U-tube viscometers

The basic design is that of the Ostwald viscometer a U-tube with two reservoir bulbs separated by a capillary, as shown in Figure 24a. The Hquid is added to the viscometer, pulled into the upper reservoir by suction, and then allowed to drain by gravity back into the lower reservoir. The time that it takes for the Hquid to pass between two etched marks, one above and one below the upper reservoir, is a measure of the viscosity. In U-tube viscometers, the effective pressure head and therefore the flow time depend on the volume of Hquid in the instmment. Hence, the conditions must be the same for each measurement. 

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). 

Figure H1.3.1 Schematic of a glass capillary (U-tube) viscometer. Contributed by Jody Renner-Nantz...

Viscometers of relatively complex geometry, for example the Ostwald glass U-tube viscometer, can be used to measure the viscosity of Newtonian liquids, which is independent of shear rate and time, after calibration with a Newtonian liquid of known viscosity. Such instruments cannot be used for Theologically characterizing non-Newtonian liquids, and therefore cannot be classed as rheometers, as geometrical complexity prevents evaluation of shear stress and shear rate at a given location independently of sample rheological behavior. 

Common-tensile Reservoir and receiving bulbs in same vertical axis U-tube viscometer with third arm... 

One of the simplest methods of examining this effect is by capillary viscometry, although automatic viscometers are commercially available. In a U-tube viscometer such as the Ubbelohde suspended level dilution model shown in Figure 9.8, the flow times of pure solvent and a polymer solution t are recorded. This is done by pipetting an aliquot of solution of known volume into bulb D. The solution is then pumped into E. The flow time t is the time taken for the solution meniscus to pass from X to y in bulb E. 

Due to the highly efficient nature of these polymer types as thickeners, viscosity-based characterisation studies are usually carried out on dilute solutions, typically at a concentration of < 1% polymer in water. Despite the non-Newtonian behaviour of these solutions (which is covered in more detail later in this chapter) useful information reflecting the character of the polymer present in solution can be obtained using cheap, simple and reliable equipment such as glass U-tube viscometers. 

Viscosity-average mol wt (using U-tube viscometer to find [ /]) Viscosity-average mol wt (using U-tube viscometer to find [ ])... 

Viscosity measurements began to be used by colloid chemists when Thomas Graham investigated the peptisation of silicic acid in the 1860 s. Soon, with the introduction of the Ostwald U-tube viscometer, viscometry became one of the most widely used methods of investigating colloidal solutions—and to a large extent it still is used for this purpose. 

As we have seen, viscometers are instruments that can either apply a force and measure a speed, or apply a speed and meastue a force, to or from a simple geometry. This might be as simple as a U-tube viscometer that meastues the time taken for a gravity-driven flow to move from one vertical position to another, or a similar situation for flow from the hole at the bottom of a carefully manufactured cup called a flow-cup, see figure 7. 

Figure 7 A schematic diagram of a simple U-tube viscometer and a flow-cup.

The viscosities of dilute polymer solutions most commonly are measured using capillary viscometers of which there are two general classes, namely U-tube viscometers and suspended-level viscometers (Fig. 3.18). A common feature of these viscometers is that a measuring bulb, with upper and lower etched marks, is attached directly above the capillary tube. The solution is either drawn or forced into the measuring bulb from a reservoir bulb attached to the bottom of the capillary tube, and the time required for it to flow back between the two etched marks is recorded. 

In U-tube viscometers, the pressure head giving rise to flow depends upon the volume of solution contained in the viscometer, and so it is essential that this volume is exactly the same for each measurement. This normally is achieved after temperature equilibration by carefully adjusting the liquid level to an etched mark just above the reservoir bulb. 

Fig. 3.18 Schematic illustrations of (a) an Ostwald U-tube viscometer (b) an Ubbelohde suspended-level viscometer, and (c) a modified Ubbelohde viscometer with a larger reservoir bulb for dilutions.

Chaige two viscometers in the manner dictated by the design of the instrument. For example, for the cross-arm or the BS U-tube viscometers for opaque liquids, filter the sample through a 7S-pm filter into two viscometers previously placed in the bath. For samples subjected to heat treatment, use a preheated filter to prevent the sample coagulating during the filtration. 

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