Relative-capillary viscosimeter

The flow cup is a relative-capillary viscosimeter. The outlet nozzle is a very short capillary, and the propulsive force is the hydrostatic pressure of the liquid, which during the tests flows under its own weight from the central outlet nozzle on the bottom of the cup. The flow cup has a defined geometry with a given volume for the sample liquid. For the measurement, the nozzle is closed, the cup is filled with the sample liquid and then the time is measured that the Uquid takes to completely drain out of the cup. From that efflux time, the kinematic viscosity v is calculated. Up to this day many different flow cups are used around the world (see Fig. 3.3). 

Even under normal ambient pressure relatively high shear rates occur at the wall of capillary viscosimeters (especially for small capillary diameters and low viscosity liquids), that can lead to a falsification of the results for so-called non-Newtonian liquids (see later in this monograph). In Tables 3.1 and 3.2 the maximum occurring shear rates for the admissible minimal running times tniin of the capillary are given for the standard Ubbelohde capillary viscosimeter (ISO 3105) and the Micro Ubbelohde capillary viscosimeter (DIN 51562). 

The relative viscosities q of a dilution series (and therefore the relation of the running times of the solution and the pure solvent in a capillary viscosimeter according to Eq. (3.38)) should lie between 1.2 and 2.5, to assure an exact analysis. These limits of the relative viscosity are shown in Fig. 4.3. The data points below the critical value of the relative viscosity of 1.2 show deviations from the linear fit and should not be included in the extrapolation to the/-axis for the determination of the intrinsic viscosity. 

Viscosity measurements are often carried out with capillary viscosimeters because they are relatively cheap and give fast but exact results. Since capillary viscosimeters operate at higher shear rates (see Tables 3.1 and 3.2) it must be checked... 

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