Viscosity

Viscosity

Viscosity, commonly depicted by the symbol μ, is a physical property of a fluid that describes its tendency/resistance to flow. A high-viscosity fluid has a low tendency to flow, whereas low-viscosity fluids flow easily. Newton’s Law of Viscosity provides a physical definition of viscosity. Power requirement to transport (e.g., to pump) a fluid depends strongly on the fluid’s viscosity. Interestingly, the viscosity of liquid decreases with increasing temperature, while viscosity of gases increases with increasing temperature. Among petroleum products, viscosity constitutes a critically important characteristic of lubricating engine oils. 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). Kinematic viscosity is expressed in units of centistokes (cSt), Saybolt Universal seconds (SUS), and Saybolt Furol seconds (SFS). Values of kinematic viscosity for pure liquid hydrocarbons are usually measured and reported at two reference temperatures, 38°C (100°F) and 99°C (210°F) in cSt. However, different reference temperatures, such as 40°C (104 °F), 50 °C (122 °F), and 60 °C(140 °F), are also used to report kinematic viscosities of petroleum fractions. The viscosity of crude oils can be measured using a standard method (ASTM D2983).