Other rotational instruments may have a stationary bob and a rotating cup. Different spindles are available for given viscosity ranges, and several rotational speeds generally are available. ![]() A particularly convenient and rapid type of instrument is a rotational viscosimeter, which utilizes a bob or spindle immersed in the test specimen and measures the resistance to movement of the rotating part. For the Saybolt instruments, measurements usually are made at 100 F and 210 F Redwood instruments may be used at several temperatures up to 250 F and values obtained on the Engler instrument usually are reported at 20 C and 50 C. Standard temperatures are adopted as a matter of convenience with these instruments. Each of these instruments uses arbitrary units that bear the name of the instrument. II, the Engler, the Saybolt Universal, and the Saybolt Furol. The viscosity of oils is expressed on arbitrary scales that vary from one country to another, there being several corresponding instruments. Several types are described, with directions for their use, by the American Society for Testing and Materials (ASTM, D-445). Many capillary-tube viscosimeters have been devised, but Ostwald and Ubbelohde viscosimeters are among the most frequently used. The usual method for measurement of viscosity involves the determination of the time required for a given volume of liquid to flow through a capillary. The approximate viscosity in centistokes at room temperature of ether is 0.2 of water, 1 of kerosene, 2.5 of mineral oil, 20 to 70 and of honey, 10,000. The sizes of the units are such that viscosities in the ordinary ranges are conveniently expressed in centistokes. ![]() To obtain the kinematic viscosity from the absolute viscosity, the latter is divided by the density of the liquid at the same temperature, i.e., kinematic viscosity = (absolute viscosity)/(density). While on the absolute scale viscosity is measured in poises or centipoises, for convenience the kinematic scale, in which the units are stokes and centistokes (1 stoke = 100 centistokes) commonly is used. ![]() The specifying of temperature is important because viscosity changes with temperature in general, viscosity decreases as temperature is raised. The basic unit is the poise however, viscosities commonly encountered represent fractions of the poise, so that the centipoise (1 poise = 100 centipoises) proves to be the more convenient unit. It is defined as the shear stress divided by the rate of shear strain. It is defined in terms of the force required to move one plane surface continuously past another under specified steady-state conditions when the space between is filled by the liquid in question. Viscosity is a property of liquids that is closely related to the resistance to flow.
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