Viscometers torsional

Coaxial (Concentric Cylinder) Viscometer, The eadiest and most common type of rotational viscometer is the coaxial or concentric cylinder instmment. It consists of two cylinders, one within the other (cup and bob), keeping the specimen between them, as shown in Figure 27. The first practical rotational viscometer consisted of a rotating cup with an inner cylinder supported by a torsion wire. In variations of this design the inner cylinder rotates. Instmments of both types ate useful for a variety of apphcations. 

The MacMichael viscometer is probably the most straightforward rotatioaal viscometer. The outer cup rotates and the inner cylinder is suspended from a torsion wire. The drag on the inner cylinder is measured as degree of twist on the wire. Wires of different stiffness are available, and the maximum viscosity is ca 10 mPa-s. The shear rate range is limited, ca 2-12, but with modification, higher shear rates can be attained. The iastmment is best... 

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

Direct Indicating Viscometer. This is a rotational type instrument powered by an electric motor or by a hand crank. Mud is contained in the annular space between two cylinders. The outer cylinder or rotor sleeve is driven at a constant rotational velocity its rotation in the mud produces a torque on the inner cylinder or bob. A torsion spring restrains the movement. A dial attached to the bob indicates its displacement. Instrument constants have been so adjusted that plastic viscosity, apparent viscosity, and yield point are obtained by using readings from rotor sleeve speeds of 300 and 600 rpm. 

As the name implies, the cup-and-bob viscometer consists of two concentric cylinders, the outer cup and the inner bob, with the test fluid in the annular gap (see Fig. 3-2). One cylinder (preferably the cup) is rotated at a fixed angular velocity ( 2). The force is transmitted to the sample, causing it to deform, and is then transferred by the fluid to the other cylinder (i.e., the bob). This force results in a torque (I) that can be measured by a torsion spring, for example. Thus, the known quantities are the radii of the inner bob (R ) and the outer cup (Ra), the length of surface in contact with the sample (L), and the measured angular velocity ( 2) and torque (I). From these quantities, we must determine the corresponding shear stress and shear rate to find the fluid viscosity. The shear stress is determined by a balance of moments on a cylindrical surface within the sample (at a distance r from the center), and the torsion spring ... 

Typical of this class of viscometer is the coaxial or Couette type of instrument described in Volume l, Section 3.7.4. The sample fluid is contained within the annular space between two coaxial cylinders, either of which may be rotated by a motor with the remaining cylinder suspended elastically in such a way that the torsional couple exerted on the latter can be measured. If the outer cylinder of radius r2 rotates with an angular velocity cou and the inner cylinder of radius r, is stationary, and the torque (or viscous drag) per unit length of cylinder exerted on the inner cylinder is T, then, for a Newtonian fluid(49) ... 

In a rotational viscometer the solution is filled in the annulus between two concentric cylinders of which either the external (Couette-Hatschek type) or the internal (Searle type) cylinder rotates and the other, which is connected to a torsion-measuring device, is kept in position. Let R, and Ro be the radii of the inner and outer cylinders, h the height of the cylinder which is immersed in the solution or its equivalent height if end effects are present, a> the angular velocity of the rotating cylinder, and T the torque (or moment of force) required to keep the velocity constant against the viscous resistance of the solution. It can be shown that the shearing stress (see, for example, Reiner, 1960) ... 

In a torsion test, a capstan-shaped specimen is twisted in a viscometer, and the generated stress and strain are measured upto the point of material fracture. Torsion produces what is called a pure stress, a condition that maintains sample shape and volume during the test. The material can fail in shear, tension, compression or in a combination mode, and the test does not dictate the mode of failure (Hamann, 1983). The main disadvantages of torsion are (1) specimen shaping and preparation are usually complex and tedious, and (2) the technique is not applicable to soft or sticky... 

Dense-phase flow properties can be predicted by measuring the viscosity and deaeration rate of a fluidized bed in a laboratory test (117). This method appears to be especially useful for evaluating the flow characteristics in standpipes. The viscosity is measured by means of a Brookfield viscometer, which consists of a cylindrical wire-screen spindle rotated about its axis by an electric motor through a torsion spring. The torque required to rotate the spindle is measured by displacement of the... 

Parts of these problems are circumvented by using the classical knife-edge surface viscometer developed by Brown et al, 1958 ) and variations of it as the double knife-edge surface viscometer and the blunt knife-edge viscometer. For each of these it is crucial that the knife just touches the monolayer, but does not break it. Therefore the knife-edge has to be non-wetting. However, placing the knife precisely in the surface remains a cumbersome manipulation. Theoretical expressions for the torque upon the torsion wire have been derived for the various types. 

In addition to the torsion plastograph mentioned above, a number of other methods are used for appraising the properties of plastic bodies in practice. Plastic strength (rigidity) is determined by means of the Rebinder penetrometer which measures the depth of penetration of a cone forced into the body under constant load. The respective quantity is related to yield point. The commercial Brabender plastograph measures the resistance of mixture to kneading. The extrusion viscometer, from which the material is extruded under pressure, is analogous to flow-out viscometers. 

If the entire temperature dependence of viscosity is to be measured, it is necessary to use several methods based on different principles. In the viscosity range 10 —10 dPa s, use is mostly made of rotary viscometers. A platinum cylinder rotates around its axis in the glass melt in a crucible, and the force required for revolving the cylinder at a certain speed is measured. In another arrangement, the external crucible is rotated while the internal one is suspended on a torsion wire. Within the same viscosity region, it is possible to measure with a counterbalanced sphere viscometer a plat inum sphere suspended on a thin wire from the balance arm is immersed in the glass melt in a crucible. The other balance arm is loaded and the speed at which the sphere is withdrawn from the melt is measured. 

Since 1970, two generic types of viscometer have received the greatest attention the first makes use of the torsional oscillations of bodies of revolution and the second is based on the rather simpler concept of laminar flow through capillaries. Both reduce the measurement of viscosity to measurements of mass, length and time. 

In the case of high pressures, different types of viscometer have been employed owing to the need to reduce the volume of fluid required. The most popular have been falling-body viscometers and torsional-crystal viscometers. Neither of these have, however, completely developed theories so that their accuracy is intrinsically limited. On the other hand, the newly developed vibrating-wire viscometer that makes use of the damping of a transverse oscillation of a thin wire enjoys a complete theory. 

Surface viscosity can be measured with a canal viscometer or with a torsion viscometer. The former instrument measures the flow of the film through a slit in a beurier on a film balance the latter, the damping caused by the presence of a film on a body moving in the surface of the liquid. Interpretation of measurements on films with a constant viscosity is difficult and would be more so in the presence of viscosity changes caused by reaction. 

In addition to those channel viscometers discussed here there is a separate type known as torsional viscometers, where surface viscosity is measured in terms of the traction on a wire, traversed lengthwise on the surface of a liquid. At this writing, none of these is known to have been subjected to a complete analysis. Joly (6) says, furthermore, calculations based on the correct equation would depend upon the exact shape of the apparatus. As these calculations have so far not been carried out in any case, the values obtained by the rotation method are incorrect and certainly too high. Nevertheless, such approaches may offer special advantages and should be investigated fully. They must be considered as separate topics, however from the channel viscometers discussed here. 

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. 

Tensions at the n-hexadecane—water interface were measured at short times 30 min) by drop-volume and pendant-drop techniques 2), and at longer times using a Wilhelmy-plate torsion balance (2) Under conditions for which protein concentration, aqueous phase volume and surface area were similar to those existing in the surface viscometer, all the pure proteins gave a steady-state tension within 5—10 h. 

Measurement of Flow Properties For the precise scientific measurement of viscosity and thixotropy in absolute units, see Rheology of Ceramic Systems . Here we shall restrict ourselves to a consideration of the torsion viscometer. 

Dilatancy cannot be measured with the usual type of torsion viscometer found on a works but the more precise rotating-cylinder instruments can measure it. When very pronounced, it can be detected qualitatively by feel . 

In Germany, standards have been published describing the Compression Shear Test (DIN 54452), Dynamic Viscosity Determination of Anaerobic Adhesives by Rotational Viscometer (DIN 54453), Initial Breakaway Test at Bonded Threads (DIN 54454), and Torsion Shear Test (DIN 54455). DIN 54455 is particularly interesting since it is one of a very few tests in which a nut and bolt (MIO) are seated to a measured torque before the anaerobic sealant is allowed to cure. 

A number of other methods are occasionally used for viscosity measurements. The most common are the parallel plate viscometer, used in the 10 -10 Pa s range, the penetration viscometer, used in the 10 -10 Pa s range, and the torsion viscometer, used in the lO -lO " Pas range. Although each of these methods has advantages under specific conditions, none have gained wide acceptance in the glass community. 

Rotation Viscometers. In rotation viscometers, a rotor moves against a stator (see Figure 9-23). The Epprecht viscometer is particularly suitable for measurements on highly viscous solutions. Here, the angle of rotation of a torsion wire on which the stator is suspended is a measure of the torque produced by the rotating rotor on the liquid. Since all the other quantities (cylinder radius, width of the slit, gap between rotor and stator, number of rotations in unit time) are kept constant, the viscosity is easily calculated. 

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