Viscosity - Dynamic Converter

Pascal Second (Pa·s)

The SI unit of dynamic viscosity. 1 Pa·s = 1 N·s/m² = 10 poise = 1,000 centipoise. Standard unit in scientific and engineering calculations worldwide.

Common values: Water at 20°C: 0.001 Pa·s; Honey: 2-10 Pa·s; Glycerin: 1.5 Pa·s; Olive oil: 0.08 Pa·s

Centipoise (cP)

One hundredth of a poise. 1 cP = 0.001 Pa·s = 0.01 poise. Most common unit in practical applications because water at 20°C = 1.002 cP (very convenient reference).

Widespread use: Industrial viscosity measurements, engine oil grades, food processing, pharmaceuticals, and chemical manufacturing.

Poise (P)

CGS unit of dynamic viscosity. 1 P = 0.1 Pa·s = 100 centipoise. Named after Jean Léonard Marie Poiseuille. Still common in older literature and certain scientific fields.

Historical context: Part of the CGS system, largely replaced by SI (Pa·s) but still used in some research and industrial settings.

Newton Second per Square Meter (N·s/m²)

Equivalent to pascal second. 1 N·s/m² = 1 Pa·s. Emphasizes the force-based definition: viscous force per unit area divided by shear rate.

Fundamental definition: Represents shear stress divided by shear rate for laminar flow.

Pound per Foot per Second (lb/(ft·s))

North American engineering unit. 1 lb/(ft·s) ≈ 1.488 Pa·s. Used in US engineering calculations for fluid dynamics and pump design.

Application: US pump specifications, HVAC design, American fluid mechanics calculations, and legacy industrial equipment.

Pound-Force Second per Square Foot (lbf·s/ft²)

Alternative North American unit. 1 lbf·s/ft² ≈ 47.88 Pa·s. Used in some US industrial specifications and pipe flow calculations.

Application: Specific US industrial standards and legacy equipment documentation.

Understanding Dynamic Viscosity

Dynamic viscosity (also called absolute viscosity) measures a fluid's resistance to flow under an applied shear force. It's defined as:

Viscosity = Shear Stress ÷ Shear Rate

Higher viscosity means the fluid flows more slowly (is "thicker"). For example:

• Low viscosity: Gases, water → flow easily
• Medium viscosity: Oils, glycerin → moderate flow resistance
• High viscosity: Honey, tar, molasses → very thick, slow to flow

Temperature Dependence - CRITICAL

Viscosity is extremely temperature-dependent! Small temperature changes can dramatically affect viscosity:

• Water: 1.79 cP at 0°C → 0.28 cP at 100°C (6× change!)
• Honey: Extremely viscous at room temperature but flows much more easily when warm
• Engine oil: Can vary by 100-1000× between cold start and operating temperature

Always specify measurement temperature when reporting viscosity! A value without temperature is meaningless for most applications.

Typical Dynamic Viscosity Values (at 20°C)

• Air (20°C): 0.000018 Pa·s (0.018 cP)
• Water (20°C): 0.001 Pa·s (1.002 cP) - reference standard
• Olive oil (20°C): ~0.08 Pa·s (80 cP)
• Motor oil SAE 10W (0°C): ~1,200 cP
• Motor oil SAE 40 (100°C): ~14 cP
• Glycerin (20°C): 1.5 Pa·s (1,500 cP)
• Honey (20°C): 2-10 Pa·s (2,000-10,000 cP)
• Peanut butter: 250-500 Pa·s
• Tar/asphalt: 1,000+ Pa·s
• Blood (37°C): 0.0035 Pa·s (3.5 cP)

Dynamic vs Kinematic Viscosity

Dynamic viscosity (this converter): Absolute measure of flow resistance = Force required to shear the fluid

Kinematic viscosity: Dynamic viscosity divided by fluid density = ν = μ/ρ

Key relationship:

Kinematic Viscosity (m²/s) = Dynamic Viscosity (Pa·s) ÷ Density (kg/m³)

For water at 20°C: kinematic viscosity = 0.001 Pa·s ÷ 998 kg/m³ = 1.002 × 10⁻⁶ m²/s = 1.002 cSt (centistokes)

Key Conversion Factors

• 1 Pa·s = 1 N·s/m² = 10 poise = 1,000 centipoise (cP)
• 1 poise = 0.1 Pa·s = 100 centipoise
• 1 centipoise = 0.001 Pa·s = 1/1000 of a pascal second
• 1 lb/(ft·s) ≈ 1.488 Pa·s ≈ 14.88 poise
• Water at 20°C = 1.002 cP (reference standard)
• For temperature effects: Use Arrhenius or Walther equations for precise calculations
• SAE viscosity grades relate to kinematic viscosity at 100°C, not dynamic viscosity