A viscometer or viscosimeter measure the viscosity and flow properties of a fluid. Viscosity arises from the internal friction of a fluid and is defined as a liquid’s resistance to flow or shear stress. Viscometers work for fluids whose viscosity does not change under varying flow conditions; rheometers must be used when the viscosity does change with flow conditions.
There are many ways to measure viscosity, but they break down into two basic methods. Either an object, such as a sphere or a rotor blade, moves through a stationary material, or the material flows through or past a stationary object. In either case, the resistance to flow is measured.
There are different types of viscometers: capillary viscometers (also known as glass or u-tube viscometers), rotational viscometers, falling sphere viscometers, falling piston viscometers, vibrational viscometers, orifice viscometers and bubble viscometers. Rotational and capillary viscometers are generally the most widely used. Capillary viscometers measure viscosity by timing how long it takes for a transparent or translucent fluid to flow between two points of a capillary tube. On the other hand, rotational viscometers use a torsion spring to measure the torque required to rotate a spindle in the material. Changing rotor speed and size allows for the measurement of different ranges of viscosity.
Viscometers have wide and varied applications in the materials science and chemical industries for materials such as oil, asphalt, plastics, wax, paints, coatings, and adhesives. They are also used for food and beverages and personal-care products such as cosmetics, shampoo, and toothpaste. Measuring viscosity is important when considering process conditions for materials that need to be pumped or piped. Viscosity also affects dipping and coating performance, which is particularly relevant to paints and inks. Viscosity is a very useful indirect measure of material properties including molecular weight and density, both of which affect flow behavior. Viscometers can therefore be used to monitor batch consistency and quality control.