centipoise
Dynamic viscosityexact by definition
The centipoise is one hundredth of a poise, exactly 0.001 Pa·s, and therefore exactly equal to the millipascal second. The relation is exact because the poise is exactly 0.1 Pa·s by the definition of the CGS system. It remains the dominant unit in the process, coatings and petroleum industries.
Watch out: Centipoise (cP) is dynamic viscosity; centistokes (cSt) is kinematic viscosity, and they are related by density: cP = cSt × specific gravity. Since water has a specific gravity of 1 and a viscosity of about 1 in both units, the two look interchangeable when you test them on water and are not. Motor oil grades, ASTM D445 measurements and most oil analysis reports are in cSt, while pump and mixer sizing usually wants cP.
| 1 cP | 0.001 Pa·s |
About the centipoise
The centipoise is one of the luckiest unit choices in engineering. Water at 20 °C has a dynamic viscosity of 1.002 cP, so any liquid's centipoise value tells you immediately how many times thicker than water it is. Light oil is 20 to 50 cP, glycerine about 1400 cP, honey a few thousand, and heavy asphalt in the millions. That ready comparison is why the process industries never moved to the Pa·s, even though the conversion is trivial: 1 cP is exactly 1 mPa·s.
The trap is the companion unit. Kinematic viscosity, symbol \(\nu\), is dynamic viscosity divided by density, and its CGS unit is the stokes, with the centistokes in everyday use. Water is about 1.004 cSt at 20 °C, almost the same number as its centipoise value, because its density is almost exactly 1 g/cm³. For any other fluid the numbers separate: an oil at 100 cSt with a specific gravity of 0.88 has a dynamic viscosity of 88 cP. Oil analysis laboratories report cSt because the standard test method, a gravity-driven capillary tube, measures kinematic viscosity directly. Pump curves, mixer power and pressure drop correlations often want the dynamic value. Converting between them requires the density at the same temperature as the viscosity measurement, and viscosity is strongly temperature dependent, so a number without a temperature attached is not usable.