Working Principle of Viscometer


A viscometer is a measuring instrument that is mainly used to measure the viscosity and other flow-related characteristics of a fluid. Viscosity is nothing but the internal resistance of the fluids to the flow. Mathematically, viscosity can be defined as the ratio of shear stress to the velocity gradient of the fluid. There are a number of viscometers available in the market; however, one of the most popular viscometer instruments is the Ostwald viscometer. Ostwald viscometer was named after a Baltic German chemist and philosopher, Wilhelm Ostwald. A viscometer is also known as a viscosimeter. An Ostwald viscometer is generally preferred to measure the viscosity of Newtonian fluids or typically the fluids whose viscosity does not change with respect to the flow rate. Calculation of viscosity or the application of a viscometer can be observed easily in pharmaceutical, cosmetic, food processing, petrochemical, and various other industries. They are also applicable while designing systems that make use of hydrodynamics and aerodynamics.

Working Principle of a Viscometer

Ostwald viscometer or viscometer is an instrument that works on the principle of Poiseuille’s law. As per Poiseuille’s law, in the case of the laminar flow of the fluids, the flow rate of the liquid can be denoted as the ratio of pressure difference with respect to the viscous resistance. The value of the viscous resistance varies directly with respect to the viscosity of the fluid and the length of the tube. Also, the viscous resistance value is inversely related to the fourth power of the radius of the tube. An Ostwald viscometer is also known as a capillary viscometer. It typically makes use of the rate of flow of liquids through a capillary tube to determine the viscosity of the fluid. For this purpose, the fluids are typically made to flow through a capillary tube between two marked points and the time taken by the fluid to flow between the two points marked on the glass tube is noted with the help of a stopwatch.

Construction of Viscometer

A viscometer is usually constructed in the shape of the English alphabet ‘U’. The material used to construct a viscometer is usually preferred to be glass. The wider limb of the viscometer is located on one side of the viscometer capillary tube and is used as the opening from which the fluid whose viscosity is required to be measured is poured. An Ostwald viscometer typically consists of two bulbs that are attached to each other with the help of a U-shaped tube. One of the bulbs is connected to the wider limb on one side and to the U-shaped tube on the other side. The second bulb is connected to the capillary tube on one side and to the rubber tube on the other side. The second bulb consists of two markings on either side. The main purpose of the two etched marks is to track the limit of the flow of the liquids through the capillary tube. These markings are generally denoted by upper mark-A and lower mark-B. The second opening of the viscometer tube that is attached to a rubber tube is used to suck the liquid inside.

Construction of Viscometer

Working of Viscometer

The working of an Ostwald viscometer is quite simple. The U-shaped glass viscometer tube is held vertically still. The first step is to pour liquid into the capillary tube through the wider limb or the pipette. The fluid is allowed to flow through the tube freely.  The liquid is poured into the tube till the bulb-1 gets properly filled with it. Once the bulb-1 gets filled with the liquid, it is sucked from the other end of the tube. The liquid begins to travel from bulb-1 to bulb-2 through the U-tube. Once the liquid manages to reach bulb-2, it is made to flow freely inside the capillary tube from the upper marking-A towards the lower marking-B. The time taken by the liquid to flow from the upper mark to the lower mark is noted with the help of a stopwatch. This time is then used to determine the value of the viscosity of the liquid.

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