Ugo... As i said before the cup viscometer are not presise as falling ball viscosimeter..

The problem with cup viscosimeter are weight of fluid and loss of heat are influencing the time of measure therefore the cup viscosimeter are not presise and has a tolerance at 20% plus minus. It is too coarse. You has not the calibrating oil to test the cup viscosimeter are within tolerance range when you are talking about calibrate the Zahn viscosimeter. Your measurement of oil can be more unprecise without you know about the measure tools are within tolerance range or not.

The falling ball viscosimeter are best since the temperature are constant and the fluid are not influenced by weight. Tolerance are better than 1%. Because the ball has a constant weight and the glass tupe are in a container with heated fluid to keep constant temperature.

http://www.geneq.com/catalog/en/visco_ball.html
http://wise.fau.edu/~blarkin/Viscometer.html
http://www.rheologysolutions.com/fallingball_techs.html