Types of viscometers

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. 

Cannon-Fenske Kinematic Bitumens, cut-back and fluxed 

Cannon-Manning vacuum Dynamic Cut-backs and soft bitumens 

Standard tar viscometer (STV) Time of flow of Bitumen emulsions and cut-back 

Saybolt Furol Engler Redwood 1 and II bitumen (s) or fluxed bitumens 

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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. 

The pH is measured using a 4% aqueous solution. Viscosity is normally measured using Brookfield viscometer. Alternatively, a capillary-type viscometer or falling ball such as Hxppler may be employed. The type of viscometer used must always be noted. 

Magnetic resonance imaging has enabled the development of a completely novel type of viscometer. This technique is based on the capacity of MRI to accurately measure velocity profiles in opaque liquids. Its potential applications include many systems of industrial relevance, such as polymer melts and slurries. The data presented here clearly show that a wide range of fluid behaviors can be measured. 

There are two main types of viscometer rotary instruments and tubular, often capillary, viscometers. When dealing with non-Newtonian fluids, it is desirable to use a viscometer that subjects the whole of the sample to the same shear rate and two such devices, the cone and plate viscometer and the narrow gap coaxial cylinders viscometer, will be considered first. With other instruments, which impose a non-uniform shear rate, the proper analysis of the measurements is more complicated. 

A type of viscometer providing a nearly uniform shear rate in the sample is that shown in Figure 3.2. The sample fills the gap between the two cylinders, one of which is rotated steadily. 

A Cannon-Fenske viscometer is a capillary type of viscometer. It utilizes the flow through a capillary tube as a means of measuring viscosity. 

Two main types of viscometers are suitable for the determination of the viscosity of a polymer melt The rotation viscometer (Couette viscometer, cone-plate viscometer) and the capillary viscometer or capillary extrusiometer. The latter are especially suitable for laboratory use since they are relatively easy to handle and are also applicable in the case of high shear rates. With the capillary extrusiometer the measure of fluidity is not expressed in terms of the melt viscosity q but as the amount of material extruded in a given time (10 min). The amount of ex-trudate per unit of time is called the melt index or melt flow index i (MFI). It is also necessary to specify the temperature and the shearing stress or load. Thus MFI/2 (190 °C)=9.2 g/10 min means that at 190 °C and 2 kg load, 9.2 g of poly-... 

Fig. 17.1 Two types of viscometers Ubbelohde (left) and Cannon-Fenske (right). The Ubbelodhe viscometer has the following components (1) fill tube, (2) capillary outlet, (3) pressure relief tube, (4) solution bulb, (5) suspended volume bulb, (6) lower flow bulb, (7) upper flow bulb, (8) upper timing mark, and (9) lower timing mark. The Cannon-Fenske Viscometer has the following components (1) fill tube, (2) capillary outlet tube, (3) solution bulb, (4) lower flow bulb, (5) upper flow bulb, (6) upper timing mark, and (7) lower timing mark.

In the paper submitted to this meeting, Fetters claims that the viscosity could not be measured if its value exceeds 10- poise. There is no inherent limit for viscosity measurements, even in conventional types of viscometers, provided that the tubes are sufficiently wide. His remark about the effect of 2.4Z dilution is again erroneous. Such an effect is clearly shown by the data of Table III of Ref. (26). 

As shown in Figure 10.3, the viscosity of milk increases sharply when milk coagulates and may be used to determine the coagulation point. Any type of viscometer may, theoretically, be used but several dedicated pieces... 

We conclude this section with a few remarks about the cone-and-plate type of viscometer, sketched schematically in Figure 4.4. In this viscometer, the fluid is placed between a stationary plate and a cone that touches the plate at its apex. This apparatus also possesses cylindrical symmetry, but this time in order to indicate a location within the fluid we must specify not only r, the distance from the axis of rotation, but also the location within the gap between the cone and the plate, as measured by 0, the angle from the vertical (see Fig. 4.4). Mathematical analysis of this apparatus leads to the result... 

The two types of viscometers differ in the following way, however. A much wider range of... 

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... 

Many modifications of the basic Ostwald geometry are employed in different situations. One example is the Cannon-Fenske routine viscometer (Fig. 6.37b) which is used in the oil industry for measuring kinematic viscosities of 0.02 m2/s and less(4<). As viscosity is sensitive to variations in temperature, these types of viscometer are always immersed in a constant temperature bath. They are not normally suitable for non-Newtonian fluids although FAROOQI and Richardson(47) have employed a capillary viscometer to characterise a power-law fluid. 

Viscosity measurements on emulsions were carried out with three types of viscometers. Figure 2 shows the flow curves of emulsions with different volume ratios of the two solutions, as measured with a Ferranti-Shirley cone-plate viscometer. The ratio between the viscosities of the two pure polymer solutions is about 3 at low shear rates but only 2 at the highest shear rates. 

There are two basic types of methylcellulose viscometers— one for cellulose derivatives of a range between 1500 and 4000 centipoises, and the other for less viscous ones. Each type of viscometer is modified slightly for the different viscosities. 

Probably the most widely used type of viscometer in the food industry is the Brookfield rotational viscometer. An example of this instrument s application to a non-Newtonian food product is given in the work of Sarava-cos and Moyer (1967) on fruit purees. Viscometer scale readings were plotted against rotational speed on a logarithmic scale, and the slope of the straight line obtained was taken as the exponent n in the following equation for pseudoplastic materials ... 

Type of viscometer Rotating Capillary type, Capillary type,... 

Although the values of Intrinsic viscosity determined with a low shear viscometer are the only ones which truly represent the Intrinsic viscosity at high molecular weights, the results from the capillary viscometer are shown In Figure 7 to give an Indication of the effect of shear In the viscosity range of the study. The values of Intrinsic viscosity are different for the two types of viscometers, but the trend of Intrinsic viscosity versus conversion Is still the same. 

A variation in the time of flow when the walls of the tube are covered with an immiscible liquid, such as oleic acid, is really due to changes of surface tension, which are iniportant with some types of viscometer. It is possible, however, that some slip may occur with some lubricating oils, due to the regular reflexion of large molecules from the walls, as contrasted with the diffuse scattering of smaller molecules by irregularities of molecular dimensions in the walls. ... 

Two types of viscometer were used by Scarpa. In the first, .the apparatus (Fig. 5.Vni E) had a small bulb C with two capillaries on each side, with two marks a and 6, and a receiver bulb D. Pressure was applied at A. In the second... 

Flow through a porous cell has been used in the viscometry of mobile liquids. The older types of viscometer used in testing lubricating oils, such as those of Redwood, 3 Engler, etc., in which a liquid flows through a small hole in the bottom of a vessel, have no scientific interest, and are being replaced by instruments of the type described above. 

A ball-and-bucket type of viscometer in which the ball is fixed and the bucket falls was used by Beal and Docksey.i A falling sphere viscometer for opaque liquids was described by McDowell and Walker, Michell s viscometer ... 

Here Ah is the pressure head, p the density of the solution, and g the acceleration of gravity. Strictly speaking. Ah in this type of viscometer varies during the measurement. It is not correct merely to take the average Ah at the beginning and ending of the measurement since the flow is faster at first than near the end. The average Ah can however be calculated by the Meissner equation... 

The time for a standard volume of fluid to flow through an orifice is measured. The orifice may be regarded as a very short eapillary. The viscometer is made up of a cup or bowl of standard dimensions with an aeeurately designed diameter orifiee in the bottom. This type of viscometer is often used for quality control of fluid foods in food processing plants where a rapid, inexpensive method is required and extreme accuracy is not needed. 

Other types of viscometers, sueh as an oseillation viscometer, that are useful for eharaeterizing Newtonian foods are also available. However, their use for characterizing non-Newtonian fluid foods can be justified only if the complex flow fields can be analyzed and expressions are derived for the shear rate and shear stress. 

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. 

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