Pascalszekundum

Pascalszekundum is the term used for the SI unit of dynamic viscosity, typically written as Pascal-second and abbreviated Pa·s. It is the measure of a fluid’s resistance to shear and represents the ratio of shear stress to shear rate in a fluid.

In SI units, dynamic viscosity μ equals 1 Pa·s when a shear stress of 1 newton per square meter produces a shear rate of 1 s−1. Equivalently, 1 Pa·s = 1 N·s/m² = kg/(m·s). The unit is used to describe how easily or how resistingly a liquid flows under an applied force. By comparison, kinematic viscosity is a related quantity, denoted ν and measured in m²/s, which equals μ/ρ where ρ is the fluid density.

Common practical values illustrate the scale: water at room temperature has a dynamic viscosity of about 0.001 Pa·s (1 mPa·s), while air is roughly 1.8×10−5 Pa·s. More viscous liquids, such as many syrups, oils, or molten polymers, have viscosities ranging from 0.01 to several pascals per second and higher, depending on temperature.

In measurement, dynamic viscosity is determined using viscometers, including capillary (e.g., Ubbelohde) and rotational types, or by falling-ball methods. The unit Pa·s is universally preferred in scientific and engineering literature, while older CGS units such as the poise (1 Poise = 0.1 Pa·s) are encountered in historical or specialized contexts. The term Pascalszekundum may appear in German-language texts, but Pa·s remains the standard designation.