ASTM D4684 for yield stress and apparent viscosity of engine oils
ASTM D4684 standard test method for determination of yield stress and apparent viscosity of engine oils at low temperature
9. Procedure
9.1 Program the temperature controller to control the mini-rotary viscometer block temperature as outlined in Table X1.1 or Table X1.2. The programmed temperature is the temperature in Table X1.1 or Table X1.2 plus the appropriate temperature correction factor determined in 8.1. Table X1.3 lists the nominal times to reach a particular test temperature.

9.2 Test Sample and Viscometric Cell Preparation:
9.2.1 Remove the nine rotors from the viscometric cells and ensure that both the cells and rotors are clean. See 9.6 for the cleaning procedure.

9.2.2 Place a 10 more or less 1.0 mL oil sample in each cell.

9.2.3 Install the rotors in the proper stators and install the upper pivots.

9.2.4 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 cross arm.

9.2.4.1 The string may be prewound around the shaft before installation of the rotor in 9.2.3.

9.2.5 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.

9.2.6 Start the programmed temperature profile. The program will warm the oil samples to 80 more or less 1°C and maintain at 80 more or less 1°C for 2 h to allow solution of any material not in true solution at room temperature.

9.2.7 At the end of the 2-h soak at 80°C, the cooling cycle starts to cool the samples in accordance with the programmed cooling sequence as programmed in 9.1.

9.2.8 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 9.3).

9.2.8.1 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.

9.2.8.2 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.

9.2.9 If the final temperature as noted in 9.2.8 is in error in either direction by more than 0.2°C, see X2.2 before starting another test.

9.2.10 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.

9.3 Measurement of the Yield Stress:
9.3.1 Beginning with the cell farthest to the left of the instrument, follow the procedure below for each cell in turn.

9.3.2 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.

9.3.3 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.)

9.3.4 Follow the instrument model specific instructions:

NOTE 2 - For users with CMRV-4 or newer instruments wishing to manually time yield stress and viscosity, follow the instructions in 9.3.4.1 and 9.4.1.1, respectively.

Model CMRV-3 or Earlier

9.3.4.1 Visually observe the rotor for movement of the crossarm. (Do not measure yield stress by way of the electronic optics.)

9.3.4.2 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 9.3.4.4.

9.3.4.3 For instruments equipped with locking pins: suspend the 10-g mass on the string, then raise the locking pin.

9.3.4.4 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 9.3.4.5. If movement is detected, record weight and proceed to 9.4.

9.3.4.5 If no movement is detected, for instruments without locking pins, hold weight assembly and add 10 g, then proceed with 9.3.4.4. 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 9.3.4.4.

Model CMRV-4 or Later Model MRV

9.3.4.6 The operator shall follow the on-screen instructions for the addition of weight increments.

9.3.4.7 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 the on-screen instructions.

9.3.4.8 If additional weight is requested, capture crossarm in locking pin, then add one additional 10 g weight, and follow the on-screen instructions. Press the flashing start button, then immediately raise the locking pin. Repeat procedure until no additional weight is requested. Proceed to 9.3.4.

9.3.4.9 For instruments without locking pins: carefully suspend and hold the 10 g weight cage on the string without jerking rotor, and follow the on-screen instructions. Press the flashing start button, and immediately release the weight cage.

9.3.4.10 If no movement is detected, then carefully weight the cage. Add next 10 g weight increment as indicated on the computer screen, without pulling on the string, and follow the on-screen instructions. Press the flashing start button, and immediately release weight cage. Repeat procedure until no additional weight is requested. Proceed with 9.3.4.

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.

9.4 Measurement of Apparent Viscosity:
9.4.1 Follow the instrument model specific instructions:

For CMRV-3 or Earlier

9.4.1.1 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.

9.4.1.2 If the first half-revolution requires less than 10 s, measure and record the time for the first three revolutions.

9.4.1.3 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.

9.4.1.4 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. Report that the viscosity is greater than the value calculated in 10.2.

9.4.1.5 If the time for the first three revolutions is less than 4 s, record the time as less than 4 s. Report that the viscosity is less than the value calculated in 10.2.

For CMRV-4 and Later

9.4.1.6 Follow the on-screen instructions, press the start button, and slowly suspend the weight on the string. Timing will automatically begin with the first movement. Do not remove weight while viscosity LED on instrument is flashing. The time and viscosity will be displayed. Proceed to 9.5.

9.5 Repeat 9.3-9.4 for each of the remaining cells in order from left to right.

9.6 Cleaning:
9.6.1 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.

9.6.2 Remove the upper rotor pivots and the rotors.

9.6.3 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.

9.6.4 Clean the rotors in a similar manner.

10. Calculation of Yield Stress and Apparent Viscosity
10.1 Yield stress is given by the following equation:
Ys = 3.5 M
where:
Ys = yield stress, Pa, and
M = applied mass, g.

10.2 The viscosity is given by the following equation when using the cell constant obtained in Eq 8:
ha = C t 3/r
where:
ha = apparent viscosity in 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.