Comments on high-accuracy viscosity measurements using capillary viscometers

The working equation of a capillary viscometer, used generally in the form v = kv - B/tau (where v is kinematic viscosity; tau is outflow time; k and B are viscometer constants), can be deduced directly from the Navier-Stokes equation. The surface tension only affects the k parameter. For a Ubbelohde-type viscometer with suspended level, the curvature of the meniscus, which compensates the influence of the mean surface tension, has been calculated. The relation often used for the capillary pressure in tubes, p(sigma) = 2sigma/R, is only approximately valid for radii R < 0.3 mm. The Knibbs method, which uses external variable pressure, provides a means of calibrating viscometers to high accuracy using only one reference liquid.