Viscosity - Dynamic Converter

Dynamic viscosity — also called absolute viscosity — is the fundamental measure of a fluid's internal resistance to flow under an applied shear force. When adjacent fluid layers move at different velocities, internal friction between those layers creates a shear stress. Dynamic viscosity (symbol μ or η) quantifies that resistance: μ = τ / (du/dy), where τ is the shear stress and du/dy is the velocity gradient (shear rate). The SI unit of dynamic viscosity is the pascal second (Pa·s), which equals 1 N·s/m² = 1 kg/(m·s). It is one of the most measured fluid properties in industry, determining how fluids flow through pipes, around immersed bodies, and through porous media.

The CGS unit, the poise (P), is named after Jean Léonard Marie Poiseuille who derived the law of viscous flow in tubes. 1 P = 0.1 Pa·s = 100 cP. In practice, the centipoise (cP) is by far the most commonly used unit in chemical engineering, petroleum engineering, and food science. The reason: water at 20°C has a dynamic viscosity of approximately 1.002 cP = 1.002 mPa·s — a very convenient reference value. Engine oils are typically 50–200 cP at operating temperature; honey is 2,000–10,000 cP; glycerol is about 1,490 cP at 20°C; blood is 3–4 cP.

In petroleum and oil refining, crude oil viscosity in centipoise (cP) or millipascal-second (mPa·s) is critical for pipeline design, pump selection, and flow assurance. Heavier crude oils (API gravity below 20°) can have viscosities of 1,000–10,000 cP at reservoir temperature. Refinery process streams — from atmospheric residues to vacuum gas oils — have viscosities spanning several orders of magnitude. Converting between cP (lab measurement), Pa·s (simulation software), and lb/(ft·s) (legacy pipeline design tools) is routine.

In polymer processing (injection molding, extrusion, film blowing), polymer melt viscosity is enormously higher — typically 100 to 100,000 Pa·s at processing conditions. Polymer viscosity is shear-rate-dependent (non-Newtonian), but at any given shear rate a single Pa·s value characterizes the flow behavior. Polymer process engineers work in Pa·s; equipment manufacturers may specify in P or kP.

In food processing, viscosity governs pump selection, pipe sizing, mixing time, and product texture. Tomato paste at 50,000 cP, ketchup at 50,000–70,000 cP, chocolate at 3,000–10,000 cP, and honey at 2,000–10,000 cP represent the range. Food engineers may work in cP for raw ingredients but need Pa·s for CFD simulation of processing equipment. The mPa·s equals 1 cP exactly, providing a convenient SI-compatible unit.

The kilogram-force second per square meter (kgf·s/m²) appears in older European engineering documentation using the metric-gravitational system. 1 kgf·s/m² = 9.80665 Pa·s. Pound-force second per square inch (lbf·s/in²) is occasionally encountered in aerospace and mechanical engineering references. Pound-force second per square foot (lbf·s/ft²) — also written as slug/(ft·s) — is the standard viscosity unit in the British Gravitational system used in US aerospace fluid mechanics: 1 lbf·s/ft² = 47.8803 Pa·s.

The poundal second per square foot (pdl·s/ft²) comes from the absolute foot-pound-second system: 1 pdl·s/ft² = 1.4882 Pa·s. Pound per foot per second (lb/(ft·s)) is another absolute FPS unit equal to 1 poundal·s/ft². Pound per foot per hour (lb/(ft·h)) = 0.0004134 Pa·s appears in some US heat transfer and mass transfer textbook correlations for gas viscosities.

The full poise SI prefix series — from exapoise (10¹⁷ Pa·s) through attopoise (10⁻¹⁹ Pa·s) — covers extremes from nuclear pasta in neutron stars to ultracold quantum gases where viscosity approaches zero. This 36-order-of-magnitude range ensures coverage for any research application.

This dynamic viscosity converter supports all 29 units — Pa·s, poise (full prefix series), kgf·s/m², N·s/m², mN·s/m², dyn·s/cm², lbf·s/in², lbf·s/ft², pdl·s/ft², g/cm/s, slug/ft/s, lb/ft/s, lb/ft/h — instant, free, precise to 12 significant digits.