Viscometers hydrostatic head

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. 

In practice we do not measure viscosity directly. Instead, what is measured is time of flow for solutions and pure solvent in a capillary viscometer, the so-called efflux time. If the same average hydrostatic head is used in all cases, and since for very dilute solutions the density differences between the different concentrations are negligible, then the ratio of the efflux time of the solution, t, to that of the pure solvent, tg, may be taken as a measure of the ratio of the viscosities, i.e. 

Hydrostatic head viscometers are capillary viscometers in which a hydrostatic head of fluid provides the... 

Capillary viscometers have been widely used in determining the viscosity of Newtonian fluids. In these viscometers, the driving force is usually the hydrostatic head of the test liquid itself, although, application of external pressure is also used in order to increase the range of measurement and allow non-Newtonian behavior to be studied. In operation, the efflux time of a fixed volume of test liquid is measured, from which the kinematic viscosity is calculated. 

Hydrostatic head viscometers. (a) In the Ostwald design, liquid is filled exactly to mark C then sucked through the narrow capillary section to above mark A. The time for the meniscus to fall from. 4 to S is proportional to viscosity. (b) The side arm of the Ubbelehde design eliminates the need to fill with a precise volume, (c) The Cannon-Fenske design reverses the flow from (a) and is used for opaque fluids. The dark meniscus rises from A to B during the timing. (from Van Wazer et al., 1963). 

Discussion—For gravity flow under a given hydrostatic head, the pressure head of a liquid is proportional to its density, p. For any particular viscometer, the time of flow of a fixed volume of fluid is directly proportional to its kinematic viscosity, v, where v = v/p, and ij is the dynamic viscosity coefficient. 

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