Viscosity describes the condition of a liquid or gas. Here you take a model that consists of two plates one on top of the other. Between these plates there is a substance that adheres to both plates. In our imagination, the substance is divided into parallel layers. One plate is now pulled in one direction, with forces acting on the layers of the material while the other plate rests. The first adjacent layer on the moving plate is dragged along with it, while the layer on the stationary plate stands still.

Now what happens to the layers in between?

Each layer transmits the speed of movement to the next layer differently. The speed naturally decreases from the moving plate to the stationary plate. This is due to the fact that the particles of the substance stand in each other’s way, so to speak, and “have to pass” each other. Electrostatic forces of attraction also act between the smallest particles. How fast or slow the layers move depends on the material properties and determines the viscosity. The condition of substances is divided into low, medium and high viscosity. One can now imagine that a substance with a high viscosity is less flowable than a substance with a low viscosity.


In practice, the unit of measurement pascal second (Pas) has become established. There is also the unit millipascal second (mPas) for substances with low viscosity. mPas is now more common.

The unit centistokes (cST) is also used (see below under flow cups).


The viscosity of a substance is also influenced by other factors. For example, and most influential, temperature. The viscosity decreases with increasing temperature. When the temperature drops, it rises again. It is therefore important to include the temperature when determining the viscosity.

Of course, the viscosity also depends on other factors, such as the specific material properties such as the shape and size of the molecules, etc.

Examples for illustration

To get a better idea of ​​the abstract, here are a few examples from everyday life. Of course, these are only approximate values, because the viscosity of some substances can vary from type to type, for example honey:

Substance Temperature Viscosity approx. in mPas
Water 20°C 1.009
Olive oil 20°C 100
Mayonnaise 20°C 2,000
Honey 20°C 10,000

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Determination of viscosity

To determine the viscosity of liquids, viscosity meters are used:

Flow cup

The flow cup is the fastest way to determine viscosity for low-viscosity liquids. It is a hollow cylinder usually made of anodized aluminum with a stainless steel outlet nozzle. The time it takes for the liquid to leave the flow cup is measured.

For the process, the test liquid is poured into the flow cup with the nozzle closed. Then we wipe off the excess liquid in the rim of the cup with a glass plate. The nozzle can then be opened and the time measurement begins. The time measurement starts when the liquid begins to emerge from the lower opening of the cup. In the case of immersion cups, the cup is immersed in the liquid and quickly pulled out again. The time measurement starts when it is pulled out. The time measurement ends when the liquid no longer flows evenly out of the opening or when it breaks. Units are seconds. This result can be converted into centistokes (cST) using a viscosity conversion disk that matches the cup.

There are various DIN and ISO cups for compliance with standards. The number (e.g. DIN4 cup) indicates the diameter of the nozzle opening. There are flow cups on the market according to DIN 53211 and more precise flow cups according to the newer DIN ISO 2431.

Rotational viscometer

However, if liquids are highly viscous, you will not get satisfactory results with a flow cup due to the slow flow behavior. More complex processes must therefore be used for highly viscous media. In the case of the rotary viscometer, the measuring liquid is placed in a gap between an inner cylinder and an outer cylinder surrounding it. Then the inner cylinder is driven by a motor while the outer one stands still. The measuring liquid decelerates the movement of the rotating cylinder, which can be recorded as a measuring signal with different means.

PTM Wiki

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