ASTM D6896 Standard Test Method for Determination of Yield Stress and Apparent Viscosity of Used Engine Oils at Low Temperature
10. Procedure
10.1 Select the cooling profile for the desired test temperature. Table X1.2 lists the nominal times to reach a particular test temperature.
10.1.1 Choose the preprogrammed temperature profile. 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 X1.3.
10.1.2 If the instrument temperature is controlled by an external controller, it will need to be programmed to follow the cooling program in Table X1.1 with adjustment for the temperature difference found in 9.1, if any.
10.2 Test Sample and Viscometric Cell Preparation:
10.2.1 Using suitable closed container, preheat the samples in an oven to 80 +/- 1°C for 2.25 h. At the end of this time, remove the samples from the oven and allow to cool for 15 min at room temperature.
10.2.2 Agitate each sample using vigorous mechanical or manual shaking for 60 s. Allow the samples to stand for a minimum of 10 min to allow for settling.
10.2.3 Remove the nine rotors from the viscometric cells and ensure that both the cells and rotors are clean. See 10.6 for the cleaning procedure.
10.2.4 Place a 10 +/- 1.0 mL oil sample in each cell.
10.2.5 Install the rotors in the proper stators and install the upper pivots.
10.2.6 Place the loop of the 700-mm long string over the crossarm at the top of the rotor shaft and wind all but 200 mm of the length of the string around the shaft. Do not overlap strings. Loop the remaining end of the string over the top bearing cover. Orient the rotor such that an end of the crossarm at the top of the rotor shaft is pointing directly forward. If available, secure crossarm with locking pin. If the rotations are manually timed, it is helpful to color one end of the crossarm.
10.2.6.1 The string may be prewound around the shaft before installation of the rotor in 10.2.5.
10.2.7 Place the housing cover over the viscometric cells to minimize the formation of frost on the cold metal parts exposed to air. In some climates it may be necessary to flush the cover with a dry gas (for example, dry air or nitrogen) to minimize the frost formation.
10.2.8 Start the programmed temperature profile.
10.2.9 The cooling cycle starts to cool the samples in accordance with the programmed cooling sequence as programmed in 10.1.
10.2.10 At the completion of the temperature profile, the temperature of the block should be within 0.2°C of the desired test temperature when measured by a thermometer other than the temperature controller in the same thermometer well used during calibration. If the block temperature is within this range, proceed with the yield stress and viscosity measurements within 30 min of the completion of the temperature profile (see 10.3).
10.2.10.1 If the final temperature of the block is 0.2 to 0.5°C warmer than the desired temperature, proceed as follows. Set the temperature controller to bring the block temperature to the correct test temperature and then hold at the correct test temperature for 30 min before proceeding. This entire temperature correction should not take longer than 1 h. The data obtained in this way are considered valid test results, otherwise the test is invalid.
10.2.10.2 If the final test temperature is more than 0.2°C cooler or more than 0.5°C warmer than the preselected test temperature, then the test is NOT VALID for the preselected temperature. FOR INFORMATION ONLY, the yield stress and viscosity may be measured without further temperature adjustment. These results are characteristic of the actual temperature, not the preselected one.
10.2.11 If the final temperature as noted in 10.2.10 is in error in either direction by more than 0.2°C, see X2.2 before starting another test.
10.2.12 With models CMRV-4 and higher, if the program reports cooling profile out of tolerance, the operation of the instrument shall be thoroughly reviewed for correct operation. With models earlier than CMRV-4, check the logged data for excessive temperature deviation. See X2.2-X2.4.
10.3 Measurement of the Yield Stress:
10.3.1 Beginning with the cell farthest to the left of the instrument, follow the procedure below for each cell in turn.
10.3.2 Align the pulley wheel with the rotor shaft for the cell to be tested, such that the string hangs past the front of the housing. Make sure that the weights clear the edge of the bench during testing.
10.3.3 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.)
10.3.4 For CMRV-3 and earlier models, follow the instructions in 10.3.5. For CMRV-4 or later models, if using the automatic timing devices, follow the instructions in 10.3.6. If manual timing measurements are used, follow the instructions in 10.3.5.
10.3.5 Visually observe the rotor for movement of the crossarm. (Do not measure yield stress by way of the electronic optics.)
10.3.5.1 For instruments not equipped with locking pins, carefully, so as not to disturb the gel structure, attach a 10-g mass to the string and gently suspend the weight on the string. Proceed to 10.3.5.3.
10.3.5.2 For instruments equipped with locking pins, suspend the 10-g mass on the string, then raise the locking pin.
10.3.5.3 If the end of the crossarm does not move at least 3 mm in 15 s (approximately twice the diameter of the crossarm or 13° of rotation) then record that the sample has yield stress. Proceed to 10.3.5.4. If movement is detected, record weight and proceed to 10.4.
10.3.5.4 If no movement is detected, for instruments without locking pins, hold weight assembly and add 10 g, then proceed with 10.3.5.3. If equipped with locking pins, lower the locking pin to re-engage crossarm. Add 10 g to the weight assembly, raise the locking pin and proceed with 10.3.5.3.
NOTE 2 - The total amount of weight available for measurement of yield stress is normally 100 g; if no movement is detected with this weight, yield stress would be recorded as >350 Pa.
10.3.6 The operator shall follow the on-screen instructions for the addition of weight increments.
10.3.6.1 For instruments with locking pins, suspend 10 g weight cage on string, press the flashing start button then immediately raise the locking pin and follow on-screen instructions.
10.3.6.2 If additional weight is requested, capture crossarm in locking pin, add one additional 10 g weight, and follow the on-screen instructions. Press the flashing start button, and immediately raise the locking pin. Repeat procedure until no additional weight is requested. Proceed to 10.4.
10.3.6.3 For instruments without locking pins, carefully suspend and hold the 10 g weight cage on the string without jerking rotor and follow on-screen instructions. Press the flashing start button, and immediately release the weight cage.
10.3.6.4 If no movement is detected, carefully weight the cage. Add next 10 g weight increment as indicated on computer screen without pulling on string and follow on-screen instructions. Press the flashing start button and immediately release weight cage. Repeat procedure until no additional weight is requested. Proceed with 10.4.
NOTE 3 - When the 10-g load is first applied, some oils may show momentary movement of the crossarm. If there is no further movement of the crossarm for 15 s, disregard the initial movement.
10.4 Measurement of Apparent Viscosity:
10.4.1 For CMRV-3 and earlier models follow the instructions in 10.4.2. For CMRV-4 or later models, if using the automatic timing devices, follow the instructions in 10.4.3. If manual timing measurements are used, follow the instructions in 10.4.2.
10.4.2 Attach a 150-g mass to the string and slowly suspend the weight on the string. Start the timer when the crossarm of the rotor shaft points directly forward and continue timing in accordance with the following constraints.
10.4.2.1 If the first half-revolution requires less than 10 s, measure and record the time for the first three revolutions, and proceed to 10.5.
10.4.2.2 If the first half-revolution requires 10 s or greater, measure and record the time for the first revolution and identify it as the time for one revolution; then proceed to 10.5.
10.4.2.3 If the first revolution has not been completed in 60 s, end the measurement. Record the time as greater than 60 s for one revolution, then proceed to 10.5, reporting that the viscosity is greater than the value calculated in 11.2.
10.4.2.4 If the time for the first three revolutions is less than 4 s, record the time as less than 4 s, then proceed to 10.5, reporting that the viscosity is less than the value calculated in 11.2.
10.4.3 Follow on-screen instructions, press start button and slowly suspend the weight on the string. Timing will automatically begin with first movement. Do not remove weight while viscosity LED on instrument is flashing. Once the time and viscosity are displayed or the viscosity LED stops flashing, proceed to 10.5.
10.5 Repeat 10.3 and 10.4 for each of the remaining cells in order from left to right.
10.6 Cleaning:
10.6.1 After all of the cells have been completed, exit the cooling program and turn on the heater to warm the viscometric cells to room temperature or somewhat higher. The temperature shall not exceed 50°C.
10.6.2 Remove the upper rotor pivots and the rotors.
10.6.3 With a vacuum, remove the oil samples and rinse the cells with an oil solvent several times, followed by two washings with acetone. Use a vacuum to remove the solvent from the cells after each rinse and allow the acetone to evaporate to dryness after the final rinse.
10.6.4 Clean the rotors in a similar manner.
11. Calculation of Yield Stress and Apparent Viscosity
11.1 Yield stress is given by the following equation:
Ys = 3.5 M
where:
Ys = yield stress, Pa, and
M = applied mass, g.
11.2 The apparent viscosity is given by the following equation when using the cell constant obtained in Eq 8:
ηa = Ct 3/r
where:
ηa = apparent viscosity, mPa·s (cP),
C = cell constant obtained in Eq 8,
t = time for number (r) of complete revolutions of the rotor, and
r = number of revolutions timed.
