Use of Ubbelohde Master viscometers as a national standard in metrological institutes

National metrological institutes are responsible for the dissemination of the units of physical quantities. Known are especially the quantities mass, length and time: e.g. there are four atomic clocks in operation in Germany, at the Physikalisch-Technische Bundesanstalt in Brunswick (PTB), which provides the time standard for legal time in Germany. In addition, more than 400 atomic clocks in over 60 institutes worldwide are compared with each other as the basis for international atomic time.

Viscometry in Metrological Institutes

Some metrological institutes are similarly responsible for the dissemination of the unit of viscosity and maintain reference viscometers as national standards for this purpose.

In general, capillary viscometers according to Ubbelohde are used for this purpose, which have a suspended level as a special feature and thus provide measured values independent of the filling quantity.

They are used for the very accurate determination of the kinematic viscosity of Newtonian (reference) fluids, which are passed on by these institutes to be able to calibrate other viscometers traceable to the national standard.

Method of measurement

In capillary viscometers, the kinematic viscosity v is measured by the flow time t of a defined volume of liquid through a measuring capillary and can be calculated using the following equation, considering the kinetic energy correction.

v = K × t             

Here K is the viscometer constant of the respective viscometer.

The viscometers are filled with the liquid to be measured and tempered in a water bath with a very high accuracy.
The accuracy of the temperature significantly determines the accuracy of the measurement method. A series of measurements of at least 3 - for metrological purposes typically 5 measurements - of the flow time is then carried out with the sample.

Special features of the master viscometers with 400 mm capillary length

In contrast to standard viscometers, the capillaries of Ubbelohde Master viscometers have a special length, at SI Analytics® the length is 400 mm.

Due to this considerably higher length compared to standard designs, the kinetic energy correction becomes particularly small and thus negligible in many cases. In general, a reduction of the Hagenbach correction leads to a higher measurement accuracy, because although this correction can be determined individually, the value of the correction has a comparatively high relative measuring uncertainty.

What can Xylem Analytics contribute to these high demands on capillary viscometry?

More details on the kinetic energy correction can be found in our Visco Handbook in Appendix A.

Due to decades of experience in manufacturing glass capillary viscometers in-house, Xylem Analytics can also calculate and manufacture special designs such as Ubbelohde master viscometers and guarantee the quality expected from such a reference measuring instrument.

This includes, for example, handcrafted optimally shaped inlet and outlet funnels as well as mergers of the capillary to keep the Hagenbach correction low. In order to achieve certain values of the viscometer constant, the right capillary can be selected from a large selection of capillary diameters and the measuring bulb volume can also be individually adapted.

SI Analytics® Ubbelohde master viscometers are therefore in use at several national metrological institutes. The complex calibration of these special viscometers is entirely the responsibility of the respective institutes.