Ostwald viscometer

An Ostwald viscometer is a simple U-shaped glass tube used to measure how thick a thin liquid is, by timing how long it takes to drain through a narrow channel. You draw the liquid up above an upper mark, then let gravity pull it down through a fine capillary and use a stopwatch to time its fall between two etched marks. A thicker liquid takes longer; a thinner one drains faster.

The instrument works by comparison: you time a reference liquid of known viscosity and density, then your sample, and the ratio of the flow times (corrected for the small density difference) gives the unknown viscosity. Because flow is driven by the liquid's own weight under gravity, the raw result is a kinematic viscosity, which can be multiplied by density to obtain dynamic viscosity.

Ostwald and related capillary viscometers (such as the Ubbelohde) are accurate, inexpensive, and standard in pharmacopoeias for clear, mobile Newtonian liquids like syrups, oils, and dilute polymer solutions. Their key limitation is that they apply a range of shear rates across the capillary that you cannot independently control, so they are unsuitable for non-Newtonian materials, where a rotational instrument is needed instead.