Viscosity describes a fluid’s resistance to flow, often called its “thickness.” This property dictates how easily a fluid deforms under force. For instance, water flows quickly, showing low viscosity, while honey pours slowly, demonstrating high viscosity.
Understanding Dynamic Viscosity
Dynamic viscosity, also known as absolute viscosity, quantifies a fluid’s internal resistance to flow under an external force. It measures the shear stress needed to achieve a certain shear rate. This internal friction arises from the interaction between adjacent fluid layers.
The SI unit for dynamic viscosity is the pascal-second (Pa·s), equivalent to one Newton-second per square meter (N·s/m²) or one kilogram per meter per second (kg·m⁻¹·s⁻¹). This unit represents the viscosity where a shear stress of one Pascal produces a shear rate of one reciprocal second.
The CGS unit for dynamic viscosity is the Poise (P), named after Jean Léonard Marie Poiseuille. One Poise equals 0.1 Pa·s. The centipoise (cP), one-hundredth of a Poise (1 cP = 0.01 P), is a more commonly used practical unit. Water at 20°C has a viscosity of approximately 1 centipoise, serving as a convenient reference.
Exploring Kinematic Viscosity
Kinematic viscosity represents dynamic viscosity divided by fluid density. It describes a fluid’s resistance to flow primarily under gravity, relevant when the fluid’s own weight drives its movement.
The SI unit for kinematic viscosity is square meter per second (m²/s). This unit reflects how quickly momentum diffuses through the fluid. Due to its size for practical applications, other units are often employed.
The CGS unit for kinematic viscosity is the Stokes (St), named after Sir George Gabriel Stokes. One Stokes equals 0.0001 m²/s (or 1 cm²/s). The centistokes (cSt), one-hundredth of a Stokes (1 cSt = 0.01 St), is a more practical submultiple. Water at 20°C has a kinematic viscosity of about 1 cSt, providing a familiar benchmark.
Converting Viscosity Units
Converting between viscosity units is often necessary for comparing data from different sources or ensuring compatibility with engineering specifications. This is crucial for accurate analysis and communication.
To convert pascal-seconds to Poise, multiply by 10 (1 Pa·s = 10 Poise). Poise can be converted to centipoise by multiplying by 100 (1 Poise = 100 centipoise).
For kinematic viscosity, convert square meters per second to Stokes by multiplying by 10,000 (1 m²/s = 10,000 Stokes). Stokes can then be converted to centistokes by multiplying by 100 (1 Stokes = 100 centistokes).
Kinematic viscosity equals dynamic viscosity divided by density. This relationship allows conversion between dynamic and kinematic measurements when fluid density is known.