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Units Of
Viscosity

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UNITS OF VISCOSITY



DYNAMIC 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)

   
air0.02motor oil SAE 20125 
acetone0.3motor oil SAE 50540 
methanol0.6castor oil986 
water1glycerin1490 
ethanol1.2pancake syrup2500 
mercury1.5maple syrup3200 
linseed oil (raw)28treacle20,000 
corn oil72peanut butter250,000 
olive oil84window putty100,000,000

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

   

Dynamic Viscosity

  

Symbol

cP Equivalent

1 kilogram-force second per square metre  kgf·s m-29 806.6501248
1 poundal second per square foot  pdl·s ft-21 488.164435
1 pound per foot hour  lb (ft·h)-10.4133789
1 pound per foot per second  lb (ft·s)-11 488.1639328
1 pound-force second per square foot  lbf·s ft-247 880.2595148
1 pound-force second per square inch (reyn)  lbf·s in-26 894 757
1 slug per foot second  slug (ft·s)-147 880.25898

KINEMATIC VISCOSITY

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”.

Degree Engler

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

    

Kinematic Viscosity

  

Symbol

cSt Equivalent

1 square centimetre per second  cm2 s-1100
1 square metre per second  m2 s-11 000 000
1 square foot per second  ft2 s-192 903.04
1 square inch per second  in2 s-1645.16

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