ASTM D943 method for oxidation characteristics of inhibited mineral oils
9. Procedure
9.1 Adjust the heating bath to a temperature high enough to maintain the oil in the oxidation test cell at the required temperature of 95 +/- 0.2°C.
9.2 Fill the empty oxidation test tube with 300 mL of the oil sample to the graduation mark. Slide the catalyst coil over the inlet of the oxygen delivery tube. If the wires are uneven at one end of the coil, position the coil so that this end is down. Place the oxygen delivery tube with the coil into the test tube. Place the condenser over the oxygen delivery tube and test tube. Place the sampling tube holder over the oxygen delivery tube. Insert the syringe sampling tube through the syringe sampling tube spacer, and into the sampling tube holder, as shown in Fig. 5. Position the bottom end of the sampling tube inside the catalyst coil. Insert a stopper in the Luer-Lok fitting end of the sampling tube. Immerse the test tube in the heating bath. Adjust the heating bath liquid level so that the tube is immersed in the liquid to a depth of 355 +/- 10 mm. Connect the condenser to the cooling water. The temperature of the outlet water should not exceed 32°C at any time during the test.
NOTE 4 - As an alternative to using the sampling tube holder and sampling tube spacer, the sampling tube may be secured in position by taping or clamping to the oxygen delivery tube using a suitable adhesive tape or clamp. The sampling tube is taped or clamped to the oxygen delivery tube at a distance of approximately 25 mm (1 in.) above the top of the condenser. The bottom of the sampling tube is positioned at 152 +/- 6 mm (6 +/- 0.25 in.) from the bottom curved end of the oxygen delivery tube.
9.3 Connect the oxygen delivery tube to the oxygen supply (see 6.9) through the flowmeter using new polyvinyl chloride flexible tubing no more than 600 mm in length. Before using, the interior of the new tubing should be rinsed with n-Heptane and blown dry with air. Adjust the rate of flow to 3 +/- 0.1 L and continue flow for 30 min.
9.4 Raise the condenser unit from the oxidation cell and add 60 mL of reagent water through the opening thus provided. The test is considered to start at this point.
9.5 Throughout the duration of the test, maintain the temperature of the oil-water mixture (sample temperature) at 95 +/- 0.2°C in each test cell with oxygen flowing. Accomplish this by maintaining the bath at the temperature that is found necessary to give the required 95°C sample temperature. The temperature of the bath is always higher than the sample temperature due to the cooling effect of the oxygen gas flow, and depends on heating bath medium, capacity, circulation, and on the number of tests cells in the bath. Measure the sample temperature by a thermometer positioned in the oxidation cell by a thermometer bracket, as in Fig. 6 (Note 5). Make temperature measurements only with new, undepleted, oil samples, and preferably with dummy cells used specifically for temperature measurement. When an actual test sample is used, remove the thermometer immediately after temperature measurement is complete. Check the temperature in this way in various parts of a multiple-cell bath to verify uniformity of temperature control. Once the required bath temperature is found, maintain at that temperature +/- 0.2°C.
NOTE 5 - With the arrangement shown in Fig. 6, the 76-mm immersion point of the thermometer is positioned at the oil surface. To allow for heating of the stem portion of the thermometer above the immersion point in the upper portion of the test cell, subtract 0.10°C from the thermometer reading to obtain the true test temperature.
9.6 Add additional reagent water to the oxidation cell as required (Note 6), at least every 2 weeks during the test, to restore the water level to the shoulder of the oxygen delivery tube. Add the water using the sampling tube and the 50-mL capacity syringe.
NOTE 6 - Under some circumstances, the level of water cannot be observed because of deposits or emulsion formation. Marking the upper oil level of the filled oxidation tube by some suitable means and maintaining this level by periodic water additions will keep the proper amount of water in the cell. The correct level for water additions may, if desired, be indicated by a movable metal strip (Fig. 7), which is clamped to the outside of the oxidation test tube by, for example, an adjustable ring-type hose clamp. To use this indicator, the lower end of the strip is set at the upper oil level when the test is started. As the test proceeds and water evaporates to cause the oil level to fall, sufficient makeup water is added, particularly just before oil samples are taken, to return the oil level to the level marked by the indicator strip. After each oil sample is taken, the indicator is moved to the new oil level before continuing the test.
9.7 Monitor the oil for signs of oxidation by periodically withdrawing samples for measurement of acid number.
9.7.1 After 500 h of test time, and every 168 h (every week) thereafter, withdraw 3 mL of test oil from the oxidation test cell (Note 7). This can be accomplished as follows. Without interrupting the oxygen flow, and without changing the position of the sampling tube, remove the sampling tube stopper and twice draw out into the syringe and return approximately 3 mL of test oil to the test cell in order to flush out the sampling tube. Then withdraw 6 mL of test oil into the syringe, hold the syringe tube vertically for 15 to 20 s to allow water to settle to the bottom of the syringe tube, and adjust the sample size to 3 mL while holding the syringe vertically. This method allows most of the water withdrawn with the test oil to be returned to the test cell. Replace the sampling tube stopper. The 3-mL sample is dispensed into a sample vial for acid number analysis by Test Methods D3339 or D5770 (Note 8). Shake the test oil sample thoroughly before taking a sample from the vial for titration.
NOTE 7 - Test results by this method are greatly affected by differences in sampling procedure. It is important to strictly adhere to the procedure and schedule for sampling for acid number. An exception is where the oil is known to have a life shorter than 500 h, the sampling is at the discretion of the operator.
NOTE 8 - Test Method D5770 is preferred but Test Method D3339 is permitted when the acid number is no greater than 0.50 mg KOH/g oil. Use Test Method D3339 when the acid number exceeds 0.50. Test Methods D3339 and D5770 use a color change to indicate the titration end point. For dark-colored oils, the use of potentiometric titration (such as Test Method D664) to measure the end point may be advantageous. It is recognized that there is no ASTM semi-micro method now available for potentiometric titration.
9.7.2 After 3000 h, take monitoring samples at 500 +/- 25 h intervals.
9.7.3 When observation indicates that the oil is nearing the end of its test life, additional samples may be taken at the discretion of the operator.
9.8 Discontinue the test when the measured acid number is 2.0 mg KOH/g or above (Note 9).
NOTE 9 - The value 2.0 mg KOH/g is taken as the limiting value when testing steam-turbine oils by this method. When oxidation is proceeding rapidly in such oils, values much higher than 2.0 are frequently encountered. At that time, the state of severe oxidation of the oil, as indicated by the high acid number, is generally confirmed by other signs of degradation such as disagreeable odor, formation of deposits, corrosion of the catalyst coil, severe darkening of the oil, and emulsification of the oil-water mixture. If such signs of oxidation are noted before the scheduled time for sampling for acid number, a sample may be taken then and the test discontinued if the measured acid number is 2.0 mg KOH/g or above.
9.8.1 If the acid number is still less than 2.0 mg KOH/g oil after 10 000 h, the test should be terminated. Report the test life and acid number as 10 000 + h (AN = x.xx) (Note 10).
NOTE 10 - Beyond 10 000 h, oil removal causes the metal catalyst coil to be exposed to the atmosphere. The effect of such exposure on oil life has not been determined, nor has the method precision been determined by cooperative testing.
10. Calculation
10.1 Calculate the hours to 2.0 mg KOH/g acid number (oxidation lifetime) H, as follows:
H = A + [(2.0 - C)/(D - C)] x (B - A)
where:
A = number of test hours when acid number was last measured below 2.0,
B = number of test hours when acid number was measured above 2.0,
C = acid number at A hours, and
D = acid number at B hours.
