Viscosity Measurement - Viscosimeters and rheometers

The choice of measurement method depends first and foremost on the quantities the user wishes to determine. For example, you may wish to :

• to obtain a point measurement of shear viscosity ;

• shear viscosity over a wide range of strain rates;

• to obtain a measurement of elongational viscosity ;

• characterize thixotropy ;

• characterize viscoelasticity...

All these quantities and properties are defined in the article "Viscosity measurement. General principles" [R 2 350] .

The user must then define the nature of the measurement: absolute or relative. At present, precise standards for absolute measurements exist only for shear viscosity. This is also the only field in which reference liquids are available for relative measurements using standardized standards.

The other selection parameters are as follows:

• the need for standardized measurement: the measuring device and its mode of operation are very often imposed; the recommended method is sometimes empirical, and can lead to considerable errors of assessment with non-Newtonian fluids;

• investment cost: as a general rule, investment in simple shear viscosity measurement alone is significantly lower than for other properties (elasticity or elongational viscosity);

• operating costs: here too, measuring shear viscosity requires less time and less qualified personnel than measuring other properties;

• the range of velocity gradients (or shear stresses) in which the measurement is to be made: in general, measurements at very low stresses can only be made with rotating equipment;

• accuracy required: this is a parameter that is not easy to capture objectively. As we shall see in the following paragraphs, measurements often have to be corrected in various ways. The value of these corrections and errors due to the fluid may be greater than that intrinsic to the instrument. As far as the latter is concerned, capillary-type instruments have the best uncertainties.

The first part of this article is devoted to the basic principles and fundamental equations of the main measurement techniques. It is important to distinguish between...