UNITS OF VISCOSITY
Poise (symbol: P) + centiPoise (symbol: cP)
Named after the French physician Jean Louis Marie Poiseuille (1799 – 1869), this is the CGS unit of viscosity, equivalent to dyne-second per square centimetre. It is the viscosity of a fluid in which a tangential force of 1 dyne per square centimetre maintains a difference in velocity of 1 centimetre per second between two parallel planes 1 centimetre apart.
Even in relation to high-viscosity fluids, this unit is most usually encountered as the centipoise (cP), which is 0.01 poise. Many everyday fluids have viscosities between 0.5 and 1000 cP.
SOME TYPICAL DYNAMIC VISCOSITIES (cP at 20°C)
|air||0.02||motor oil SAE 20||125|
|acetone||0.3||motor oil SAE 50||540|
|linseed oil (raw)||28||treacle||20,000|
|corn oil||72||peanut butter||250,000|
|olive oil||84||window putty||100,000,000|
Pascal-second (symbol: Pa.s) + milliPascal-second (symbol: mPa.s)
This is the SI unit of viscosity, equivalent to newton-second per square metre (N·s m–2). It is sometimes referred to as the poiseuille (symbol Pl).
One poise is exactly 0.1 Pa·s. One poiseuille is 10 poise or 1000 cP, while 1 cP = 1 mPa·s (one millipascal-second).
TABLE OF EQUIVALENTS
|1 kilogram-force second per square metre||kgf·s m-2||9 806.6501248|
|1 poundal second per square foot||pdl·s ft-2||1 488.164435|
|1 pound per foot hour||lb (ft·h)-1||0.4133789|
|1 pound per foot per second||lb (ft·s)-1||1 488.1639328|
|1 pound-force second per square foot||lbf·s ft-2||47 880.2595148|
|1 pound-force second per square inch (reyn)||lbf·s in-2||6 894 757|
|1 slug per foot second||slug (ft·s)-1||47 880.25898|
Stokes (symbol: St) + centiStokes (symbol: cSt)
This is the CGS unit, equivalent to square centimetres per second (cm2 s–1). One stokes is equal to the viscosity in poise divided by the density of the fluid in grams per cubic centimetre (g·cm–3). It is most usually encountered as the centistokes (cSt), equal to 0.01 stokes.
Saybolt Seconds Universal (SSU)
This is the time for 60 millilitres (ml) of fluid to flow through the calibrated orifice of a Saybolt Universal viscometer at a specified temperature, as prescribed by test method ASTM D 88. For higher viscosities, SSF (Saybolt Seconds Furol) is used. “Furol” comes from “fuel and road oil”.
This is the ratio of the time of flow of 200 ml of fluid to the time of flow of 200 ml of water at the same temperature in a standardized Engler viscosity meter.
TABLE OF EQUIVALENTS
|1 square centimetre per second||cm2 s-1||100|
|1 square metre per second||m2 s-1||1 000 000|
|1 square foot per second||ft2 s-1||92 903.04|
|1 square inch per second||in2 s-1||645.16|
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