ASTM D3829 Predicting the Borderline Pumping Temperature of Engine Oil
10. Yield Stress and Viscosity Measurement Procedure
10.1 Viscometric Cell Preparation:
10.1.1 If the cells are not clean, clean according to 10.7 cleaning procedure.
10.1.2 Place 10 mL ± 0.2 mL of test oil sample into a clean cell.
10.1.2.1 All cells should contain a fluid and rotor; if there are less than a full set of samples to run, fill each of the unused cells with 10 mL of a typical test sample.
10.1.3 Repeat 10.1.2 until all test samples are in their cells.
10.1.4 Place each rotor in its cell, and place upper pivot pin in position, including those for any unused cells.
NOTE 7 - Before inserting the rotors in the cells, inspect each rotor to be sure that the shaft is straight, that the rotor surface is smooth and free from dents, scratches, and other imperfections. For rotors with a bearing point at the bottom of the shaft, ensure that the point is sharp and centered on the rotor shaft. If these conditions are not met, repair or replace the rotor.
10.1.5 Optional - If available for instrument, install a cell cap on all cells, including any unused cells.
10.1.6 For each cell, except any unused ones, place a loop of the nominal 700 mm long string over the crossbar. Hang the string over the timing wheel with a small weight attached such as a large paper clip. Wind the string around the shaft until the end is about 100 mm below the wheel. Do not overlap
NOTE 8 - The strings can be pre-wound around the shafts before they are installed in 10.1.4.
10.1.6.1 Engage the locking pin to prevent the rotor from turning.
10.1.6.2 Lay the remaining string over the top of the bearing plate letting it hang over the back of the plate.
10.1.6.3 Repeat 10.1.6 until all cells with samples to be measured are prepared.
10.1.7 Place the housing cover over the viscometric cells.
10.1.8 Connect the dry gas supply to the housing cover, as noted in 6.6. Set the dry gas flow to approximately 1 L/h. Increase or decrease the flow as necessary to minimize frost or moisture condensation around the cells.
10.2 Select the cooling profile for the desired test temperature and follow the instrument instructions to initiate the program. Table X1.1 shows cooling program requirements for each test temperature.
10.3 Place the thermometer in the thermowell at least 30 min prior to completion of the test. See Note 5. The same thermowell location is to be used for all measurements and must be the same one as was used for calibration.
10.4 At the completion of the cooling profile, check the time-temperature plot for the run to ensure that the time-temperature profile is within tolerance and that the test temperature as measured in the thermowell is within ±0.2 °C of the final test temperature. Both of these checks may be done automatically by the control software incorporated in some instruments. Final test temperature is to be verified independently from the instrument's temperature control using a thermometer that has been in the thermowell for at least 30 min prior to reaching the test temperature. See Note 5. If the final test temperature is more than 0.1 °C from the set point on two consecutive runs, the instrument's temperature control must be recalibrated according to 9.1.
10.5 If the temperature profile is within tolerance, proceed with measurements. If not, then abort the test and recalibrate instrument's temperature control as in 9.1.
10.6 Measurement of the Yield Stress and Viscosity:
10.6.1 Immediately prior to starting measurements, take the cell housing cover off the instrument.
10.6.2 Yield Stress Determination - Starting with the cell farthest to the left while facing the instrument, use the following procedure for each cell in turn, bypassing the unused cells.
10.6.2.1 Align the pulley wheel with the rotor shaft of the cell to be tested.
10.6.2.2 Hang the string over the timing wheel such that the string hangs past the front of the housing. Make sure that the disk holder clears the edge of the bench during testing. (Do not allow the rotor shaft to turn.)
10.6.2.3 Suspend the disk holder from the string.
10.6.2.4 For instruments with automatic timing, start timing and then release the locking pin. For manual timing, start timing immediately after the locking pin is disengaged.
10.6.2.5 Observe whether the end of the crossbar moves more than 3 mm in 15 s. (This 3 mm is approximately twice the diameter of the crossbar.) An alternative procedure is the use of a marked rotation of the timing wheel equivalent to a crossbar movement of 3 mm.
10.6.2.6 Electronic or timing wheel motion-sensing devices, which are available on some instruments, are suitable alternatives to direct observation.
10.6.2.7 If rotor movement of more than 3 mm in 15 s is observed in 10.6.2.5, remove the disk holder from the end of the string, and proceed to 10.6.3.
10.6.2.8 If a rotor movement of less than 3 mm in 15 s is observed in 10.6.2.5, stop timing and lift the disk holder so it is not supported by the string. Then add an additional 10 g disk to disk holder.
NOTE 9 - As additional disks are added to the disk holder it is necessary to suspend the holder with the additional disks from the string and restart timing without the use of the locking pin for the remainder of the yield stress assessment. When using software available for some instruments, ensure that the mass applied is the mass requested by the program.
10.6.2.9 Carefully and gently, suspend the disk holder from the string and start timing.
10.6.2.10 Repeat steps in 10.6.2.8 and 10.6.2.9 until the accumulated mass causes rotation of the rotor. At this point, remove the disk holder from the string.
10.6.2.11 If no rotation is observed with a total of 100 g, record that the yield stress is >350 Pa, and proceed with 10.6.3.
10.6.3 Viscosity Determination:
10.6.3.1 Gently suspend the 150 g mass from the string.
10.6.3.2 If the applied mass moves the rotor, as soon as the cross-arm is clear of the locking pin, reengage the locking pin. Allow rotation to continue until the cross-arm contacts the locking pin causing rotation to stop. If no appreciable rotation occurs, terminate the test and proceed to 10.6.3.7.
NOTE 10 - Yield stresses exceeding the stress exerted by 150 g have been encountered.
10.6.3.3 When using instruments capable of timing rotation automatically, initiate viscosity measurement by starting timing, then release the locking pin. When timing manually, start timing immediately after the locking pin is disengaged.
10.6.3.4 Stop the timer after three revolutions of the rotor from point of release. When the time for one revolution is greater than 60 s, time only one revolution.
NOTE 11 - The timing of three revolutions may be done automatically.
10.6.3.5 After completing three revolutions (one revolution if the time is greater than 60 s), remove mass from string.
10.6.3.6 Record both the time and the number of revolutions timed.
10.6.3.7 If no rotation occurs with the application of the 150 g mass, record the result for that sample as being "Too Viscous To Measure" (TVTM).
10.6.3.8 Repeat 10.6.2 - 10.6.3.7 for the remaining cells to be measured.
10.7 Cleaning:
10.7.1 When all measurements have been completed, set the instrument to warm to room temperature or somewhat above. Cleaning cells above a temperature of 55 °C is not recommended.
10.7.2 When the desired cleaning temperature is reached:
10.7.2.1 For instruments with non-removable cells, remove strings, rotors, and cell caps, when used, then proceed with 10.7.3.
10.7.2.2 For instruments with removable cells, either follow instructions for non-removable or remove cells from instrument. The removable cells are to be cleaned by generally following the instructions in 10.7.3.
10.7.3 Cleaning Cells:
10.7.3.1 Remove oil samples from cells by using a vacuum hose.
10.7.3.2 Using an appropriate solvent, rinse, the cells at least three times with approximately 15 mL of an appropriate solvent for each rinse. Then rinse once with acetone.
10.7.3.3 Remove traces of residual solvent by flushing cell with dry air or preferably with a vacuum hose to prevent contamination with house air. (Warning - When flushing cells with air, be sure that the air is clean and free from oil, water and other contaminants as these could be left in the cell. House air is frequently contaminated.)
10.7.4 Clean rotors with appropriate sample solvent, and dry.
10.8 To calculate the BPT (Section 11), it is necessary to determine a sample's viscosity at either 5 degrees Celsius above or below the first measurement. Therefore for BPT repeat Section 9 on a fresh sample at the appropriate test temperature.
