ASTM D2112 for oxidation stability of inhibited mineral insulating oil
ASTM D2112 standard test method for oxidation stability of inhibited mineral insulating oil by pressure vessel
9. Procedure
9.1 Charging - Weigh 50 more or less 0.5 g of oil sample into the container, add 5 mL of distilled water, and cover with a 2-in. (51-mm) watch glass or a 2 1/4-in. (57.2-mm) PTFE disk with one or four holes and retaining spring. If rinse water is present in the container, compensate for it by using less added water based on the water retention determined in 8.3. Add 5 mL of distilled water to the vessel and slide the test specimen container and cover lid into the vessel body (see Note 3). Apply a thin coating of silicone stopcock grease to the O-ring vessel seal located in the gasket groove of the vessel lid to provide lubrication, and insert the lid into the vessel body. Place the vessel cap over the vessel stem, and tighten by hand. Cover the threads of the gage-nipple with a thin coating of stopcock grease or TFE-fluorocarbon, or both, and screw the gage into the top-center tap of the vessel stem. Flush the vessel twice with oxygen supplied to the vessel at 90 psi (620 kPa) and release to the atmosphere. Adjust the regulating valve on the oxygen supply tank to 90 psi at a room temperature of 25°C (77°F). For each 2.8°C (5.1°F) above or below this temperature, add or subtract 1 psi (7 kPa) unit to attain the required initial pressure. Fill the vessel to this required pressure and close the inlet valve securely by hand. If desired, test the vessel for leaks by immersion in water (see Note 4). Prepare a duplicate test specimen in exactly the same way.
NOTE 3 - The water between the vessel well and the test specimen container aids heat transfer.
NOTE 4 - If the vessel was immersed in water to check for leaks, dry the outside of the wet vessel by any convenient means such as an air blast or a towel. Such drying is advisable to prevent subsequent introduction of free water into the hot oil bath, which would cause sputtering.
9.2 Oxidation - Bring the heating bath to the test temperature of 140°C while the stirrer is in operation. Insert the vessels into the rotating carriages and note the time. If an auxiliary heater is used, keep it on for the first 5 min of the run and then turn it off (see Note 5). Allow the bath temperature to level out at the test temperature; this must occur within 10 min after the vessels are inserted. Maintain the test temperature within more or less 0.1°C (see Note 6).
NOTE 5 - The time for the bath to reach the operating temperature after insertion of the vessels may differ for different apparatus assemblies and should be observed for each unit. The objective is to find a set of conditions that does not permit a drop of more than 2°C after insertion of the vessels and allows the vessel pressure to reach a plateau within 15 min as shown in Curve A of Fig. 3.
NOTE 6 - Maintaining the correct temperature within the specification limits of 60.1°C during the entire test run is the most important single factor ensuring good repeatability and reproducibility of test results.
9.3 Keep the vessels completely submerged and maintain rotation continuously and uniformly throughout the test. A standard rotational speed of 100 more or less 5 rpm is required; any appreciable variations in this speed could cause erratic results. If a dial gage is used, take readings every 5 min.
9.4 The test is complete after the pressure drops more than 25 psi (172 kPa) below the maximum pressure. The 25-psi pressure drop usually, but not always, coincides with an induction-type "period of rapid pressure drop". When it does not, the operator should question whether a valid experiment has been produced.
NOTE 7 - A typical experiment is shown in Fig. 3 as Curve A. The maximum pressure expected to be reached within 30 min; a pressure plateau is established and an induction-type pressure drop is observed. Curve B, in which there is a gradual decrease in pressure before the induction break is recorded, is more difficult to evaluate. The gradual decrease in pressure could be due to a vessel leak; however, some synthetic fluids will generate this type of curve. If a leak is suspected, repeat the test in a different vessel. If the same type of curve is derived when the test is repeated, the experiment is likely valid.
9.5 After termination of the test, remove the vessels from the oil bath, dip briefly into and swirl around in a bath of light mineral oil or detergent and water to wash off the adhering bath oil. Rinse off the vessels with hot water, then immerse in cold water to bring them quickly to room temperature. Allow the vessel to fully cool before bleeding off excess oxygen pressure and opening the vessel.
NOTE 8 - A hazardous situation can arise when excess oxygen is bled off immediately upon removal of the vessel from the bath since it may be accompanied by hot oil and steam. (See 6.7).
