Falling ball viscometers

Falling ball viscometers are based on Stokes law, which relates the viscosity of a Newtonian fluid to the velocity of the falling sphere. If a sphere is allowed to fall freely through a fluid, it accelerates until the viscous force is exactly the same as the gravitational force. The Stokes equation relating viscosity to the fall of a soHd body through a Hquid may be written as equation 34, where ris the radius of the sphere and d are the density of the sphere and the hquid, respectively g is the gravitational force and p is the velocity of the sphere. 

Fig. 31. Falling ball viscometer. Lines /q and t are the timing lines for velocity deterrninations.

Weber, W. 1956. Systematic investigation of falling ball viscometers with inclined tubes. Kolloid Z. 147, 14-28. 

Kv Capillary and falling ball viscometer constant m2/s2 l2t2... 

The viscosity of emulsions obtained from two mutually incompatible polymers dissolved in a common solvent was studied by a falling ball viscometer, a cone-plate viscometer, and a capillary viscometer. The two polymers are polyacrylonitrile and polyurethane, and the solvent is N-methyl-pyrrolidone. The measurements are compared with theory, and a model is proposed for the development of a stationary pressure flow of an emulsion in a capillary. 

Figures 4 and 5 represent the viscosities of the emulsions measured with a falling-ball viscometer. When one ball is used (a steel ball 0.8 mm in diameter), only one value of the viscosity is measured for each emulsion and not a flow curve. The emulsions in Figure 4 are rich in PU solution, and those in Figure 5 are rich in PAN solution.

Figure 4. Measurements with a falling-ball viscometer. Ratio between viscosities of emulsions and matrix vs. volume fraction of dispersed PAN solutions.

Falling-Ball Viscometer. Taylor (5) has developed a formula for the viscosity of emulsions ... 

Other methods Falling ball viscometers Newtonian 6.2.3... 

Another method to determine viscosity uses the falling-ball viscometer. Determine viscosity by noting the time it takes for a ball to fall through the distance between two marks on a tube filled with the unknown liquid (the tube is generally in a constant-temperature bath). Use balls of different weights to measure a wide range of viscosities. Calculate the viscosity by using manufacturer-supplied constants for the ball used. These instruments can be quite precise for Newtonian liquids, that is, liquids that do not have viscosities that vary with flow (more correctly, shear) rate. 

The viscosity of Newtonian liquids can be measured simply, by one-point determinations with viscometers, such as rotational, capillary, or falling ball viscometers. For non-Newtonian materials, measurement of... 

In the falling ball viscometer, the time for a standard sphere to fall a standard distance through a liquid at its limiting velocity is measured. The diameter of the ball is... 

The falling ball viscometer consists of a wider tube containing the test sample. The viscosity of Newtonian-behaving materials is obtained by measuring the time for a ball of known density and size to fall through the sample. The resistance of the liquid against the fall of the ball (w) is ... 

Texture analyzers are also used to assess deformability of a fluid, using penetration force vs. depth profiles, etc. These instruments in addition to Brookfield and Haake viscometers are common QC metrics. Other methods include viscosity flow cups and bubble or falling ball viscometers, and several relevant standard test methods include ASTM D1200, DIN/ISO 2431, ASTM D5125, BS3900 Part A6, ASTM D1545, and ASTM D1725. 

In the case of the falling-ball viscometer, details may be found in Ref. 8. Reference 9 provides detailed coverage of the falling-needle viscometer. 

Y. I. Cho, The Study of non-Newtonian Flows in the Falling Ball Viscometer, Ph.D. thesis, University of Illinois at Chicago, 1979. 

In this instrument a liquid is caused to rotate in an outer cylinder, and it causes a torque to be applied to the torsion wire attached to the inner cylinder. The. viscosity is calculated from the torque, the apparatus being calibrated. Another device for measuring viscosity is the falling-ball viscometer (Figure 11.16e). The viscosity is calculated from the time reciuired for the ball to fall from one position to another. 

Falling ball viscometers and capillary viscometers are not generally used for testing paints. Flow cups are, however, special capillary viscometers with a short capillary in which the force of gravity acts on the paint. 

Example 2.3. The viscosity of transparent liquids can be measured with a falling ball viscometer. A glass tube is filled with the liquid of interest. A ball is allowed to drop through the liquid in the tube and the time it takes the ball to travel between two lines separated by an precisely defined distance is measured. The results (in s) were 12.4, 11.6, 13.9, 11.8, 12.4, 10.0, 11.6, 12.8, 11.5, and 11.9. 

The next example combines the concepts of uncertainty propagation as well as confidence intervals. The problem is related to measuring the viscosity of a transparent fluid in a falling ball viscometer, in which the time it takes a ball to cross two lines separated by a distance is measured. The ball falls through a tube filled with the fluid and is assumed to reach a steady velocity by the time it reaches the first Une. The velocity is determined by the geometry and density of the ball as well as the density and, most importantly, the viscosity of the liquid ... 

Example 2.5. The viscosity of a new polymer was measured at 85 °C with a falling ball viscometer. Nine measurements were made and the time it took the ball to travel 300 mm was measured to be ... 

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