 1) using the oxidation induction time reported in 9.1.2, thus,
A = oxidation induction time/exp(17 500/test temperature, K).
The estimated oxidation induction time can be used as a guide for choosing appropriate alternative test temperatures for a grease. The estimated oxidation induction time is not a part of the report for this test method.
