10. Calibration
10.1 Calibration of Manual CCS
10.1.1 On start-up of a new instrument or when any part of the viscometric cell or drive component (motor, belt, tachometer-generator, and so forth) is replaced, determine the required motor drive current. Initially, recheck the drive current (as described in 10.1.2) monthly until the change in drive current in consecutive months is less than 0.020 A and every three months thereafter.
10.1.2 Drive Current Determination - Plug the tachometer into the CAL jack, where fitted with a CAL jack. Run the 3500 mPa•s, -20°C viscosity standard at -20°C as described in Section 11. When the drive motor is turned on, establish a speed meter reading of 0.240 more or less 0.010 by adjustment of the current adjust dial. Keep this current setting constant for all subsequent calibration and test sample runs at all temperatures. When the current setting must be changed to maintain a dial reading of 0.240 more or less 0.010 units with the 3500 mPa•s reference oil at -20°C, recalibrate the instrument by either procedure described in 10.1.3.
10.1.3 Calibration Procedure - At each test temperature, calibrate with the oils listed for that temperature in Table 1 by using the procedure described in Section 11.
10.1.3.1 When only a narrow viscosity range of test liquids is to be measured, use a minimum of three calibration oils spanning the narrow viscosity range of the oils to be tested.
10.1.4 Preparation of Calibration Curves - Plot the viscosity of the calibration oils as a function of speed meter readings, and draw a smooth curve. The use of log-log coordinates or special linearized graph paper have been found suitable for this purpose. Take care to get the best fit to the points found; careless use of commercial drawing curves can lead to excessive errors. See Fig. 4 for a typical curve. Use the equation in 10.1.4.1 as an alternative method to this graphical method.
10.1.4.1 Alternatively Expressing Calibration Results by Equation - Calibration data over a limited viscosity range are well represented by the following equation:
h 5 b0/N 1 b1 1 b2N
where:
h = viscosity,
b0, b1, b2 = constants determined with a minimum of three calibration oils, and
N = observed speed indicator reading.
10.1.4.2 When more than three pairs of data are available, regress these data to the following equation to determine the values of the constants b0, b1, and b2:
hN = b0 + b1N + b2N2
10.1.5 When check runs of a calibration oil do not fall within more or less 5 % of the values calculated from the calibration curve, recheck the calibration of the temperature sensor or rerun the calibration oils.
NOTE 5 - A separate curve or equation is intended for each temperature. However, if the calibration data at two or more temperatures fit a single curve or equation without a bias, a single curve or equation may be used for these temperatures.
10.2 Calibration of Automated CCS
10.2.1 On start-up of a new instrument or when any part of the viscometric cell or drive component (motor, belt, and so forth) is replaced, determine the required motor current. Initially, recheck the motor current (as described in 10.2.2) monthly until the change in motor current in consecutive months is less than 0.020 A and every three months thereafter.
10.2.2 Motor Current Determination - Run the 3500 mPa•s, -20°C viscosity standard at -20°C as described in Section 12. When the drive motor is turned on, establish a speed reading of 0.240 more or less 0.005 (displayed as SPEED on the computer monitor) by adjustment of the CURRENT ADJUST DIAL. Keep this current setting constant for all subsequent calibration and test sample runs at all temperatures. When the current setting must be changed to maintain a speed reading of 0.240 more or less 0.005 units with the 3500 mPa•s reference oil at -20°C, recalibrate the instrument by the procedure described in 10.2.3.
10.2.3 Calibration Procedure - At each test temperature, calibrate with the oils listed for that temperature in Table 1 by using the procedure described in Section 12.
10.2.3.1 When only a narrow viscosity range of test liquids is to be measured, use a minimum of four calibration oils spanning the narrow viscosity range of the oils to be tested.
10.2.4 Calibration Equation - The computer program regresses the calibration data over the viscosity range at each calibration temperature as is described in 10.1.4.1. There must be a minimum of four pairs of calibration data (viscosity and speed) for each temperature.
10.2.5 When check runs of a calibration oil do not fall within more or less 5 % of the values calculated from the stored calibration equation, recheck the calibration of the temperature sensor or rerun the calibration oils.
