The Italian primary kinematic viscosity standard: The viscosity scale

The base for all viscosity measurements is the internationally accepted viscosity of double-distilled water at 20 °C. Using this water and liquids of ever greater viscosity, INRiM derives by a step-up procedure the constants for its master “glass capillary” Ubbelohde viscometers, and these are used in their turn to determine the viscosity of undetermined liquids and reference materials in the kinematic viscosities range from about 0.4 mm2/s to above 700,000 mm2/s with temperatures from 10 °C to 150 °C. From the results obtained on recalibration of the INRiM viscometers and the comparisons made with other independent laboratories, it is concluded that the present scale is realized with an uncertainty of about 0.2% at low viscosities rising to about 0.8% at high viscosities. This enable INRiM to provide traceability to Italian viscosity measurements as well as calibrating various types of viscometers for the highest measurement accuracy i.e. glass capillary, cups, rotational. This paper addresses the relevant procedure on the realisation of the Italian kinematic viscosity scale (viscosity standard) which was conducted over a number of years. The different sources of error in the viscometer calibration, mainly due to the contribution by the kinetic energy of the fluid stream, surface tension variations and, finally, buoyancy are considered. Special emphasis is also given on the uncertainty evaluation and in the international activities which provide evidence of the present measurement capabilities of the Italian laboratory and ensures smaller measurement uncertainties in the future.