Understanding viscosity
Resistance to flow, in pascals per second.
The dynamic vs kinematic distinction, why centipoise and stokes still exist, and what motor oil grades actually mean.
Dynamic viscosity.
A fluid's internal resistance to shear. SI unit pascal-second (Pa·s); CGS unit poise (P), with the more common centipoise (cP) = 0.001 Pa·s. Water at 20°C is 1 cP — the unit's natural anchor. Honey is ~10,000 cP; motor oil is 100-500 cP at operating temperature; molten glass is 10¹² cP. Eight orders of magnitude across common substances.
Kinematic viscosity.
Dynamic viscosity divided by density: ν = μ/ρ. Units: m²/s (SI) or stokes (St), with centistokes (cSt) = 1 mm²/s. Why both: dynamic viscosity is what determines force/shear stress, but kinematic is what determines flow behaviour for given pressure differences. Pipeline calculations, lubrication, motor oil specifications all use kinematic viscosity. Water at 20°C: 1 cSt; SAE 30 oil at 40°C: ~100 cSt.
SAE motor oil grades.
The "5W-30" on a motor oil bottle is two viscosity grades. The "5W" is the low-temperature (winter) grade: maximum viscosity at cold cranking. "30" is the high-temperature grade: kinematic viscosity at 100°C. A multigrade oil flows easily when cold (for cold starts) and maintains adequate viscosity when hot. 5W-30 at 100°C is ~10-13 cSt; the same oil at -30°C might be 6000+ cP dynamic. The SAE J300 spec defines all the ranges; oil engineers blend base oils plus polymeric viscosity-index improvers to hit them.
A worked conversion.
A datasheet reports a fluid at 250 cP dynamic, density 850 kg/m³. SI dynamic: 0.25 Pa·s. Kinematic: 0.25 / 850 = 2.94 × 10⁻⁴ m²/s = 294 cSt. Imperial lubrication often uses SUS (Saybolt Universal Seconds), an older flow-time-from-orifice measurement, with empirical conversions to cSt. For modern work, the SI / poise / stokes triangle is enough.
cP to cSt via density
ν = μ / ρ
Same fluid, two viscosities — one dynamic, one kinematic.
250 cP / 850 kg/m³ = 294 cSt
= 0.25 Pa·s ≡ 2.94 × 10⁻⁴ m²/s
Temperature dependence is huge.
Most fluids' viscosity drops dramatically as temperature rises. Water goes from 1.79 cP at 0°C to 0.28 cP at 100°C — a 6× change. Motor oil can vary 10-50× across its operating range. Honey is solid-thick at room temperature and pours easily when warmed. Always check the temperature attached to a viscosity number; the value alone is meaningless without it.