Viscometers glass capillary

Gla.ss Ca.pilla.ry Viscometers. The glass capillary viscometer is widely used to measure the viscosity of Newtonian fluids. The driving force is usually the hydrostatic head of the test Hquid. Kinematic viscosity is measured directly, and most of the viscometers are limited to low viscosity fluids, ca 0.4—16,000 mm /s. However, external pressure can be appHed to many glass viscometers to increase the range of measurement and enable the study of non-Newtonian behavior. Glass capillary viscometers are low shear stress instmments 1—15 Pa or 10—150 dyn/cm if operated by gravity only. The rate of shear can be as high as 20,000 based on a 200—800 s efflux time. 

Fig. 24. (a) Ostwald glass capillary viscometer, (b) Cannon-Fenske viscometer, and (c) Ubbelohde viscometer. 

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. 

Osteoporosis biomarkers, 17 649 Ostromislensky, Ivan, 25 628 Ostwald color system, 7 309 Ostwald glass capillary viscometer, 21 728 Ostwald ripening, 10 124... 

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). 

For low-viscosity Newtonian foods that can flow under nornial gravitational force, glass capillary viscometers can be used. Recalling that the magnitude of viscosity can be obtained by dividing the equation for shear stress by that for shear rate, one can derive from the Hagen-Poiseuille equation ... 

In glass capillary viscometers, both entrance and exit effects depend on the kinetic energy of the fluid stream in the capillary. Based on extensive experimental data, the kinematic viscosity, v = r]/p, data with a correction term for kinetic energy is expressed ... 

It is important that magnitudes of t and tgt are determined with care at a specific temperature, and that magnitude of viscosity of the standard fluid jst is reliable. Glass capillary viscometers are not suitable for liquids that deviate substantially from Newtonian flow or contain large size particles or a high concentration of suspended solids. In this case, the viscosity is directly proportional to the time of flow t. 

As stated in Chapter 1, for the determination of intrinsic viscosity, [ ], of a polymer, viscosity values of several dilute solutions, when the relative viscosities ( / s) of the dispersions are from about 1.2 to 2.0, are determined. To facilitate such measurements, the so called Ubbelohde glass capillary viscometer is used that has a large reservoir to permit several successive dilutions of a polymer solution (Figure 3-19). Because intrinsic viscosity measurement is important, the test procedure for using the Ubbelohde viscometer is outlined here in brief (Cannon Instrument Co., 1982). 

Equation 3B.16 is the basis for calculation of viscosity of a Newtonian fluid using glass capillary viscometer. Itshouldalso be recognized that(4g/jrro) = ilQ/nEf) gives the shear rate for Newtonian fluids but not for non-Newtonian fluids and it is called pseudo shear rate. Additional steps are required to obtain an expression for the true shear rate. 

Viscosity is usually determined at different temperatures (e.g., 25°C/77°F, and 100°C/212°F) by measuring the time for a volume of liquid to flow under gravity through a calibrated glass capillary viscometer (ASTM D-445). 

The use of these empirical procedures is being superseded by the more precise kinematic viscosity method (ASTM D-445, IP 71), in which a fixed volume of fuel flows through the capillary of a calibrated glass capillary viscometer under an accurately reproducible head and at a closely controlled temperature. The result is obtained from the product of the time taken for the fuel to flow between two etched marks on the capillary tube and the calibration factor of the viscometer and is reported in centistokes. Because the viscosity decreases with increasing temperature the temperature of test must also be reported if the viscosity value is to have any significance. For distillate fuel oils the usual test temperature is 38°C (100°F). 

The most common viscosity test is the kinematic viscosity method (ASTM D445, IP-71, DIN 51566 and ISO 3104). Note that lubricant viscosity is discussed in detail in the next chapter. The kinematic viscosity is the product of the time of flow and the calibration factor of the instrument. The test determines the kinematic viscosity of liquid lubricants by measuring the time taken for a specific volume of the liquid to flow through a calibrated glass capillary viscometer under specified driving head (gravity) and temperature conditions. The test is usually performed at a lubricant temperature of 40°C and/or 100°C to standardize the results obtained and allow comparison among different users. 

Glass capillary viscometers operating under the force of gravity are instruments of low shear stress and shear rate and are mostly used in dealing with liquids of relatively low viscosities (0.4 x 10 to 16 x 10 square meter per second 0.4 to 16,000 centistokes). Although this may seem like an enormous range, many lubricating oils have viscosities well above 50 x 10 square meter per second (50,000 centistokes) at 253 K (-20 C). 

TABLE 4-1. DIMENSIONS AND VISCOSITY RANGES FOR CANNON-FENSKE GLASS CAPILLARY VISCOMETERS... 

Viscosity of Newtonian liquids ean be measured by calibrated glass capillary viscometer. Kinematic (the resistance to flow of a fluid under gravity) and dynamic (the ratio between the applied shear stress and the rate of shear of a liqitid) viscosities can be calculated from measured time of flow using the following equations ... 

By performing the test, the time for a fixed volume of the liquid to flow through the capillary of a calibrated glass capillary viscometer under an accurately reproducible head and at... 

Experimentally, the viscosity of dilute polymer solutions is, in most cases, determined with glass capillary viscometers, making application of the Hagen-Poiseuille s law for laminar flow of liquids. The time required for a specific volume of a liquid to flow through a capillary of... 

The values R, V. 1 can be made constant by using a capillary of known dimensions for all the liquids. A glass capillary viscometer can serve the purpose. If we measure the time of flow of two different liquids Uirough the same viscometer, then according to Poiseuille equation, the ratio of the viscosity coefiicients of the two liquids is given by... 

One method which is a variation on this theme and which provides useful data on polymer characteristics is the determination of an intrinsic viscosity value. Glass capillary viscometers (see Figure 3.2) are widely used for such measurements. This exercise involves measuring the viscosity of the polymer in solution at one or more concentrations and comparing this with the result for the solvent alone. A number of terms have been defined for such studies as follows ... 

In the first capillary rheometer, Hagen (1839 Figure 6.1.1) controlled pressure by a gravity head. This is still the case with the common glass capillary viscometers. As indicated in Figure... 

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.)... 

For accurate measurements on mineral oils, glass capillary viscometers are used. The driving force is provided by the head of the test oil flowing vertically down the tube and is given by... 

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