Viscometers falling-level

The falling-level viscometer is a simple device that may be used to determine flow curves for relatively dilute polymer solutions in a single experiment. It consists of a vertical buret of diameter D connected to a horizontal capillary tube of length L and radius R. The buret is filled with the solution, which is then allowed to drain through the capillary. Data are h vs. t, where h is the height of the fluid in the buret above the capillary. It may be assumed that flow in the capfllary is laminar, that all resistance to flow is in the capillary, and that entrance and exit effects are negligible. The fluid has a known density p. Describe clearly how to proceed from the data, h vs. t, to the flow curve, T vs. y. Show all necessary equations. 

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... 

The falling sphere instrument is representative of this class of viscometer and is employed with higher viscosity liquids. A stainless steel ball is allowed to sink through a column of the liquid under test, and the time taken to fall from one designated level to another at its terminal falling velocity is recorded. In order that the terminal... 

Many types of apparatus used by earlier investigators as vis-cometerss are only of historical interest. A handy simple apparatus which is often used in laboratories (although it has some defects) is that known as tht Ostwald viscometer (Fig. l.VIII E). The time taken for the level to fall from a to 6 by way of the capillary be is found, the liquid being first sucked from the bulb A above the mark a by a piece of rubber tubing attached to B. The apparatus must be enclosed in some type of constant temperature bath Ostwald used an inverted bell-jar filled with water... 

Viscosity is usually measured by the time it takes a fluid to flow through a tube under standard conditions. Figure 5.2 is a schematic of a glass Ostwald viscometer. The fluid to be measured is poured into the left tube. The unit is immersed in a constant temperature water bath and when a constant temperature is reached, the fluid is drawn up into the right tube. The time required for the level to fall from 4 to 5 is measured with a stopwatch The quantity p/p = v of the liquid, called kinematic viscosity is proportional to the time required for outflow. Such a device... 

Fig. 3.16 Schematic illustrations of different viscometers. The flow time is determined from the time taken for the level of the liquid to fall from A to B. (a) Ostwald viscometer, (b) Ubbelohde or suspended-level viscometer.

FIGURE 7.7 Ostwald viscometer. Viscosity is determined by measuring the time rrequired for fluid level to fall from the upper to the lower mark. 

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