Notes on Units

Viscosity can be measured using dynamic or kinematic viscometers. A dynamic viscometer is one where the shear rate can both controlled and measured (e.g a cone and plate viscometer). It is the only type of viscosity measurement that is relevant to materials where the viscosity is related to the shear rate (i.e. non Newtonian materials). The classical definition of dynamic viscosity is the ratio of shear stress to the rate of shear under steady flow conditions.

Dynamic viscosity can be measured in units of poise which is defined as the force in dynes between parallel plates 1 square cm in area, 1 cm apart, moving with a relative velocity of 1 cm / second. Transposing this to SI units gives the pascal second (Pa s) where

1 Pa s = 10 poise
1 millipascal second (mPa s) = 1 centipoise (cps)

A kinematic viscometer is one where the shear rate can neither be controlled nor measured, e.g. capillary viscometers.

Kinematic viscosity is defined as the dynamic viscosity divided by the density of the material. This results in units with dimensions of area per unit of time. When the units are mm² / s, they are known as centistokes (cSt). In SI units, kinematic viscosity is measured in m² / s

The Krebs unit is a measure of viscosity unique to the Stormer viscometer as defined in ASTM D 562. The instrument consists of a paddle-type rotor which, in its original form, is driven (via a cord and pulley arrangement) by a falling weight. The Standard defines the viscosity of a paint in terms of the weight required to produce a rotation of 200 revolutions per minute. The Standard also includes charts which enable the weight and rotational speed data to be converted to Krebs units.

The method is intended for measurements on DIY and trade paints applied by brush or roller and consequently is limited to viscosities from ~ 2 to 50 poise. It is also a convenient method for measuring the viscosity of a sample under low shear conditions.