Fiber Elongation Viscometers

Mergules viscometer Fiber elongation Beam bending... 

Figure 9.14 a) Schematic cross section of viscometer, b) Fiber elongation method. 

The determination of viscosity from the couple exerted on concentric cylinders is used in many viscometers. For silicate glasses, a wide range of techniques are used for measuring viscosity. For low viscosities (1 Pa s to 1 MPa s), a rotating spindle technique, based on the above principle, is often used. For higher viscosities, techniques based on fiber elongation, compression of a cylinder and be im-bending are utilized. 

FIGURE 21.5 Schematic illustration of instruments used to measure viscosity (a) a viscometer (b) the fiber elongation method. 

The most widely used viscometers are based on measurements of the rate of elongation of a fiber of known dimensions under a known load. This method can be used for viscosities ranging from 10 to 10 Pa s. This method is also used for the determination of the Littleton softening and annealing reference points. Since the method requires formation of a long fiber for a specimen, it is well suited for many easily-worked commercial glasses, but difficulties in the formation of good fibers from many experimental compositions often limit the use of this method for basic research studies. 

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