Viscometers, types

Viscometer type Lowest viscosity Highest viscosity Shear rate range (s 1)... 

The technique involving an evacuated capillary viscometer is limited to the measurement of polymer solutions having viscosities less than ca. 103 poise a point which has been made previously by Hadjichristidis and Roovers 132). This is a consequence of the viscometer type (Ubbelohde) which has been used in these determinations. Despite this practical limitation, it has been stated 78) that there is no inherent limit for viscosity measurements, even in conventional types of viscometers provided that the tubes are sufficiently wide. However, the limitations of operating with an evacuated viscometer, and the flow behavior of high viscosity (> 103 poise) polymer solutions, clearly reveals that the foregoing claim is unrelated to reality. 

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. 

Obviously a viscosity index recommended for characterization of bio-oil stability [2], using a viscometer type available in a participating laboratory , cannot be correctly applied due to tendency of bio-oils, non-Newtonian liquids, to phase separation into thin oil, thick tar and solid admixtures [1]. However, the ESR method allows to reveal considerable difference between the properties of the samples prior and after ageing It was shown (Table 3) that the concentrations of paramagnetic centers m samples taken after ageing from the top and the bottom of oil storage vessel differed significantly, e.g. m the cases of oils from IWC and Aston installations, by 10 and 800 times, respectively. 

A number of Fann or Baroid rheometers have been used for the rig site measurement of fluid rheology (62, 63). The design of the rheometers is similar and the various viscometer types differ largely in the control of shear rate. Early models of the concentric cylinder rheometer were limited to two shear rate measurements made at rotation speeds of r = 300 rpm and r = 600 rpm. Later models have extended the number of rotational speeds (shear rates) at which the torque can be measured, enabling a more complete rheogram to be constructed. 

This device is really the sliding-plate device wrapped around a cylinder. Mathematical corrections are needed, to make the readings of this viscometer correspond exactly to those of the sliding-plate visc ometer [2], but these are generally small see Prob, 15.11. This type of viscomker is suited to newtonian or nonnewtonian fluids with or without time dependence. Several other comparable viscometer types are known [2]. ... 

There are various types of efflux or cup viscometers, two of which are shown in Table 4.1. The standard tar viscometer type (STV), also called efflux viscometer, is recommended to be used for bituminous emulsions (CEN EN 12846-1 2011) or for cut-back and fluxed bituminous binders (CEN EN 12846-2 2011). An efflux viscometer (STV) is shown in Figure 4.6. 

Viscosity was measured after storage at 4°C. Samples were allowed to warm to room temperature before viscosity was measured with a Brookfield viscometer, type T, spindle C, 100 rpm. 

In Chapter 4 the development of axisymmetric models in which the radial and axial components of flow field variables remain constant in the circumferential direction is discussed. In situations where deviation from such a perfect symmetry is small it may still be possible to decouple components of the equation of motion and analyse the flow regime as a combination of one- and two-dimensional systems. To provide an illustrative example for this type of approximation, in this section we consider the modelling of the flow field inside a cone-and-plate viscometer. 

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... 

The concentric cylinder viscometer described in Sec. 2.3, as well as numerous other possible instruments, can also be used to measure solution viscosity. The apparatus shown in Fig. 9.6 and its variations are the most widely used for this purpose, however. One limitation of this method is the fact that the velocity gradient is not constant, but varies with r in this type of instrument, as noted in connection with Eq. (9.26). Since we are not considering shear-dependent viscosity in this chapter, we shall ignore this limitation. 

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). 

Solution Polymers. Methacryhc solution polymers are usually characterized by thek composition, soHds content, viscosity, molecular weight, glass-transition temperature, and solvent type. The compositions of methacryhc polymers are most readily determined by physicochemical methods such as spectroscopy, pyrolytic gas—Hquid chromatography, and refractive index measurements. The soHds content is determined by dilution followed by solvent evaporation to constant weight. Solution viscosities are most conveniendy determined with a Brookfield viscometer. Methods for estimating molecular weights by intrinsic viscosity are available (103). 

Viscometers may be separated into three main types capillary, rotational, and moving body. There are other kinds, usually designed for special apphcations. For any given type there usually is a choice of several different instmments. The choice depends on the particular requirements of the investigator and the price range. 

The Cannon-Fenske viscometer (Fig. 24b) is excellent for general use. A long capillary and small upper reservoir result in a small kinetic energy correction the large diameter of the lower reservoir minimises head errors. Because the upper and lower bulbs He on the same vertical axis, variations in the head are minimal even if the viscometer is used in positions that are not perfecdy vertical. A reverse-flow Cannon-Fen ske viscometer is used for opaque hquids. In this type of viscometer the Hquid flows upward past the timing marks, rather than downward as in the normal direct-flow instmment. Thus the position of the meniscus is not obscured by the film of Hquid on the glass wall. 

AH glass capillary viscometers should be caUbrated carefully (21). The standard method is to determine the efflux time of distilled water at 20°C. Unfortunately, because of its low viscosity, water can be used only to standardize small capillary instmments. However, a caUbrated viscometer can be used to determine the viscosity of a higher viscosity Hquid, such as a mineral oil. This oil can then be used to caUbrate a viscometer with a larger capillary. Another method is to caUbrate directly with two or more certified standard oils differing in viscosity by a factor of approximately five. Such oils are useful for cahbrating virtually all types of viscometers. Because viscosity is temperature-dependent, particularly in the case of standard oils, temperature control must be extremely good for accurate caUbration. 

Linear equations of the type u = ct — C, where c and C are constants, relate kinematic viscosity to efflux time over limited time ranges. This is based on the fact that, for many viscometers, portions of the viscosity—time curves can be taken as straight lines over moderate time ranges. Linear equations, which are simpler to use in determining and applying correction factors after caUbration, must be appHed carefully as they do not represent the tme viscosity—time relation. Linear equation constants have been given (158) and are used in ASTM D4212. 

The equations and methods for determining viscosity vary greatly with the type of instmment, but in many cases calculations may be greatly simplified by calibration of the viscometer with a standard fluid, the viscosity of which is known for the conditions involved. General procedures for calibration measurement are given in ASTM D2196. The constant thus obtained is used with stress and shear rate terms to determine viscosity by equation 25, where the stress term may be torque, load, or deflection, and the shear rate may be in rpm, revolutions per second (rps), or s F... 

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. 

Dyna.mic Viscometer. A dynamic viscometer is a special type of rotational viscometer used for characterising viscoelastic fluids. It measures elastic as weU as viscous behavior by determining the response to both steady-state and oscillatory shear. The geometry may be cone—plate, parallel plates, or concentric cylinders parallel plates have several advantages, as noted above. 

Another type of rotational viscometer is the hehcal-screw rheometer (176). This iastmment is basically a screw-type metering pump that does not pump. The measure of force is the pressure difference resulting from the rotational motion. It is possible to use a bank of pressure transducers of different sensitivities to measure viscosity over a wide range. The iastmment can be used for high temperature rheometry and to foUow polymerkation, shear and heat degradation, and other developments. 

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. 

The Nametre Rotary B rotational viscometer measures torque in terms of the current needed to drive the d-c motor at a given speed while a material is under test. The standard sensors are coaxial cylinders or Brookfield disk-type spindles, but a cone—plate system is also available. The viscosity range for the coaxial cylinder sensors is 5 to 5 x 1(T mPa-s, and the maximum shear rate is 200. ... 

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