ASTM D6821 Standard Test Method for Low Temperature Viscosity of Drive Line Lubricants in a Constant Shear Stress Viscometer
10. Procedure
10.1 Viscometric Cell Preparation:
10.1.1 If the cells are not clean, clean according to 10.8 cleaning procedure.
10.1.2 Place a 10 mL +/- 1 mL oil sample into 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 then place upper pivot pin in position, including those for any unused cells.
NOTE 6 - Caution: The rotors used with this test method are physically different than those used with other mini-rotary viscometer (MRV) test methods. The rotors have a white band on upper part of the shaft to identify the rotor type while in the instrument. DO NOT use the larger diameter rotors as they are used with Test Methods D3829, D4684, and D6896. For laboratories using both rotors, ensure that the rotors for this method have a visible identifying mark placed on the shaft to minimize the wrong rotors being used in a test. This could be at the top of the rotor above the crossarm or between the cell caps and below where the string wraps around the shaft.
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 - 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 windings.
NOTE 8 - The string may be pre-wound around the shaft before installation of the rotor in step 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.
NOTE 9 - 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 a typical test sample.
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 6.5. 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.
10.2.1 If the profile is not available, enter it using the custom profile part of the software program. The instrument manual provides instructions on adding custom profiles. The entries for a custom program will be found in Table X1.1.
10.3 Place the thermometer in thermowell at least 30 min prior to completion of the cooling profile (see Note 4). The thermowell used must be the same one used during 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. See Note 4. If the final test temperature deviates by 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 the instrument's temperature control as in 9.1.
10.6 Measurement of the Yield Stress (Optional):
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.
10.6.3 Align the pulley wheel with the shaft of the cell to be tested.
10.6.4 Hang the string over the timing wheel such that the string hangs past the front of the housing. Make sure that the weights clear the edge of the bench during testing.
10.6.5 Remove the string from the upper bearing support and carefully place it over the pulley wheel so as not to disturb the test oil. (Do not allow the rotor shaft to turn.)
10.6.6 Carefully suspend the 2.5 g weight holder from the string.
10.6.7 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.
10.6.8 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.
10.6.9 Electronic or timing wheel motion-sensing devices, which are available on some instruments, are suitable alternatives to direct observation.
10.6.10 If rotor movement of more than 3 mm in 15 s is observed in 10.6.8, record the total mass, remove weights from the end of the string, and proceed to 10.7.
10.6.11 If a rotor movement of less than 3 mm in 15 s is observed in 10.6.8, stop timing and lift the weight holder so it is not supported by the string, then add an additional 2.5 g weight segment to weight holder.
NOTE 10 - As additional weight segments are added to the weight holder, it is necessary to suspend the holder with the additional weights 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.
10.6.12 Carefully and gently, suspend the weight holder with the additional weights from the string and start timing.
10.6.13 Repeat steps 10.6.8 through 10.6.12 until the accumulated weights causes rotation of the rotor. At this point, remove all the weights from the string.
10.6.14 Ifno rotation is observed with a total of 20 g, record that the yield stress is >90 Pa, and proceed with 10.7.
10.7 Measurement ofApparent Viscosity:
10.7.1 Gently suspend the 20 g mass from the string. (Weight holder plus 7 weight segments.)
10.7.2 If the applied mass of 20 g moves the rotor, then as soon as the cross-arm clears 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 10.7.7.
10.7.3 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.
10.7.4 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.
10.7.5 After completing three revolutions (one revolution if the time is greater than 60 s), remove mass from string.
10.7.6 Record both the time and the number of revolutions timed.
10.7.7 If no rotation occurs with the application of a 20 g mass, record the result for that sample as being "Too Viscous To Measure" (TVTM).
10.7.8 Repeat 10.6 (optional) and 10.7 for each of the remaining cells in order from left to right.
10.8 Cleaning:
10.8.1 After all the cells have been completed, turn off the cooling program. Start the warm up program to raise the viscometer cells to not more than 50 °C.
10.8.2 Once the viscometer has warmed up, remove the upper rotor pivots and rotors.
10.8.3 With vacuum, remove the oil samples. Rinse the cells with an oil solvent several times using the vacuum to remove each rinse from the cells. Additional rinses with acetone can be done to remove any traces of solvent or water, again using the vacuum to remove the rinse from the cells. Allow the solvent to evaporate before beginning a new test.
10.8.4 Clean the rotors in a similar manner.
11. Calculations
11.1 Yield Stress (Optional):
11.1.1 Calculate the yield stress from the following equation:
Sri = 4.5 M
where:
Sri = yield stress, Pa, and
M = mass applied to initiate rotation, g.
11.1.2 Report the yield stress as being less than the result rounded to the nearest Pa.
11.2 Apparent Viscosity:
11.2.1 The viscosity is given by the following equation when using the cell constant obtained in Eq 8:
ηa = Ct3/r
where:
ηa = apparent viscosity in mPa·s,
C = cell constant obtained in Eq 8,
t = time for number (r) of complete revolutions of the rotor, and
r = number of revolutions timed.
