ISO 3104 Petroleum products - Transparent and opaque liquids - Determination of kinematic viscosity and calculation of dynamic viscosity
7 Calibration and verification
7.1 Verify the viscometer calibration following the laboratory procedure using a certified viscosity reference standard (5.5). If the measured kinematic viscosity does not agree within +/- 0.35 % of the certified value, recheck each step in the procedure, including thermometer and viscometer calibrations, to locate the source of error. Table 1 in ISO 3105 gives details of standards available.

NOTE 10 The most common sources of error are caused by particles of dust lodged in the capillary bore and temperature measurement errors. It should be appreciated that a correct result obtained on a standard oil does not preclude the possibility of a counterbalancing combination of the possible sources of error.

7.2 The calibration constant, C, is dependent upon the gravitational acceleration at the place of calibration and this shall therefore be supplied by the standardization laboratory, together with the instrument constant. Where the acceleration of gravity, g, differs by more than 0.1 %, correct the calibration constant as follows:
C2 = (g2/g1)C1
where the subscripts 1 and 2 indicate respectively the standardization laboratory and the testing laboratory.