EVO EXPERT Manual
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In these fluids, the viscosity increases with time when the fluid is subjected to a constant
speed gradient.
These substances tend to return to their previous viscosity once the speed gradient ceases
to be applied.
These fluids are not very common.
NOTE:
The turbulent behaviour of a fluid can produce falsely high results in
viscosity tests. Normally, turbulent behaviour is due to an excessively high
rotation speed in relation to the viscosity of the sample (see detailed Warning
further on).
FACTORS AFFECTING VISCOSITY
There are many variables that affect the rheological properties of products, so it is very important
to take the following factors into account.
Temperature
Temperature is one of the most obvious factors affecting rheological behaviour.
It is essential to consider the effects of temperature on viscosity in the evaluation of materials that
are subject to changes in temperature during its use or other processes. Some examples of this are
motor oils, greases and adhesives.
Shear Rate
When a fluid is subjected to variations in the speed gradient during its process or use, it is essential
to know its viscosity at the projected speed gradients.
Examples of materials, which are subjected to and affected by important variations in speed
gradient during its process or use, are: paints, cosmetics, liquid latex, some food products such as
ketchup and blood in the human circulatory system.
Measurement conditions
The measurement conditions of a material during its viscosity reading can have a considerable
effect on the results of this measurement. Consequently, it is important to be careful and control
the environment and conditions of any sample subjected to analysis.
Variables such as the type of viscometer, the speed/spindle combination, the sample’s container,
the absence or presence of a spindle protector, the temperature of the sample and the sample
preparation techniques, etc, can affect not only the precision of the reading but also the real
viscosity of the sample.
Time
Ageing under the same speed gradient conditions affects tixotropical and reopectical fluids.
In some fluids the action of time combined with the proportion of the shear is very complex. In
these cases, one can observe, with time, a return to the original fluid state.
Previous conditions
The conditions that the sample is subjected to before the viscosity reading can significantly affect
the results, especially with heat-sensitive fluids or ageing.
Thus, the storage condition and the sample preparation techniques should be conceived to minimize
effects on the viscosity measurements.
Composition and additives
A material’s composition is a determining factor in its viscosity. When the composition is altered,
whether this is by changing substance proportions that compose it or adding other substances,
important changes can be observed in their viscosity.
For example, adding solvent to printing ink reduces the viscosity of the ink and other types of
additives are used to control the rheological properties of paints.
