Viscosity Conversion

Dynamic Viscosity 

Viscosity describes a fluids resistance to flow. Dynamic viscosity (sometimes referred to as Absolute viscosity) is obtained by dividing the Shear stress by the rate of shear strain. The units of dynamic viscosity are: Force / area x time The Pascal unit (Pa) is used to describe pressure or stress = force per area This unit can be combined with time (sec) to define dynamic viscosity. 

Centipoise (cP) is commonly used to describe dynamic viscosity because water at a temperature of 20°C has a viscosity of 1.002 Centipoise. This value must be converted back to 1.002 x 10 -3 Pa s for use in calculations.

Kinematic Viscosity 

Sometimes viscosity is measured by timing the flow of a known volume of fluid from a viscosity measuring cup. The timings can be used along with a formula to estimate the kinematic viscosity value of the fluid in Centistokes (cSt). The motive force driving the fluid out of the cup is the head of fluid. This fluid head is also part of the equation that makes up the volume of the fluid. Rationalizing the equations the fluid head term is eliminated leaving the units of Kinematic viscosity as area / time.

The kinematic viscosity can also be determined by dividing the dynamic viscosity by the fluid density.