What is Kinematic Viscosity and Kinematic Viscometer?

What is Kinematic Viscosity?

What is kinematic viscosity? How is it measured? How does temperature affect viscosity? And how can you make sure your viscosity measurements are accurate?

The definition of viscosity is a measure of a liquid's resistance to movement, it means how thick or thin the liquid is, how easily it flows.

Kinematic Viscosity specifically is a measure of resistance to the liquid moving. This is different to Dynamic viscosity, which measures the resistance of another object moving through the liquid.

The time taken for the liquid to flow is measured – this could be the time it takes to travel through a capillary, or through a cup with a hole in the base.

The most commonly used unit is centiStokes (cSt) but some methods may report in mm2/s

The impact of temperature on viscosity is huge – for every °C change, the viscosity can change by 2 to 10% (depending on the liquid).

You can see this effect in action by warming honey – at room temperature this is thick and slow to pour (high viscosity) but if you heat it in a pan you can see it becomes thin and fast to pour (low viscosity).

This means that accurately controlling the temperature of the liquid during testing is vital to ensure consistent results which can be compared across different batches or products.

A viscosity bath is used to maintain samples at a steady and accurate temperature for Kinematic Viscosity testing to ensure that temperature is constant, and not a variable.

Kinematic Viscosity:

Kinematic viscosity is the ratio of - absolute (or dynamic) viscosity to density - a quantity in which no force is involved. Kinematic viscosity can be obtained by dividing the absolute viscosity of a fluid with the fluid mass density like

• ν = μ / ρ                                     

  where

• ν = kinematic viscosity (m2/s)

• μ = absolute or dynamic viscosity (N s/m2)

• ρ = density (kg/m3)

In the SI-system the theoretical unit of kinematic viscosity is m2/s - or the commonly used Stoke (St) where

• 1 St (Stokes) = 10-4 m2/s = 1 cm2/s

Stoke comes from the CGS (Centimetre Gram Second) unit system.

Since the Stoke is a large unit it is often divided by 100 into the smaller unit centiStoke (cSt) - where

• 1 St = 100 cSt

• 1 cSt (centiStoke) = 10-6 m2/s = 1 mm2/s

• 1 m2/s = 106 centiStokes

Kinematic Viscosity - Online Converter

The specific gravity for water at 20.2oC (68.4oF) is almost one, and the kinematic viscosity for water at 20.2oC (68.4oF) is for practical purpose 1.0 mm2/s (cStokes). A more exact kinematic viscosity for water at 20.2oC (68.4oF) is 1.0038 mm2/s (cSt).

Kinematic viscosity of common liquids and fluids 

A conversion from absolute to kinematic viscosity in Imperial units can be expressed as

• ν = 6.7197 10-4 μ / γ                               

  where

• ν = kinematic viscosity (ft2/s)

• μ = absolute or dynamic viscosity (cP)

• γ = specific weight (lb/ft3)