Rotating cylinder viscometers

Rotating cylinder viscometers consist of two concentric cylinders, corresponding to the bob and the crucible, one cylinder of which is rotated at a constant speed. The viscosity is determined from measurement of the torque generated. A schematic drawing is shown in Fig. 25. This method is most popular for measuring viscosity at high temperatures and is suitable for melts with high viscosity. The viscosity is calculated from... 

Figure 25. Schematic drawing of rotating cylinder viscometer. (Reprinted from Ref 124. Crown Copyright 1992. Reproduced by permission of the Controller of Her Majesty s Stationary Office.)...

FIGURE 10.3 Schematic of rotating-cylinder viscometer. Torque C measured on inner cylinder (radius R,). Outer cylinder rotates with angular velocity 12. 

A commonly employed instrument for measuring the viscosity of a liquid is the Oswald viscosimeter or viscometer, illustrated in Figure 11.16c. One measures the time that it takes, for a quantity of liquid to pass through the tube, from one position to another, under the force of its own weight. Usually the instrument is calibrated by the use of a liquid of known viscosity.f Another apparatus used for viscosity measurements is the Couette rotating-cylinder viscometer (see Figure 11.16d). 

The formula was established for slop glazes by A. Brongniart (Traiti des Arts Ciramiques, Vol.l, p.249,1854). Brookfield Viscometer. An electrically operated rotating cylinder viscometer in which the drag is recorded directly on a dial it has been used in the testing of vitreous-enamel slips (/. Amer. Ceram. Soc., 31,18,1948). 

Stormer Viscometer. A rotating cylinder viscometer of a type that has found considerable use in USA for the determination of the viscosity and thixotropy of clay slips. (Industr. Engng. Chem., 1,317,1909.)... 

Once the values of T Jind co are known, q can be calculated by equation (23). The constants of the viscometer appearing in the equation can be detennined by accurate measurements. Alternatively, two liquids whose viscosities are known can be used for calibration. The type of flow obtained in this type of coaxial rotating cylinder viscometer is simpler than that in a crapUlaiy viscximeter. The only difficulty lies in controlling the temperature of the system. 

The rotating cylinder viscometer is the most popular type of viscometer, because it gives accurate and precise data. This viscometer can measure non-Newtonian as well as Newtonian flow characteristics. 

The rotating cylinder viscometer consists of two concentric cylinders, outer and inner, as shown in Figure 12-9. The inner cylinder is himg by a torsion wire or spring. The outer cylinder can be rotated at a desired angular velocity. In the measmements, the liquid sample is placed between the onter cylinder and inner cylinder. When the onter cylinder is rotated. 

The viscosity can be obtained from formula (12-12) by measuring Tas a function of Q. The measurement by the rotating cylinder viscometer provides the flow curve, cr s.y. The shear stress, a, acting on the cylindrical layer located at a distance r from the central axis is expressed by the formula ... 

Dynamic rheological measurements will be briefly described. In dynamic measurements an oscillatory microscopic strain is given and the corresponding stress is measured. This method gives information on viscoelastic behavior even in the region where no viscous flow takes place. The measurements can be carried out by using the rotating cylinder viscometer or the cone and plate viscometer. 

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