![]() Absolute measuring systems include rotation and oscillation rheometers that use concentric cylinders, plate-plate or cone-plate systems to determine the flow behaviour of Newtonian and non-Newtonian fluids. All the pressure-driven and drag flow viscometers described above are relative measuring systems. The calculation of parameters such as shear stress, shear rate and viscosity is not recommended in relative measuring systems, but raw data such as torque or speed can be used instead 19. The former have a standard “two plates” geometry and defined shear conditions in a relatively narrow shearing gap, whereas the latter do not meet the conditions of the two-plates model 17. Viscometers can also be classed as absolute or relative measuring systems. Mixer-type rheometers with various rotating stirrers are particularly suitable for the analysis of building materials and foodstuffs containing large dispersed particles, e.g. Different geometries are available (cylinders, disks, cones, pins and T-bars) and such devices are often used for quality assurance 22– 24. Spindles are immersed in the sample and rotated at a specified rate to measure the resulting torque 19. Drag flow viscometers include spindles and mixer-type rheometers. Falling sphere viscometers additionally require transparent samples, unless variants based on induction or electromagnetic fields are available 19– 21. In all three cases, flow is induced by gravitational force and measurement is dependent on weight and density, and can only be determined for Newtonian fluids with low viscosity 3. Typical pressure-driven viscometers include flow cups, falling sphere viscometers and glass capillary viscometers. The most common are pressure-driven and drag flow viscometers 16. ![]() Several devices have been developed to measure the viscous, viscoelastic and elastic properties of liquids and solids 17– 19. ![]() The viscosity of crude oils can be measured using a standard method (ASTM D2983).The rheological behaviour of viscous liquids is relevant in the food 1– 4, pharmaceutical 5– 9 and cosmetic 10– 12 industries, as well as technical chemistry 13– 15 and production engineering 16. However, different reference temperatures, such as 40☌ (104 ☏), 50 ☌ (122 ☏), and 60 ☌(140 ☏), are also used to report kinematic viscosities of petroleum fractions. Values of kinematic viscosity for pure liquid hydrocarbons are usually measured and reported at two reference temperatures, 38☌ (100☏) and 99☌ (210☏) in cSt. Kinematic viscosity is expressed in units of centistokes (cSt), Saybolt Universal seconds (SUS), and Saybolt Furol seconds (SFS). Viscosity of liquids is usually measured in terms of kinematic viscosity, which is defined as the ratio of absolute (dynamic) viscosity to absolute density (ν = μ/ρ). Among petroleum products, viscosity constitutes a critically important characteristic of lubricating engine oils. Interestingly, the viscosity of liquid decreases with increasing temperature, while viscosity of gases increases with increasing temperature. Power requirement to transport (e.g., to pump) a fluid depends strongly on the fluid’s viscosity. Newton’s Law of Viscosity provides a physical definition of viscosity. A high-viscosity fluid has a low tendency to flow, whereas low-viscosity fluids flow easily. ![]() Viscosity, commonly depicted by the symbol μ, is a physical property of a fluid that describes its tendency/resistance to flow. ![]()
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