Viscosity measurement reveals the degree of resistance to flow of a liquid or gaseous substance, usually in terms of its unwillingness to flow under conditions where it is deformed by shear or extension. A viscometer is the instrument used to measure the fluidity or relative viscosity of the material.
Almost all fluids (with the notable exception of the liquid form of the helium isotopes helium-3 and helium-4) have some degree of resistance to flow, and the measurement is used to determine the degree of viscosity over a spectrum ranging from from unstressed resistance (called ideal fluid) to total resistance.
The study of this property of substances is known as rheology, and researchers are engaged in measuring viscosity to determine this often important property of a particular liquid or gas. This property is measured with a viscometer to provide this data, which is important for quality control applications in the production of a wide variety of products, including inks, paints, motor oils, foods, pharmaceuticals, and cosmetics, to name a few. only some.
Viscosity is most commonly expressed as a figure called the coefficient of viscosity; however, there are several different types of coefficients that can be used. The specific viscosity coefficient used to indicate the results obtained from viscosity measurement depends to a large extent on the type of stress applied to the fluid, as well as the particular qualities of the fluid in question.
The different coefficients used to express the results of a viscometer include absolute (or dynamic) viscosity, kinematic viscosity (which is absolute viscosity divided by density), volumetric (or apparent) viscosity, shear viscosity, and viscosity extensional. Each of these coefficients is more or less suitable for specific fluids, with absolute and shear viscosity being the best known and most widely used when measuring viscosity.
There are many different types of rheometers and other instruments used to measure the viscosity of substances that can be grouped into five broad categories based on the style of measurement used. The rotational method measures this characteristic by immersing the rotors in a sample and measuring the torque. A vibroviscometer with an adjustable amplitude transducer immersed in the sample is used; The current used to drive this transducer is then measured. The capillary method measures the pressure difference at either end of a capillary through which the sample is allowed to flow. The falling ball method of measuring viscosity involves taking a measurement of the amount of time required for an object (usually a sphere or cylinder) to fall through the sample material. The cup method determines the viscosity of a sample by measuring the time it takes for a sample to come out of a hole in a container.
Since the temperature of a liquid and its viscosity are correlated, a viscometer or viscometer must be used under carefully controlled conditions of temperature to produce accurate results. Even a small change in temperature can have enough of an effect to negatively affect the result of a quality control test or production process. Measuring viscosity is a demanding but essential task for a wide range of scientific research, as well as for the manufacturing of a variety of products for the business-to-business and consumer markets.