Viscometers, common types

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. 

This unit describes a method for measuring the viscosity (r ) of Newtonian fluids. For a Newtonian fluid, viscosity is a constant at a given temperature and pressure, as defined in unit hi. i common liquids under ordinary circumstances behave in this way. Examples include pure fluids and solutions. Liquids which have suspended matter of sufficient size and concentration may deviate from Newtonian behavior. Examples of liquids exhibiting non-Newtonian behavior (unit hi. i) include polymer suspensions, emulsions, and fruit juices. Glass capillary viscometers are useful for the measurement of fluids, with the appropriate choice of capillary dimensions, for Newtonian fluids of viscosity up to 10 Pascals (Newtons m/sec 2) or 100 Poise (dynes cm/sec 2). Traditionally, these viscometers have been used in the oil industry. However, they have been adapted for use in the food industry and are commonly used for molecular weight prediction of food polymers in very dilute solutions (Daubert and Foegeding, 1998). There are three common types of capillary viscometers including Ubelohde, Ostwald, and Cannon-Fenske. These viscometers are often referred to as U-tube viscometers because they resemble the letter U (see Fig. HI.3.1). 

The Ostwald U-tube instrument is the most common type of viscometer based upon this principle (Fig. 6.37a and Table 6.7). The viscometer is filled with liquid until the liquid level is such that the bottom of the meniscus in the right-hand limb coincides with the mark C. The fluid is drawn up the left-hand limb to a level about 5 mm above A and then released. The time t taken for the bottom of the meniscus to fall from A to B is recorded. The dynamic viscosity p is determined from Poiseuille s law (Volume 1, equation 3.30 and Section 9.4.3) which under the... 

A variety of laboratory instruments have been used to measure the viscosity of polymer melts and solutions. The most common types are the coaxial cylinder, cone-and-plate, and capillary viscometers. Figure 11 -28 shows a typical flow curve for a thermoplastic melt of a moderate molecular weight polymer, along with representative shear rate ranges for cone-and-plate and capillary rheometers. The last viscometer type, which bears a superficial resemblance to the orifice in an extruder or injection molder, is the most widely used and will be the only type considered in this nonspecialized text. 

Figure 4.15 Common types of glass viscometers, (a) Ostwald viscometer (b) Ubbelohde suspended-level viscometer (c) A modified Ubbelohde suspended-level viscometer (see text for description).

The Ubbelohde viscometer is the most common type of viscometer used for the determination of the intrinsic viscosity. It was originally introduced in 1937 [33] and is shown in Figure 17.6. 

Viscosity is a property sometimes difficult to assess, as figures can be measured on any one of several types of viscometer. One common type is the rotating disk viscometer, which must be used in a container big enough to eliminate wall effects. The main alternative is the cup-and-bob viscometer, where the viscous drag of the liquid between stationary and rotating concentric cylinders is indicated by a spring-loaded pointer moving over a dial. 

There is a great number of apparatuses (viscometers) for measuring viscosity of bitumen or bituminous binders. These viscometers could be classified as (a) rotational, (b) capillary, (c) cup or efflux and (d) sliding plate viscometers. The most common types of viscometers used for determining the viscosity of bitumen and bituminous binders are presented in Table 4.1. Table 4.1 also provides information on the type of viscometer used per bituminous binder. 

Dilute-solution viscosity (solution viscosity) n. (1) A catchall term that can mean any of the interrelated and quantitatively defined viscosity ratios of dilute polymer solutions or their absolute viscosities. (2) The kinematic viscosity of a solution as measured by timing the rate of efflux of a known volume of solution, by gravity flow, through a calibrated glass capillary that is immersed in a temperature-controlled bath. Two common types of viscometer are the Ostwald-Fenske and Ubbelohde. From the viscosities of the solution rj and the solvent 7o, and the solution concentration c, five frequently mentioned viscosities (viscosity ratios, actually) can be derived, as follows ... 

Viscosities for molecular weight determination are usually measured in glass capillary viscometers, in which the solution flows through a capillary under its own head. Two common types, the Ostwald and Ubbelohde, are sketched in Figure 5.6. (Since polymer solutions are non-Newtonian, intrinsic viscosity must be defined, strictly speaking, in terms of the zero-shear or lower-Newtonian viscosity (see Chapter 14). This is rarely a problem, because the low shear rates in the usual glassware viscometers give just that. Occasionally, however, extrapolation to zero-shear conditions is required.)... 

There are several types of devices which measure viscosity but we will only show the most common type here, the Ostwald viscometer (Figure 2.3). 

Rotating cone viscometers are among the most commonly used rheometry devices. These instruments essentially consist of a steel cone which rotates in a chamber filled with the fluid generating a Couette flow regime. Based on the same fundamental concept various types of single and double cone devices are developed. The schematic diagram of a double cone viscometer is shown in... 

Slurry Viscosity. Viscosities of magnesium hydroxide slurries are determined by the Brookfield Viscometer in which viscosity is measured using various combinations of spindles and spindle speeds, or other common methods of viscometry. Viscosity decreases with increasing rate of shear. Fluids, such as magnesium hydroxide slurry, that exhibit this type of rheological behavior are termed pseudoplastic. The viscosities obtained can be correlated with product or process parameters. Details of viscosity deterrnination for slurries are well covered in the Hterature (85,86). 

These two instmments form a relatively inexpensive package that allows the characterization of a large number of materials over a wide range of viscosities and shear rates. Brookfield has also developed a digital Stormer-type viscometer (ASTM D562), Model KU-1, which is an improvement over the old manual Stormer. This low shear (- 50 ) viscometer is commonly used to test house paints. 

In shear studies, the most commonly used type of device for the generation of well-defined flow fields is the rotational viscometer. The use of these devices for the rheological characterization of liquids is well established [137]. Compared with the capillary and jet devices (Sects. 5.1 and 5.2), rotational viscometers allow the investigation of the effects of continuous rather than intermittent shearing. 

A wide variety of viscometers suitable for liquids are currently available, often with computer control. Many quality control laboratories use simple, cheap, robust instrumentation, which performs quite adequately in a day to day context. However these instruments can have a very narrow range and do not always give well-defined shear rates. This makes them less suitable for research and development work and we will not consider them further here. Figure 3.3 shows schematically the two main instrument types in common use controlled stress, where the stress is applied electrically via a motor leaving us to measure the strain and... 

Fig. 6.37. Common capillary viscometers (a) simple Ostwald type (b) Cannon-Fenske type...

The most common dynamic method is oscillatory testing, in which the sample is subjected to a sinusoidal oscillatory strain, and the resulting oscillatory stress measured. The more sophisticated rotational viscometers have the additional capability of dynamically testing liquid-like materials using small angle oscillatory shear. A parallel disc viscometer can be set up for testing solid-like materials (e.g., butter), in oscillatory shear. Some UTM-type solids rheometers, in which the moving crosshead can be made to reciprocate sinusoidally, can be used to test solid-like materials in oscillatory deformation in compression, tension or shear. 

Viscometers include the capillary, coaxial cylindrical, cone-and-plate, falling-ball type, etc. The capillary and, to a lesser extent but of increasing importance, the coaxial cylindrical viscometers, in numerous modifications, are the two most commonly used in scientific laboratories. 

The calculation of power follows methods described previously, using the estimated weight average density, p, in the expression for Reynolds number and power. The viscosity for a Newtonian system is best measured by using a paddle-type rotating bob viscometer. The Metzner-Otto method is commonly used to determine the Re for a non-Newtonian system, where viscosity is a function of shear rate. This method consists of determining the mean shear rate from y = KN, where N is the stirring speed in rps, K is 10 for a propeller, and y is the mean shear rate is in s. Viscosity will not influence the calculation of power when Re > 200. 

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