Rotational paddle viscometer

The rotational paddle viscometer measures the apparent viscosity of bitumen emulsions with viscosities between 30 and 1500 mPa-s (centipoises) at 50 C. 

The apparatus consists of a paddle that rotates at 100 rpm and measures viscosity in cen-tipoise. The preset temperature and rotational speed allow for an automated and consistent determination of an emulsified asphalt viscosity within a short time. 

The rotational paddle viscometer is used when, rarely, the apparent viscosity of a bitumen emulsion needs to be determined. A detailed description of the viscosity test by rotational paddle viscometer can be found in ASTM D 7226 (2011). 

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In a rotating paddle viscometer, such as the Krebs-Stormer viscometer commonly used in the paint industry, the shear rate and shear stress caimot be calculated, although it is possible to estimate them by comparison with results from other instmments. 

ASTM D 7226. 2011. Test method for determining the viscosity of emulsified asphalts using a rotational paddle viscometer. West Conshohocken, PA ASTM International. 

A number of investigators have measured the apparent viscosity of fluidized bed using methods available for ordinary liquids. For example, Matheson et al. (1949) and Furukawa and Ohmae (1958) employed a rotating paddle viscometer and found that a fully fluidized bed had a viscosity from 0.5 to 20 poises. The viscosity increases with the size of the particles in the bed. Other researchers, Kramers (1951), Dickman and Forsythe (1953), and Schugerl et al. (1961), used viscometers of slightly different designs and obtained practically similar results. For small shear stress, the fully fluidized beds behave as newtonian fluids. The viscosity of the fluidized bed is very high at close to minimum fluidization condition and decreases sharply with increases in gas flow. 

The calculation of power follows methods described previously, using the estimated weight average density, p, in the expression for Reynolds number and power. The viscosity for a Newtonian system is best measured by using a paddle-type rotating bob viscometer. The Metzner-Otto method is commonly used to determine the Re for a non-Newtonian system, where viscosity is a function of shear rate. This method consists of determining the mean shear rate from y = KN, where N is the stirring speed in rps, K is 10 for a propeller, and y is the mean shear rate is in s. Viscosity will not influence the calculation of power when Re > 200. 

OtherRota.tiona.1 Viscometers. Some rotational viscometers employ a disk as the inner member or bob, eg, the Brookfield and Mooney viscometers others use paddles (a geometry of the Stormer). These nonstandard geometries are difficult to analy2e, particularly for an infinite bath, as is usually employed with the Brookfield and the Stormer. The Brookfield disk has been analy2ed for Newtonian and non-Newtonian fluids and shear rate corrections have been developed (22). Other nonstandard geometries are best handled by determining iastmment constants by caUbration with standard fluids. 

Some rotational viscometers employ a rotating disc, bar, paddle or pin at a constant speed (or series of constant speeds). It is extremely difficult to obtain tme shear stress, and the shear rate usually varies from point to point in the rotating member. In particular, the velocity field of a rotating disc geometry can be considerably distorted in viscoelastic fluids. Nevertheless, because they are simple to operate and give results easily, and their cost is low, they are widely used in the food industry. While they may be useful for quality control purposes, especially Newtonian foods, the reliability of their values should be verified by comparison with data obtained with well defined geometries (capillary/tube, concentric cylinder, and cone-plate). 

Flow properties. Slugging tendency of a fluidized bed increases with increased Stormer viscosity (207). The modified Stormer viscometer used ill this work is provided with a paddle that is rotated in the fluidized bed (by means of a weight attached to a string passing over a system of pulleys). Viscosity is measured by determining the weight required to spin the paddle at 200 r.p.m. Stormer viscosity of the bed increases with decreased gas velocity and with increased size and density of particles. Viscosity of coarse catalyst is decreased, within limits, by addition of fines. 

Rotational Viscometers. These relatively complex instruments can be used in the steady state or in an oscillatory, dynamic mode. Some are useful up to the glassy state of the polymer. The working mechanism, in all cases, is one part that moves past another. Designs include concentric cylinders (cup and bob), cone-and-plate, parallel-plate, and disk, paddle, or rotor in a cylinder. 

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