ASTM D7098 Standard Test Method for Oxidation Stability of Lubricants by Thin-Film Oxygen Uptake (TFOUT) Catalyst B
10. Procedure
10.1 Weighing and Mixing Sample and Catalyst Components:
10.1.1 Place the clean glass sample container onto the precision balance and tare.
10.1.2 Weigh 1.500 +/- 0.001 g of oil sample into the container and tare.
10.1.3 Add 0.045 +/- 0.001 g of the soluble metal catalyst mixture into the glass sample container and tare.
10.1.4 Add 0.030 +/- 0.001 g each of the fuel component, nitro-paraffin and reagent water to the glass sample container and tare each time. It is easiest to add the distilled water last and place on top of the oil sample.
10.1.5 Just prior to inserting the glass sample container into the pressure vessel, thoroughly mix the catalyst components within the sample container by hand-rotation (approximately five rotations) and proceed immediately to 10.2. Delay may result in variation of results.
10.2 Pressure Vessel Assembly and Charging - Immediately and rapidly assemble and charge the pressure vessel in accordance with apparatus type (see A1.2 or A2.7).
NOTE 8 - Avoid releasing the oxygen too rapidly by decreasing the pressure to atmospheric in no less than 1 min to avoid possible foaming and overflow of the sample from the glass sample container.
10.3 Oxidation - Before starting the test, bring the heating bath to the test temperature at 160°C and insert the pressure vessel(s) in accordance with apparatus type (see A1.3 or A2.8).
10.3.1 Allow the bath temperature to level out at the test temperature, which must occur within 15 min after insertion of the pressure vessel. Maintaining the test temperature within the specified limits of 160 +/- 0.3°C during the entire test run is the most important single factor ensuring both repeatability and reproducibility of test results. If the test temperature cannot be maintained as specified, the test results shall not be considered valid.
NOTE 9 - The time for the bath to reach the operating temperature after insertion of the pressure vessel may differ for different apparatus assemblies and shall be observed for each unit (a unit may carry one, two, three, or four pressure vessels). The objective is to find a set ofconditions, which does not permit a drop of more than 2°C after insertion of the pressure vessel(s) and allows the pressure vessel pressure to reach plateau within 15 min.
10.4 Keep the pressure vessel completely submerged and maintain continuous and uniform rotation throughout the test. A standard rotational speed of 100 +/- 5 r/min is required; any variation in this speed could cause erratic results.
10.5 Monitor the pressure of the pressure vessel preferably using a strip chart or some other form of electronic data collection program. If a dial pressure gauge is used, make readings at least every 5 min. (The maximum pressure must be reached within 15 min.) After a test period (the induction time), the pressure decreases because ofoxygen absorption by oil (the break point).
10.5.1 When the oil reaches the break point, the pressure decreases rapidly as oxygen is absorbed rapidly by the test oil. The test can be terminated as soon as sufficient information has been collected to form a tangent to the decreasing pressure trace (see 10.6) or, if desired, continued until pressure decreases to some further level.
NOTE 10 - The pressure within the pressure vessel increases at the beginning because of gas expansion accompanying the temperature increase of the pressure vessel. Following this rise, the pressure reaches a plateau as shown in Fig. 1. This pressure may gradually drop slightly during the test. A gradual decrease of the pressure is not unusual and does not invalidate the test. The time between initiating the test and the break point is called the oxidation induction time.
NOTE 11 - If a break in pressure does not occur within 300 to 500 min, the operator may elect to terminate the test. A slow decrease in pressure may also indicate a small leak from the pressure vessel, which is why it is a good practice to occasionally determine whether a slow leak is present.
10.6 Record the time at which the pressure starts to decrease rapidly at the break point (Point B, Fig. 1), which is marked as the intersection of the tangent of the pressure plateau line during the final 20 min before the break point and the tangent of the pressure decrease line following the break point as shown in Fig. 1.
