A sphere of known size and density is allowed to descend through the liquid. Stokes' law is the basis of the falling-sphere viscometer, in which the fluid is stationary in a vertical glass tube. Otherwise the change in driving head, which in turn changes the shear rate, will produce a different viscosity for the two bulbs.įalling-sphere viscometers Creeping flow past a sphere The use of two timings in one viscometer in a single run is only possible if the sample being measured has Newtonian properties. This also allows the viscometer to have more than 1 set of marks to allow for an immediate timing of the time it takes to reach the 3rd mark, therefore yielding 2 timings and allowing subsequent calculation of determinability to ensure accurate results. Such classifications exist so that the level can be determined even when opaque or staining liquids are measured, otherwise the liquid will cover the markings and make it impossible to gauge the time the level passes the mark. Reverse-flow viscometers have the reservoir above the markings, and direct-flow are those with the reservoir below the markings. ![]() Such viscometers can be classified as direct-flow or reverse-flow. By multiplying the time taken by the factor of the viscometer, the kinematic viscosity is obtained. The time required for the test liquid to flow through a capillary of a known diameter of a certain factor between two marked points is measured. Most commercial units are provided with a conversion factor. The calibration can be done using a fluid of known properties. The time taken for the level of the liquid to pass between these marks is proportional to the kinematic viscosity. Two marks (one above and one below the upper bulb) indicate a known volume. In use, liquid is drawn into the upper bulb by suction, then allowed to flow down through the capillary into the lower bulb. Above there is a bulb, with it is another bulb lower down on the other arm. In one arm of the U is a vertical section of precise narrow bore (the capillary). Another version is the Ubbelohde viscometer, which consists of a U-shaped glass tube held vertically in a controlled temperature bath. These devices are also known as glass capillary viscometers or Ostwald viscometers, named after Wilhelm Ostwald. Standard laboratory viscometers for liquids Ostwald viscometers measure the viscosity of a fluid with a known density. These values are used for calibrating certain types of viscometers. s and its kinematic viscosity (product of flow time × factor) is 1.0022 mm 2/s.The flow conditions must have a sufficiently small value of Reynolds number for there to be laminar flow.Īt 20 ☌, the dynamic viscosity (kinematic viscosity × density) of water is 1.0038 mPa The drag caused by relative motion of the fluid and a surface is a measure of the viscosity. In general, either the fluid remains stationary and an object moves through it, or the object is stationary and the fluid moves past it. Viscometers can measure only constant viscosity, that is, viscosity that does not change with flow conditions. Thus, a rheometer can be considered as a special type of viscometer. For liquids with viscosities which vary with flow conditions, an instrument called a rheometer is used. Scientific instrument used to measure viscosityĪ viscometer (also called viscosimeter) is an instrument used to measure the viscosity of a fluid.
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