ASTM D2440 for oxidation stability of mineral insulating oil
8. Preparation of Oxygen Supply
8.1 Oxidize the oil by contact with oxygen of 99.4 % minimum purity conditioned as follows:
8.1.1 Use metal or glass tubing to deliver the oxygen to the oxygen delivery tube. Eliminate rubber connections if possible. If used, the rubber should be at room temperature. Limit the length of rubber tubing exposed to the oxygen flow to 13 mm. Do not use rubber connections in contact with the liquid of the heating bath or under conditions where the temperature of the rubber will be above room temperature. Where it is necessary to use rubber connections, only sulfur-free rubber is permissible. FOA/USP Nalgene tubing has been found satisfactory in this application.
8.1.2 Dry the oxygen by forcing it through a solid desiccant of high moisture-absorbing capacity. Arrange the desiccant in the drying tower to a depth of 205 to 254 mm. Change the desiccant when the indicator begins to change colors from moisture absorption. If an indicator is not used, change the desiccant at least weekly.
8.1.3 After passing oxygen through the drying tower, admit it directly to the receptacle containing the oil to be tested. Do not preheat the oxygen.
8.1.4 Determine the rate of oxygen supply with an electronic flow meter, calibrated rotameter, or soap bubble buret, and adjust the flow rate to deliver the conditioned oxygen at a rate of 1.0 more or less 0.1 L/h to each tube of oil being tested.
9. Preparation of Oil Receptacle
9.1 Wash each oil receptacle thoroughly, first with acetone and then with soap and water, and rinse in acid solution. The following reagents have been found suitable: chromic acid, aqua regia, and ammonium persulfate. Wash each receptacle free of acid, using tap water, and finally rinse with distilled water. Dry in an oven at 105 to 110°C for at least 3 h, cool to room temperature in a desiccator, and keep the receptacle in the desiccator until ready for use.
10. Preparation of Copper Catalyst
10.1 Immediately before use, polish the copper wire with silicon carbide abrasive cloth and wipe free of abrasive with a clean dry cloth.
10.2 Wind a 300-mm length of the polished wire into a helical coil approximately 16 mm in outside diameter and 50 mm in height. Clean the coil thoroughly with chloroform or acetone, air dry, and insert immediately into the oil receptacle. Handle the clean copper coil only with clean tongs to avoid contamination.
10.3 Commercially available, prepackaged, preformed coils that meet the requirements described in this test method may be used as an alternative method of catalyst preparation. Clean the coil in a 10 % HCl solution for 30 s, rinse three times with tap water and with distilled water, chloroform or acetone and air dry immediately before use. Preformed coils were used in a 1997 Round Robin Study and found suitable for this application.
11. Preparation of Glass Filtering Crucible
11.1 Clean the glass filtering crucible and dry in an air oven at 105 to 110°C until it has reached constant mass. Cool and store in a desiccator and, when needed, weigh to the nearest 0.0001 g.
12. Conditioning of Test Specimen
12.1 Filter the oil test specimen by gravity at ambient pressure or at reduced pressure, through acid-free filter paper to remove traces of sediment, fiber, and excess water. Alternatively, filter the test specimen under vacuum through a 0.45 µm mixed ester or cellulose filter. Discard the first 25 mL of the oil filtrate. Carefully protect the filtrate during the filtration against dust and other contaminations.
