Capillary viscometer with a pressure sensor: a subject for student projects

We describe a vacuum-based pressure-detecting capillary viscometer for the viscosity determination of Newtonian fluids without known density. The viscometer operates on the principle that the variation in air pressure of the vacuum vessel p (t) replaces the flow rate and pressure drop measurements which are usually required for the operation of a capillary tube viscometer. The mathematical expression for p(t), found in the terms of the Lambert-W function, is used to fit the experimental data for viscosity determination. The results for viscosities of distilled water and 50 wt.% glycerol aqueous solution obtained under ambient temperature condition were compared to reference data and a good agreement was observed. The viscometer is suitable for undergraduate laboratories due to its low cost and simplicity in experimental setup. Moreover, the experimental with the vacuum vessel setup provides an in-depth understanding of fluid flow.