The increase in uncertainty throughout the viscosity scale being the principal disadvantage of capillary viscometry, the BNM-LNE decided to develop an absolute falling-ball viscometer making it possible to cover a wide range of viscosity while keeping a weak uncertainty. The measurement of viscosity then rests on the speed limit measurement of falling ball, corrected principal identified effects (edge effects, inertial effects, etc). An experimental bench was developed in order to reach a relative uncertainty of the order of 10 -3 to the measure of viscosity. This bench must allow to observe the trajectory of the ball inside a cylindrical tube filled with liquid for which the viscosity is to be measured, and to obtain the variations in speed throughout the fall in order to determine the area where the speed limit is reached.

The restrictions inherent in the use of the conventional reference in capillary viscometry, the viscosity of water at 20 C, have led to increased interest in absolute viscometers. The viscosity laboratory of the Bureau National de Métrologie, Laboratoire National d'Essais (BNM-LNE) has begun the study of an absolute falling-ball viscometer. The principal difficulties with this system are the measurement of the velocity of the ball and the corrections to compensate for the effects exercised on the ball. Results from a preliminary experiment indicate that uncertainties better than those of capillary viscometers can be obtained. The use of a velocity measurement video system should further reduce the uncertainties.