Viscometer - Wikipedia, The Free Encyclopedia

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  12/9/13Viscometer - Wikipedia, the free encyclopediaen.wikipedia.org/wiki/Viscometer1/8 Viscometer From Wikipedia, the free encyclopedia A viscometer  (also called viscosimeter ) is an instrument used to measure the viscosity of a fluid. For liquidswith viscosities which vary with flow conditions, an instrument called a rheometer is used. Viscometers onlymeasure under one flow condition.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 measur e of the viscosity.The flow conditions must have a sufficiently small value of Reynolds number for there to be laminar flow.At 20.00 degrees Celsius the viscosity of water is 1.002 mPa·s and its kinematic viscosity (ratio of viscosity todensity) is 1.0038 mm 2 /s. These values are used for calibrating certain types of viscometer. Contents 1 Standard laboratory viscometers for liquids1.1 U-tube viscometers1.2 Falling sphere viscometers2 Falling Piston Viscometer 3 Oscillating Piston Viscometer 4 Vibrational viscometers5 R otational viscometers5.1 Electromagnetically Spinning Sphere Viscometer (EMS Viscometer)5.2 Stabinger viscometer 6 Bubble viscometer 7 Micro-Slit Viscometers8 Miscellaneous viscometer types9 See also10 References11 External links Standard laboratory viscometers for liquids U-tube viscometers These devices also are known as glass capillary viscometers or Ostwald viscometers , named after WilhelmOstwald. Another version is the Ubbelohde viscometer, which consists of a U-shaped glass tube held verticallyin 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, with it is another bulb lower down on 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 aboveand 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 aconversion factor, or can be calibrated by a fluid of known properties. The time required for the test liquid toflow through a capillary of a known diameter of a certain factor between two marked points is measured. Bymultiplying the time taken by the factor of the viscometer, the kinematic viscosity is obtained.  12/9/13Viscometer - Wikipedia, the free encyclopediaen.wikipedia.org/wiki/Viscometer2/8 Ostwaldviscometersmeasure theviscosity of afluid with aknown density.Creeping flow past a sphere. 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 existsso that the level can be determined even when opaque or staining liquids are measured, otherwise the liquid willcover the markings and make it impossible to gauge the time the level passes the mark. This also allows theviscometer to have more than 1 set of marks to allow for an immediate timing of the time it takes to reach the3rd mark, therefore yielding 2 timings and allowing for subsequent calculation of Determinability to ensureaccurate results. Falling sphere viscometers Stokes' law is the basis of the falling sphere viscometer, in which the fluid is stationary in a vertical glass tube. Asphere of known size and density is allowed to descend through the liquid. If correctly selected, it reachesterminal velocity, which can be measured by the time it takes to pass two marks on thetube. Electronic sensing can be used for opaque fluids. Knowing the terminal velocity, thesize and density of the sphere, and the density of the liquid, Stokes' law can be used tocalculate the viscosity of the fluid. A series of steel ball bearings of different diameter arenormally used in the classic experiment to improve the accuracy of the calculation. Theschool experiment uses glycerine as the fluid, and the technique is used industrially tocheck the viscosity of fluids used in processes. It includes many different oils, and polymer liquids such as solutions.In 1851, George Gabriel Stokes derived an expression for the frictional force (also calleddrag force) exerted on spherical objects with very small Reynolds numbers (e.g., verysmall particles) in a continuous viscous fluid by changing the small fluid-mass limit of thegenerally unsolvable Navier-Stokes equations:where: is the frictional force, is the radius of the spherical object, is the fluid viscosity, and is the particle's velocity.If the particles are falling in the viscous fluid by their own weight, then a terminal velocity,also known as the settling velocity, is reached when this frictional force combined with the buoyant force exactly balance the gravitational force. The resulting settling velocity (or terminal velocity) is given by:where: V   s  is the particles' settling velocity (m/s) (vertically downwards if ,upwards if ), is the Stokes radius of the particle (m),  g   is the gravitational acceleration (m/s 2 ),  ρ  p  is the density of the particles (kg/m 3 ),  12/9/13Viscometer - Wikipedia, the free encyclopediaen.wikipedia.org/wiki/Viscometer3/8  ρ  f   is the density of the fluid (kg/m 3 ), and is the (dynamic) fluid viscosity (Pa s). Note that Stokes flow is assumed, so the Reynolds number must be small.A limiting factor on the validity of this result is the roughness of the sphere being used.A modification of the straight falling sphere viscometer is a rolling ball viscometer which times a ball rolling downa slope whilst immersed in the test fluid. This can be further improved by using a patented V plate whichincreases the number of rotations to distance traveled, allowing smaller more portable devices. This type of device is also suitable for ship board use. Falling Piston Viscometer Also known as the Norcross viscometer after its inventor, Austin Norcross. The principleof viscosity measurement in this rugged and sensitive industrial device is based on a pistonand cylinder assembly. The piston is periodically raised by an air lifting mechanism, drawingthe material being measured down through the clearance (gap) between the piston and thewall of the cylinder into the space which is formed below the piston as it is raised. Theassembly is then typically held up for a few seconds, then allowed to fall by gravity,expelling the sample out through the same path that it entered, creating a shearing effect onthe measured liquid, which makes this viscometer particularly sensitive and good for measuring certain thixotropic liquids. The time of fall is a measure of viscosity, with the clearance between the piston and inside of the cylinder forming the measuring orifice. The viscosity controller measures the time of fall(time-of-fall seconds being the measure of viscosity) and displays the resulting viscosity value. The controller cancalibrate the time-of-fall value to cup seconds (known as efflux cup), Saybolt universal second (SUS) or centipoise.Industrial use is popular due to simplicity, repeatability, low maintenance and longevity. This type of measurement is not affected by flow rate or external vibrations. The principle of operation can be adapted for many different conditions, making it ideal for process control environments. Oscillating Piston Viscometer Sometimes referred to as electromagnetic viscometer or EMV viscometer, was invented at Cambridge Viscosity(Formally Cambridge Applied Systems) (http://www.cambridgeviscosity.com) in 1986. The sensor (see figure below) comprises a measurement chamber and magnetically influenced piston. Measurements are takenwhereby a sample is first introduced into the thermally controlled measurement chamber where the pistonresides. Electronics drive the piston into oscillatory motion within the measurement chamber with a controlledmagnetic field. A shear stress is imposed on the liquid (or gas) due to the piston travel and the viscosity isdetermined by measuring the travel time of the piston. The construction parameters for the annular spacing between the piston and measurement chamber, the strength of the electromagnetic field, and the travel distanceof the piston are used to calculate the viscosity according to Newton’s Law of Viscosity.  12/9/13Viscometer - Wikipedia, the free encyclopediaen.wikipedia.org/wiki/Viscometer4/8 The oscillating piston viscometer technology has been adapted for small sample viscosity and micro-sampleviscosity testing in laboratory applications. It has also been adapted to measure high pressure viscosity and hightemperature viscosity measurements in both laboratory and process environments. The viscosity sensors have been scaled for a wide range of industrial applications such as small size viscometers for use in compressors andengines, flow-through viscometers for dip coating processes, in-line viscometers for use in refineries, andhundreds of other applications. Improvements in sensitivity from modern electronics, is stimulating a growth inoscillating piston viscometer popularity with academic laboratories exploring gas viscosity. Vibrational viscometers Vibrational viscometers date back to the 1950s Bendix instrument, which is of a class that operates bymeasuring the damping of an oscillating electromechanical resonator immersed in a fluid whose viscosity is to bedetermined. 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 bemeasured by one of several methods:1. 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.2. Measuring the decay time of the oscillation once the excitation is switched off. The higher the viscosity,the faster the signal decays.3. Measuring the frequency of the resonator as a function of phase angle between excitation and responsewaveforms. 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 measuringthe viscosity of a fluid whose flow behaviour is not known before hand.Vibrating viscometers are rugged industrial systems used to measure viscosity in the process condition. Theactive part of the sensor is a vibrating rod. The vibration amplitude varies according to the viscosity of the fluid inwhich the rod is immersed. These viscosity meters are suitable for measuring clogging fluid and high-viscosityfluids, including those with fibers (up to 1,000 Pa·s). Currently, many industries around the world consider theseviscometers to be the most efficient system with which to measure the viscosities of a wide range of fluids; bycontrast, rotational viscometers require more maintenance, are unable to measure clogging fluid, and requirefrequent calibration after intensive use. Vibrating viscometers have no moving parts, no weak parts and thesensitive part is very small. Even very basic or acidic fluids can be measured by adding a protective coating suchas enamel, or by changing the material of the sensor to a material such as 316L stainless steel.
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