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
8. Calibration and Standardization
8.1 Calibrate the temperature sensor while attached to the temperature controller. The sensed temperature shall be verified using a reference thermometer noted in 5.3 at a minimum of three temperatures. Make these temperature measurements at least 5°C apart to establish a calibration curve for this combination of temperature sensor and controller.
NOTE 1 - All temperatures in this test method refer to the actual temperature and not necessarily the indicated temperature.
8.2 The calibration of each viscometric cell (viscometer constants) can be determined with the viscosity standard and the following procedure at -20°C:
8.2.1 Use steps 9.2-9.2.5.
8.2.2 Program the temperature controller to cool the mini-rotary viscometer block to -20°C within 1 h or less, then start the program.
8.2.3 Allow the oil in the cells to soak at -20 more or less 0.2°C for at least 1 h, making small temperature adjustments, if necessary, to maintain the test temperature.
8.2.4 At the end of the soak period, record the temperature reading (test temperature) and remove the cover of the viscometer cell.
8.2.5 Proceed to 9.3.1-9.3.3.
8.2.6 Perform step 9.4.1.
8.2.7 Repeat 8.2.5 and 8.2.6 for each of the remaining cells, taking the cells in order from left to right.
8.2.8 Calculate the viscometer constant for each cell (rotor/stator combination) with the following equation:
C = ho/t
where:
ho = viscosity of the standard oil, cP (mPa•s) at -20°C,
C = cell constant with 150 g mass, Pa, and
t = time in seconds for three complete revolutions.
8.2.9 If any cell has a calibration constant more than 10 % higher or lower than the average for the other cells, the fault may be a problem with rotor operation. Examine rotor for damage, and recalibrate the instrument.
8.3 If corrected values for controller temperature and thermometer deviate by more than the tolerance, use X2.2 to assist in determining the fault.
