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A viscometer (also called viscosimeter) is an instrument used to measure the viscosity of a fluid. For liquids with viscosities which vary with flow conditions, an instrument called a rheometer is used. Viscometers only measure under one flow condition. For other uses, see Viscosity (disambiguation). ...
A fluid is defined as a substance that continually deforms (flows) under an applied shear stress regardless of the magnitude of the applied stress. ...
Rheometers are viscometers which are able to measure visco-elastic properties of materials rather than viscosity alone. ...
In general, either the fluid remains stationary and an object moves through it, or the object is stationary and the fluid moves past it. The drag caused by relative motion of the fluid and a surface is a measure of the viscosity. The flow conditions must have a sufficiently small value of Reynolds number for there to be laminar flow. In fluid mechanics, the Reynolds number is the ratio of inertial forces (vsÏ) to viscous forces (μ/L) and consequently it quantifies the relative importance of these two types of forces for given flow conditions. ...
Laminar flow (bottom) and turbulent flow (top) over a submarine hull. ...
At 20.00 degrees Celsius the viscosity of water is 1.002 mPa·s and its kinematic viscosity (ratio of viscosity to density) is 1.0038 mm2/s. These values are used for calibrating certain types of viscometer. The pascal second (symbol Pa·s) is the SI unit of dynamic viscosity. ...
Standard Laboratory Viscometers for Liquids
These viscometers measure the viscosity of a fluid with a known density.
U-tube viscometers These are also known as Ostwald viscometers or glass capillary viscometers. They basically consist of a glass tube in the shape of a U held vertically in a controlled temperature bath. In one arm of the U is a vertical section of precise narrow bore (the capillary). Above this is a bulb. There is another bulb lower down in the other arm. In use, liquid is drawn into the upper bulb by suction, then allowed to flow down through the capillary into the lower bulb. Two marks (one above and one below the upper bulb) indicate a known volume. The time taken for the level of the liquid to pass between these marks is proportional to the kinematic viscosity. Most commercial units are provided with a conversion factor, or can be calibrated by a fluid of known properties. Friedrich Wilhelm Ostwald (commonly just Wilhelm Ostwald) (September 2, 1853 - April 4, 1932) was a German chemist. ...
The time it takes for the test liquid to flow through a capillary of a known diameter of a certain factor between 2 marked points is measured. By multiplying the time taken by the factor of the viscometer, the kinematic viscosity is obtained. The viscometers are usually placed in a constant temperature water bath as temperature affects viscosity.
Such viscometers are also classified as direct flow or reverse flow. Reverse flow viscometers have the reservoir above the markings and direct flow are those with the reservoir below the markings. Such classifications exists 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. 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 for subsequent calculation of Determinability to ensure accurate results.
Falling Sphere Viscometers In these the fluid is stationary in a vertical glass tube. A sphere of known size and density is allowed to descend through the liquid. If correctly selected, it reaches terminal velocity, which can be measured by the time it takes to pass two marks on the tube. Electronic sensing can be used for opaque fluids. 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. For other uses, see Terminal velocity (disambiguation). ...
In 1851, George Gabriel Stokes derived an expression for the frictional force exerted on spherical objects with very small Reynolds numbers (e. ...
Vibrating viscometers Vibrating viscometers are rugged industrial systems used to measure viscosity in the process condition. The active part of the sensor is a vibrating rod. The vibration amplitude varies according to the viscosity of the fluid in which the rod is immersed. These viscosity meters are suitable for measuring clogging fluid and high-viscosity fluids even with fibers (up to 1,000,000 cP). Currently, many industries around the world consider these viscometers as the most efficient system to measure viscosity of any fluid, contrasted to rotational viscometers, which require more maintenance, inability to measure clogging fluid, and frequent calibration after intensive use. Vibrating viscometers has no moving parts, no weak parts and the sensitive part is very small. Actually even the very basic or acid fluid can be measured by adding a special coating or by changing the material of the sensor to a material such as 316L, SUS316, Hastelloy, or enamel.
Rotation viscometers Rotational viscometers uses the idea that the torque required to turn an object in a fluid, can indicate the viscosity of that fluid.
The common Brookfield-type viscometer determines the required torque for rotating a disk or bob in a fluid at known speed.
'Cup and bob' viscometers work by defining the exact volume of sample which is to be sheared within a test cell, the torque required to achieve a certain rotational speed is measured and plotted. There are two classical geometries in "cup and bob" viscometers, known as either the "Couette" or "Searle" systems - distinguished by whether the cup or bob rotates. The rotating cup is preferred in some cases, because it reduces the onset of Taylor vortices, but is more difficult to thermostat accurately.
'Cone and Plate' viscometers use a cone of very shallow angle in bare contact with a flat plate. With this system the shear rate beneath the plate is constant to a modest degree of precision and deconvolution of a flow curve; a graph of shear stress (torque) against shear rate (angular velocity) the viscosity in a straightforward manner.
Stabinger viscometer
 Stabinger viscometer (SVM 3000) By modifying the classic Couette rotational viscometer, an accuracy comparable to that of kinematic viscosity determination is achieved. The internal cylinder in the Stabinger Viscometer is hollow and specifically lighter than the sample, thus floats freely in the sample, centered by centrifugal forces. The formerly inevitable bearing friction is thus fully avoided. The speed and torque measurement is implemented without direct contact, by a rotating magnetic field and an eddy current brake. This allows for a previously unprecedented torque resolution of 50 pNm and an exceedingly large measuring range from 0.2 to 20,000 mPa.s with a single measuring system. A built-in density measurement based on the oscillating U-tube principle allows the determination of kinematic viscosity from the measured dynamic viscosity employing the relation Image File history File links No higher resolution available. ...
Image File history File links No higher resolution available. ...
Look up cylinder in Wiktionary, the free dictionary. ...
Centrifugal force (from Latin centrum centre and fugere to flee) is a term which may refer to two different forces which are related to rotation. ...
For other uses, see Friction (disambiguation). ...
This article does not cite any references or sources. ...
For other senses of this word, see torque (disambiguation). ...
Magnetic field lines shown by iron filings In physics, the space surrounding moving electric charges, changing electric fields and magnetic dipoles contains a magnetic field. ...
Eddy current brakes at the Intamin roller coaster Goliath in Walibi World (Netherlands) Eddy current brake, like conventional friction brakes, are responsible for slowing an object, such as a roller coaster. ...
For other senses of this word, see torque (disambiguation). ...
For other uses, see Density (disambiguation). ...
Digital density measuring principle Digital density meter with oscillating U-tube installed With the oscillating U-tube principle, the density of liquids and gases is determined based on an electronic measurement of the frequency of oscillation, from which the density value is calculated. ...
For other uses, see Viscosity (disambiguation). ...
 The Stabinger Viscometer was presented for the first time by Anton Paar GmbH at the ACHEMA in the year 2000. The measuring principle is named after its inventor Dr. Hans Stabinger.
Stormer viscometer The Stormer viscometer is a rotation instrument used to determine the viscosity of paints, commonly used in paint industries. It consists of a paddle-type rotor that is spun by an internal motor, submerged into a cylinder of viscous substance. The rotor speed can be adjusted by changing the amount of load supplied onto the rotor. For example, in one brand of viscometers, pushing the level upwards decreases the load and speed, downwards increases the load and speed.
The viscosity can be found by adjusting the load until the rotation velocity is 200 rotations/minute. By examining the load applied and comparing tables found on ASTM D 562, one can find the viscosity in Krebs units (KU), unique only to the Stormer type viscometer. ASTM International is an international voluntary standards organization that develops and produces technical standards for materials, products, systems and services. ...
This method is intended for paints applied by brush or roller.
Miscellaneous viscometer types Other viscometer types use bubbles, balls or other objects. Viscometers that can characterize fluids with non-newtonian behavior are usually called rheometers or plastometers. Look up bubble in Wiktionary, the free dictionary. ...
Rheometers are viscometers which are able to measure visco-elastic properties of materials rather than viscosity alone. ...
A plastometer is a tool used to determine the flow properties of plastic materials. ...
Vibrational viscometers date back to the 1950s Bendix instrument, which is of a class that operates by measuring the damping of an oscillating electromechanical resonator immersed in a fluid whose viscosity is to be determined. The resonator generally oscillates in torsion or transversely (as a cantilever beam or tuning fork). The higher the viscosity, the larger the damping imposed on the resonator. The resonator's damping may be measured by one of several methods:
- Measuring the power input necessary to keep the oscillator vibrating at a constant amplitude. The higher the viscosity, the more power is needed to maintain the amplitude of oscillation.
- Measuring the decay time of the oscillation once the excitation is switched off. The higher the viscosity, the faster the signal decays.
- Measuring the frequency of the resonator as a function of phase angle between excitation and response waveforms. The higher the viscosity, the larger the frequency change for a given phase change.
The vibrational instrument also suffers from a lack of a defined shear field, which makes it unsuited to measuring the viscosity of a fluid whose flow behaviour is not known before hand.
In the I.C.I "Oscar" viscometer, a sealed can of fluid was oscillated torsionally, and by clever measurement techniques it was possible to measure both viscosity and elasticity in the sample.
References - British Standards Institute BS ISO/TR 3666:1998 Viscosity of water
- British Standards Institute BS 188:1977 Methods for Determination of the viscosity of liquids
- ASTM International (ASTM D7042)
- http://www.pra.org.uk/viscosityoils/notes-units.htm
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