Viscometry, used to determine the viscosity of a substance, allows us to estimate such important parameters of materials in a solution or a melt as, for example, the degree of dispersion of fillers, the existence of polymeric bonds between their particles, etc. Viscosity (dynamic viscosity) η is the coefficient of proportionality between shear rate æ and shear stress σ in the Newton’s equation σ = ηæ describing the so-called Newtonian system, i.e. the system in which the shear stress σ is linearly dependent on the shear rate æ. In non-Newtonian systems, the effective viscosity is determined, i.e. the ratio of the shear stress σ to the shear rate æ that depends on η and æ. In the SI system, the viscosity is measured in pascal-seconds (Pa·s). Poise is also used as a measurement unit (1 P = 0,1 Pa·s).

The three most widespread methods of measuring viscosity are the capillary, rotation and vibration methods. In a capillary viscometre the viscosity is determined by fluid flow rate in a capillary; in a vibrational viscometre – by resistance to oscillatory motion of a body immersed in the fluid or by the intensity of absorption of oscillations; in a rotary viscometre – by the change in torque at a given angular velocity. In laboratories the three following methods are also used to measure viscosity: falling ball method, penetration method and plastometry. In the “falling ball” method the viscosity is determined by the rate of uniform falling of a ball in a viscous fluid, using the Stokes' formula. The penetration method and plastometry are used for studying high-viscosity fluids; in the first case a solid body (e.g. cylinder, cone or sphere) is pressed into the fluid, and the speed of its motion or the applied force is used to determine the value of viscosity; in the second case the measured parameter is the shear flow of the fluid between plane-parallel plates shifting against each other, or the spreading of the material induced by squeezing of the plates.

To date a large number of diverse viscometre designs have been developed and are used for measuring viscosity in a wide range – from 10^-5 Pa·s (gases) up to 10¹² Pa·s (polymer systems).