In 1851, George Gabriel Stokes derived an expression for the frictional force (also called drag force) exerted on spherical objects with very small Reynolds numbers (e.g., very small particles) in a continuous viscous fluid by changing the small fluid-mass limit of the generally unsolvable Navier–Stokes equations:į = 6 π r η v, It includes many different oils and polymer liquids such as solutions. The school experiment uses glycerol as the fluid, and the technique is used industrially to check the viscosity of fluids used in processes. A series of steel ball bearings of different diameter are normally used in the classic experiment to improve the accuracy of the calculation. Knowing the terminal velocity, the size and density of the sphere, and the density of the liquid, Stokes' law can be used to calculate the viscosity of the fluid. Electronic sensing can be used for opaque fluids. If correctly selected, it reaches terminal velocity, which can be measured by the time it takes to pass two marks on the tube. 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. ![]()
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |