ASTM D6821 Standard Test Method for Low Temperature Viscosity of Drive Line Lubricants in a Constant Shear Stress Viscometer
8. Sampling
8.1 A representative sample of test oil free from suspended solid 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
9.1 Temperature Control Calibration Procedure - Calibrate the MRV temperature control by comparing the instrument's displayed temperature to a thermometer in the thermowell. The thermometer used shall meet the requirements in 6.4.
9.1.1 Place 10 mL of a typical test fluid and rotors in each cell then place cover on instrument. Cell caps maybe used if available.
9.1.2 Place the thermometer in the thermowell. This thermowell is to be used for all temperature measurements below 25 °C.
NOTE 4 - Prior to inserting the thermometer or DCT probe in the thermowell, place several drops (~3) of a heat transfer fluid such as 50/50 water/ethylene glycol mix, CCS reference oil CL100, or a dewaxed low viscosity mineral oil in the thermowell.
9.1.3 Make these temperature measurements at least 5 °C apart and include both -5 °C and the lowest test temperature used. Establish a calibration curve using at least three comparisons of the thermometer and the instrument's temperature control. Make at least two temperature measurements at every calibration temperature with at least 10 min between observations.
9.1.4 Follow the instrument manufacturers instructions for correcting the instrument's measured temperature. Alternatively, establish a correction equation between thermometer and the instrument's measured temperature then adjust each temperature of the cooling program by the offset determined with the correction equation.
NOTE 5 - All temperatures in this test method refer to the actual temperature and not necessarily the indicated temperature.
9.2 Viscometer Cell Calibration - The calibration cell constant for each viscometric cell (viscometer constants) is to be determined at -25 °C with the viscosity calibration oil.
9.2.1 Following the steps in 10.1, prepare the cells for calibration using the calibration oil as the sample.
9.2.2 Use either the calibration temperature profile for the instrument or, alternatively, the cooling profile given in Test Method D3829 for a -25 °C test temperature and follow the owner's manual instructions for the instrument to initiate the cooling profile program.
9.2.3 Allow the samples to soak at -25 °C +/- 0.2 °C for at least 1 h.
9.2.4 Place the thermometer in the thermometer well at least 30 min prior to executing 9.2.6. See Note 4. This thermowell location is to be used for calibration and temperature monitoring during the test procedure.
9.2.5 At the completion of the temperature profile and soak period, check that the test temperature is within +/- 0.1 °C of the desired calibration temperature with a thermometer.
9.2.6 Perform step 10.7.
9.2.7 Repeat 9.2.6 for each of the remaining cells in numerical order.
9.2.8 Calculate the viscometer constant for each cell (rotor/stator combination) with the following equation:
C = ηo/t
where:
ηo = viscosity of the calibration oil, mPa·s at -25 °C,
C = cell constant for a 20 g mass, mPa, and
t = time for three complete rotor revolutions, s.
