Falling ball measurement, viscosity

Plazek et al.t measured the viscosities of a poly(dimethyl siloxane) sample of = 4.1 X 10 over a range of temperatures using the falling-ball method. Stainless steel (P2 = 7.81 g cm" ) balls of two different diameters,... 

The pH is measured using a 4% aqueous solution. Viscosity is normally measured using Brookfield viscometer. Alternatively, a capillary-type viscometer or falling ball such as Hxppler may be employed. The type of viscometer used must always be noted. 

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.

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... 

Subsequent falling-ball experiments performed with suspended rods (Graham et al., 1987 Milliken et al., 1989) replacing the spheres revealed significant and systematic differences between quiescent values of the suspension viscosity and those derived from Couette and capillary viscometer flow measurements. This is attributed by Graham, Mondy, and co-workers to fundamental differences in the distributions of rod orientations characterizing the quiescent and sheared suspensions. 

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 ... 

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. 

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 ... 

The specific gravity of the oil in the drop ball viscometer is 0.8 and its viscosity equals /x = 10000 500 cP. A persons pulse is used as a timer with a rate of 60 beats per minute and an uncertainty of 2 beats per minute. The falling ball drops a distance of 20 cm but the uncertainty of the measurement is 1 cm. The experimenta is repeated five times and the number of beats each time is 21,20,23,22,22. 

The falling-ball and the rolling-ball viscometers, shown in Fig. 20, are also simple devices for viscosity measurement. However, they do not qualify as rheometers but as indexers. 

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