Rising bubble viscometer

Air-bubble viscometer n. An instrument used to measure the viscosities of oils, varnishes and resin solutions by matching the rate of rise of an air bubble in the sample liquid with the rate of rise in one of a series of standard liquids, whose viscosities are known. The Gardner-Holt bubble viscometer is such an instrument (Paul N. Gardner, Company Inc., 316 N. E. Fifth Street, Pompano Beach, FI, www.gardco.com). 

Tail . ( ) Highest boiling solvent fraction. (2) Elongated, somewhat pointed extension of the lower portion of the rising bubble in a bubble tube viscometer, characteristic of a varnish or resin solution that is near or approaching gelation or which has a peculiar rheological characteristic. 

In a bubble viscometer, a liquid streams downward in the ring-shaped zone between the glass wall of a sealed tube and a rising air bubble. The rate at which the bubble rises is a direct measure of the kinematic viscosity. The rate of bubble rise is compared with a set of calibrated bubble tubes containing liquids of known viscosities. Bubble viscometers are shown in Figure 7-30. 

Viscosity can also be determined from the rising rate of an air bubble through a Hquid. This simple technique is widely used for routine viscosity measurements of Newtonian fluids. A bubble tube viscometer consists of a glass tube of a certain size to which Hquid is added until a small air space remains at the top. The tube is then capped. When it is inverted, the air bubble rises through the Hquid. The rise time in seconds may be taken as a measure of viscosity, or an approximate viscosity in mm /s may be calculated from it. In an older method that is commonly used, the rate of rise is matched to that of a member of a series of standards, eg, with that of the Gardner-Holdt bubble tubes. Unfortunately, this technique employs a nonlinear scale of letter designations and may be difficult to interpret. 

It is clear then, that the measurement of non-Newtonian materials presents special challenges for a viscometer. Many industrial viscometers designed to give a single point determination have a deceptively simple operating principle. Examples include the speed at which a liquid flows out of a container through a known orifice, a bubble rises in a column of fluid, or a ball falls in a column of fluid. These simple devices are actually very complex in terms of the shear field that is generated. The shear field is the variation of shear stress or shear rate as a function of position within the... 

With floppier viscometers, the time required for a rolling ball to run along an inclined tube is measured. In Cochius tubes, the time taken for an air bubble to rise is a measure of the viscosity. Here, the true viscosities, shear stresses, and shear gradients are also difficult to determine. 

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