Kinematic Viscosity measurements are used in quality control and characterization tests in a wide range of fields.
We’ve explored some of the most commonly asked questions about viscosity testing; what is kinematic viscosity? How is it measured? How does temperature affect viscosity? And how can you make sure your viscosity measurements are accurate?
What is viscosity?
The definition of viscosity is; a measure of a liquid’s resistance to movement.
Put simply, this means how thick or thin the liquid is – how easily it flows.
What is Kinematic Viscosity?
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.
How is Kinematic Viscosity measured?
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.
What units is Kinematic Viscosity reported in?
The most commonly used unit is centiStokes (cSt) but some methods may report in mm2/s
What are some examples viscosity values?
Below you can see the viscosity of some common liquids – ranging from low viscosities (i.e. very thin liquid, flows and pours fast) to high viscosity (i.e. very thick liquid, slow to move when it is poured)
How does temperature affect viscosity?
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.
Viscosity Baths are available in a range of specifications – models that heat, high temperature models for extended heating, or refrigerated models to run tests below room temperature.
Some also have features to automatically calculate the viscosity, or even detect the results by optical sensors. These all help to ensure more accurate results.
How is Kinematic Viscosity measured?
After equilibrating to the test temperature, the liquid is flowed through a capillary viscometer and the time is recorded.
The unique constant of the capillary viscometer is also taken into account, with the final result calculated by the equation;
V = C x t
Where V = Kinematic Viscosity, C = Calibration constant of the viscometer, t = time.
The time recorded is that taken for the liquid to flow through the capillary section of the viscometer, from one marker line to the other;
The temperature and viscometer type can be changed to suit the sample being analysed, or to meet standard test protocols such as ASTM or IP specifications.
The sample flowing through the capillary viscometer can be detected either manually or using an automatic system, sometimes known as optical detection system.
How can you test a Kinematic Viscosity system?
To ensure that equipment and operators are working accurately, viscosity standards should be measured regularly. These are liquids with a set and known viscosity, to evaluate if the equipment and operator are accurate.