viskosimeters

Viskosimeters, or viscometers, are instruments that measure the viscosity of liquids, a property that describes resistance to flow. They typically report dynamic viscosity in pascal-seconds (Pa·s) or, for liquids with known density, kinematic viscosity in square meters per second (m^2/s). Measurements depend strongly on temperature and, for non-Newtonian fluids, on shear rate, so controlled temperature and well-defined test conditions are essential.

Most viscometers operate on one of several principles. Capillary viscometers determine viscosity from the flow time of a liquid through a narrow tube under gravity or pressure (e.g., Ubbelohde or Ostwald types). Rotational viscometers measure the torque required to rotate a spindle or plate at a set speed, covering a wide viscosity range and enabling measurements at defined shear rates. Falling-ball viscometers infer viscosity from the time it takes a ball to fall through a liquid column. Other approaches include ultrasonic viscometry and rheometers, which can measure viscosity as a function of shear rate and temperature for complex fluids.

Common applications include lubricants, oils, paints, polymers, foods, cosmetics, and pharmaceuticals. In industry and research, viscometry supports quality control, process monitoring, and material characterization.

Key considerations in obtaining reliable results include proper sample preparation, clean and calibrated instrumentation, and temperature control. For non-Newtonian fluids, a viscosity vs. shear-rate curve is typically required, and report should specify test temperature, shear rate, instrument geometry, and any corrections for density or pressure.

Standards issued by organizations such as ASTM and ISO provide test methods and reference fluids for different viscometers. These standards promote comparability across laboratories and applications.