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Viscosity- the resistance of a fluid to shear or tensile stress- is an indispensable measurement in fluid analysis. Viscosity can be thought of as a resistance to flow, with highly viscous fluids being more resistant to flow. Understanding the role of viscosity, and measuring it, is key to correctly analyzing many engineering situations. Using lubricating fluids that become more viscous at high temperatures, for example, could be disastrous in a car engine. It is therefore sometimes important to be able to measure viscosity. Engineers can do this by using viscosity measurement devices. The most commonly used devices are viscometers and rheometers. This article explains the differences between the two devices and when each one is used.
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Viscometers are used to measure viscosity in most circumstances. They work for fluids whose viscosity does not change under varying flow conditions; rheometers must be used when the viscosity does change with flow conditions. Viscometers usually work by comparing a stationary object and a fluid flow, or vice versa. Hence, a viscometer could be placed in a fluid flow or moved through a stationary fluid. The flow must have a Reynolds number in the laminar region in order to record accurate values. The measure of the resistance is taken by measuring the drag resistance during relative motion through the fluid. There are several types of viscometers available, some for use in laboratories and others used as portable viscosity testers.
These viscometers are often used in laboratory settings. Users can obtain the dynamic viscosity by measuring how long it takes the fluid to flow between two points of a capillary of known radius; it is necessary to know the fluid's density to calculate viscosity in this manner.
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Falling Sphere Viscometers
As the name implies, these viscometers use a falling sphere to measure viscosity. The time taken for the falling sphere, whose density and radius are known, to move between two markings is measured, and then users can calculate viscosity. This model is also typically used in the lab. They work on principles derived from Stokes' Law, which gives drag force on a sphere.
Falling Piston Viscometers
Falling Piston viscometers operate on similar principles as the falling sphere viscometers, except that they measure resistance to a piston moving through a fluid. These devices are very long-lasting and simple to operate, and require little maintenance. For this reason, they are very popular in industry.
Rotational viscometers measure the resistance of fluids to torque. There are several types of rotational viscometers: the Stabinger viscometer was developed in 2000, while the Stormer viscometer is commonly used to measure viscosity of paints. The Stabinger viscometer uses a proprietary unit, the Krebs Unit (KU).
Bubble viscometers measure the time it takes for bubbles to rise through a liquid. These viscometers are most often used for resins or varnishes. These viscometers are fast, and very useful for measuring viscosity in the field. Models that use the alphabetical comparison method measure viscosity in stokes, which is equal to 1 cm2s-1.
Rheometers are most useful for non-newtonian fluids; that is, fluids whose viscosity isn't described by a single value. Larger forces typically induce larger viscosity in non-newtonian fluids. There are several commercial rheometers on the market. For forces below 10 pascals, ThermoFisher's CaBER is popular. Tthe Cambridge Polymer Group's FiSer can be used for values from 1 to 1000 pascals, while the Gottfert Rheotens is rated for values above 100 Pa and the Xpansion Instruments Sentmanat extensional rheometer is rated for over 10 kPa.
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Why to Measure Viscosity
Engineers may want to measure viscosity in any application that involves fluid flow, particularly in design. Because viscosity can change dramatically with temperature, it is important to understand what will happen to lubricants at high temperatures and pressures or low temperatures. Failure to do so could result in design errors. Engineers developing new lubricants or other fluids may also want to measure the fluid's viscosity in a laboratory setting.
Engineers in the field may also need to measure viscosity. They can do this with any of a number of portable viscosity testers, or with larger industrial models. Viscosity is important in many commercial applications, such as consumer products like shampoo, and viscometers are used extensively in quality control.
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