viskometri

Viskometri, or viscometers, are instruments that measure viscosity, the resistance of a liquid to flow. Viscosity is reported in pascal-seconds (Pa·s) or centipoise (mPa·s), and measurements are temperature-dependent, requiring control of testing temperature and conditions.

Two main families exist: capillary viscometers and rotational viscometers. Capillary viscometers determine viscosity from the time a liquid takes to flow through a narrow tube, providing kinematic viscosity with a calibration constant. Examples include Ostwald and Ubbelohde viscometers. Rotational viscometers impose shear by rotating a spindle or cone and measure the resulting torque to obtain viscosity; they are suitable for both Newtonian and non-Newtonian fluids.

Other methods include falling-ball viscometers, based on the rate at which a ball falls through the liquid.

Applications span lubricants and oils, paints and coatings, foods, cosmetics, pharmaceuticals, and polymers. In polymer science, viscosity data help infer molecular weight and processability, often alongside more comprehensive rheology.

Standards from ASTM and ISO define testing conditions and reporting. Common references include ASTM D445 for kinematic viscosity and ISO methods for various fluids. Accurate measurements require temperature control, clean samples, calibration with reference fluids, and appropriate selection of viscometer type for the fluid behavior.

Limitations include dependence on shear rate and history, especially for non-Newtonian fluids, and potential measurement errors from air entrainment, contaminants, or poor temperature control.