ASTM D6896 Standard Test Method for Determination of Yield Stress and Apparent Viscosity of Used Engine Oils at Low Temperature
8. Sampling
8.1 A representative sample of test oil free from suspended granular material and water is necessary to obtain valid viscosity measurements. If the sample in its container is received below the dew-point temperature of the room, allow the sample to warm to room temperature before opening the container.
9. Calibration and Standardization
9.1 Calibrate the temperature sensor in place while attached to the temperature controller. The sensed temperature shall be verified using a reference thermometer specified in 6.4 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. For instruments using an independent temperature controller, see X2.1 for calibration guidance.
NOTE 1 - All temperatures in this test method refer to the actual temperature as measured in the left thermowell and not necessarily the indicated temperature.
9.2 The calibration of each viscometric cell (viscometer constants) can be determined with the viscosity standard and the following procedure at -20°C.
9.2.1 Use steps 10.2.3-10.6.
9.2.2 Program the temperature controller to cool the minirotary viscometer block to -20°C within 1 h or less, then start the program.
9.2.3 Allow the oil in the cells to soak at -20 +/- 0.2°C for at least 1 h, making small temperature control adjustments, if necessary, to maintain the test temperature.
9.2.4 At the end of the soak period, record the temperature reading of the measuring device in the left thermowell (test temperature) and remove the cover of the viscometer cell.
9.2.5 Perform step 10.4.1.
9.2.6 Repeat 9.2.5 for each of the remaining cells, taking the cells in order from left to right.
9.2.7 Calculate the viscometer constant for each cell (rotor/stator combination) with the following equation:
C = ηo/t
where:
ηo = viscosity of the standard oil, cP (mPa·s) at -20°C,
C = cell constant with 150 g mass, Pa, and
T = time for three complete revolutions, s.
9.2.8 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 instrument.
9.3 If corrected values for controller temperature and thermometer deviate by more than the tolerance, use X2.2 to assist in determining the fault.
9.4 Oven - Check the calibration of the temperature sensing device by appropriate methods. The temperature should be constant at 80 +/- 1°C.
