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A viscometer (also called viscosimeter) is an instrument used to measure the
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
of a
fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...
. For liquids with viscosities which vary with
flow conditions In fluid measurement, the fluid's flow conditions (or flowing conditions) refer to quantities like temperature and static pressure of the metered substance. The flowing conditions are required data in order to calculate the density of the fluid at ...
, an instrument called a
rheometer A rheometer is a laboratory device used to measure the way in which a dense fluid (a liquid, suspension or slurry) flows in response to applied forces. It is used for those fluids which cannot be defined by a single value of viscosity and t ...
is used. Thus, a rheometer can be considered as a special type of viscometer. Viscometers only measure 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 measure of the viscosity. The flow conditions must have a sufficiently small value of
Reynolds number In fluid mechanics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be dom ...
for there to be
laminar flow In fluid dynamics, laminar flow is characterized by fluid particles following smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral mi ...
. At 20°C, the
dynamic viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
(kinematic viscosity × density) of water is 1.0038 mPa·s and its
kinematic viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
(product of flow time × factor) is 1.0022mm2/s. These values are used for calibrating certain types of viscometers.


Standard laboratory viscometers for liquids


U-tube viscometers

These devices are also known as glass capillary viscometers or Ostwald viscometers, named after
Wilhelm Ostwald Friedrich Wilhelm Ostwald (; 4 April 1932) was a Baltic German chemist and philosopher. Ostwald is credited with being one of the founders of the field of physical chemistry, with Jacobus Henricus van 't Hoff, Walther Nernst, and Svante Arrhen ...
. Another version is the
Ubbelohde viscometer An Ubbelohde type viscometer or suspended-level viscometer is a measuring instrument which uses a capillary based method of measuring viscosity. It is recommended for higher viscosity cellulosic polymer solutions. The advantage of this instrumen ...
, which consists of a U-shaped glass tube held vertically in a controlled temperature bath. In one arm of the U is a vertical section of precise narrow bore (the capillary). Above there 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 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. The calibration can be done using a fluid of known properties. Most commercial units are provided with a conversion factor. 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. By multiplying the time taken by the factor of the viscometer, the kinematic viscosity is obtained. Such viscometers can be 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 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. This also allows the viscometer to have more than 1 set of marks to allow , therefore yielding 2 timings and allowing subsequent calculation of determinability to ensure accurate results. The use of two timings in one viscometer in a single run is only possible if the sample being measured has Newtonian properties. Otherwise the change in driving head, which in turn changes the shear rate, will produce a different viscosity for the two bulbs.


Falling-sphere viscometers

Stokes' law In 1851, George Gabriel Stokes derived an expression, now known as Stokes' law, for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers in a viscous fluid. Stokes' law is derived by ...
is the basis of the falling-sphere viscometer, in which 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 Terminal velocity is the maximum velocity (speed) attainable by an object as it falls through a fluid ( air is the most common example). It occurs when the sum of the drag force (''Fd'') and the buoyancy is equal to the downward force of grav ...
, 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 Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
of the liquid, Stokes' law can be used to calculate the
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
of the fluid. A series of steel ball bearings of different diameter are normally used in the classic experiment to improve the accuracy of the calculation. The school experiment uses
glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
as the fluid, and the technique is used industrially to check the viscosity of fluids used in processes. It includes many different oils and
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
liquids . In 1851,
George Gabriel Stokes Sir George Gabriel Stokes, 1st Baronet, (; 13 August 1819 – 1 February 1903) was an Irish English physicist and mathematician. Born in County Sligo, Ireland, Stokes spent all of his career at the University of Cambridge, where he was the Luc ...
derived an expression for the frictional force (also called
drag force In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding flu ...
) exerted on spherical objects with very small
Reynolds number In fluid mechanics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be dom ...
s (e.g., very small particles) in a continuous
viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the in ...
fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...
by changing the small fluid-mass limit of the generally unsolvable
Navier–Stokes equations In physics, the Navier–Stokes equations ( ) are partial differential equations which describe the motion of viscous fluid substances, named after French engineer and physicist Claude-Louis Navier and Anglo-Irish physicist and mathematician Geo ...
: : F = 6 \pi r \eta v, where : ''F'' is the frictional force, : ''r'' is the radius of the spherical object, : ''\eta'' is the fluid viscosity, : ''v'' is the particle 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 Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...
exactly balance the
gravitational force In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
. The resulting settling velocity (or
terminal velocity Terminal velocity is the maximum velocity (speed) attainable by an object as it falls through a fluid ( air is the most common example). It occurs when the sum of the drag force (''Fd'') and the buoyancy is equal to the downward force of grav ...
) is given by : V_\text = \frac \frac, where: : is the particle settling velocity (m/s), vertically downwards if , upwards if , : is the Stokes radius of the particle (m), : is the
gravitational acceleration In physics, gravitational acceleration is the acceleration of an object in free fall within a vacuum (and thus without experiencing drag). This is the steady gain in speed caused exclusively by the force of gravitational attraction. All bodie ...
(m/s2), : is the
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
of the particles (kg/m3), : is the
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
of the fluid (kg/m3), : is the (dynamic) fluid
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
(Pa·s). Note that Stokes flow is assumed, so the
Reynolds number In fluid mechanics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be dom ...
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 down a slope whilst immersed in the test fluid. This can be further improved by using a patented V plate, which increases the number of rotations to distance traveled, allowing smaller, more portable devices. The controlled rolling motion of the ball avoids turbulences in the fluid, which would otherwise occur with a falling ball. This type of device is also suitable for ship board use.


Falling-ball viscometer

In 1932, Fritz Höppler was granted a patent for the falling-ball viscometer, named after him – the worldwide first viscometer to determine the dynamic viscosity. More other world-firsts viscometers developed by Fritz Höppler in Medingen (Germany) are the ball pressure types consistometer and rheoviscometer, see = ball pressure viscometer.


Falling-piston viscometer

Also known as the Norcross viscometer after its inventor, Austin Norcross. The principle of viscosity measurement in this rugged and sensitive industrial device is based on a piston and cylinder assembly. The piston is periodically raised by an air lifting mechanism, drawing the material being measured down through the clearance (gap) between the piston and the wall of the cylinder into the space formed below the piston as it is raised. The assembly 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 on the measured liquid, which makes this viscometer particularly sensitive and good for measuring certain
thixotropic Thixotropy is a time-dependent shear thinning property. Certain gels or fluids that are thick or viscous under static conditions will flow (become thinner, less viscous) over time when shaken, agitated, shear-stressed, or otherwise stressed ( ...
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 can calibrate the time-of-fall value to cup seconds (known as efflux cup),
Saybolt universal second Saybolt universal viscosity (SUV), and the related Saybolt FUROL viscosity (SFV), are specific standardised tests producing measures of kinematic viscosity. ''FUROL'' is an acronym for ''fuel and road oil''. Saybolt universal viscosity is specified ...
(SUS) or
centipoise The poise (symbol P; ) is the unit of dynamic viscosity (absolute viscosity) in the centimetre–gram–second system of units (CGS). It is named after Jean Léonard Marie Poiseuille (see Hagen–Poiseuille equation). The centipoise (1 cP = 0 ...
. 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 An industrial process control in continuous production processes is a discipline that uses industrial control systems to achieve a production level of consistency, economy and safety which could not be achieved purely by human manual control. ...
environments.


Oscillating-piston viscometer

Sometimes referred to as electromagnetic viscometer or EMV viscometer, was invented a
Cambridge Viscosity (Formally Cambridge Applied Systems)
in 1986. The sensor (see figure below) comprises a measurement chamber and magnetically influenced piston. Measurements are taken whereby a sample is first introduced into the thermally controlled measurement chamber where the piston resides. Electronics drive the piston into oscillatory motion within the measurement chamber with a controlled magnetic field. A shear stress is imposed on the liquid (or gas) due to the piston travel, and the viscosity is determined 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 distance of the piston are used to calculate the viscosity according to Newton's Law of Viscosity. The oscillating-piston viscometer technology has been adapted for small-sample viscosity and micro-sample viscosity testing in laboratory applications. It has also been adapted to high-pressure viscosity and high-temperature 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 and engines, flow-through viscometers for dip coating processes, in-line viscometers for use in refineries, and hundreds of other applications. Improvements in sensitivity from modern electronics, is stimulating a growth in oscillating-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 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 beforehand. 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, including those with fibers (up to 1000 Pa·s). Currently, many industries around the world consider these viscometers to be the most efficient system with which to measure the viscosities of a wide range of fluids; by contrast, rotational viscometers require more maintenance, are unable to measure clogging fluid, and require frequent calibration after intensive use. Vibrating viscometers have no moving parts, no weak parts and the sensitive part is typically small. Even very
basic BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
or
acidic In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...
fluids can be measured by adding a protective coating, such as enamel, or by changing the material of the sensor to a material such as 316L
stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's r ...
. Vibrating viscometers are the most widely used inline instrument to monitor the viscosity of the process fluid in tanks, and pipes.


Quartz viscometer

The quartz viscometer is a special type of vibrational viscometer. Here, an oscillating quartz crystal is immersed into a fluid and the specific influence on the oscillating behavior defines the viscosity. The principle of quartz viscosimetry is based on the idea of W. P. Mason. The basic concept is the application of a piezoelectric crystal for the determination of viscosity. The high-frequency electric field that is applied to the oscillator causes a movement of the sensor and results in the shearing of the fluid. The movement of the sensor is then influenced by the external forces (the shear stress) of the fluid, which affects the electrical response of the sensor. The calibration procedure as a pre-condition of viscosity determination by means of a quartz crystal goes back to B. Bode, who facilitated the detailed analysis of the electrical and mechanical transmission behavior of the oscillating system. On the basis of this calibration, the quartz viscosimeter was developed which allows continuous viscosity determination in resting and flowing liquids.


Quartz Crystal Microbalance

The
quartz crystal microbalance A quartz crystal microbalance (QCM) (also known as ''quartz microbalance'' (QMB), sometimes also as ''quartz crystal nanobalance'' (QCN)) measures a mass variation per unit area by measuring the change in frequency of a quartz crystal resonator. The ...
functions as a vibrational viscometer by the piezoelectric properties inherent in quartz to perform measurements of conductance spectra of liquids and thin films exposed to the surface of the crystal. From these spectra, frequency shifts and a broadening of the peaks for the resonant and overtone frequencies of the quartz crystal are tracked and used to determine changes in mass as well as the
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
,
shear modulus In materials science, shear modulus or modulus of rigidity, denoted by ''G'', or sometimes ''S'' or ''μ'', is a measure of the elastic shear stiffness of a material and is defined as the ratio of shear stress to the shear strain: :G \ \stackre ...
, and other viscoelastic properties of the liquid or thin film. One benefit of using the quartz crystal microbalance to measure viscosity is the small amount of sample required for obtaining an accurate measurement. However, due to the dependence viscoelastic properties on the sample preparation techniques and thickness of the film or bulk liquid, there can be errors up to 10% in measurements in viscosity between samples. An interesting technique to measure the viscosity of a liquid using a quartz crystal microbalance which improves the consistency of measurements uses a drop method. Instead of creating a thin film or submerging the quartz crystal in a liquid, a single drop of the fluid of interest is dropped on the surface of the crystal. The viscosity is extracted from the shift in the frequency data using the following equation \Delta f = -f_0^\sqrt where f_0 is the resonant frequency, \rho_l is the density of the fluid, \mu_Q is the shear modulus of the quartz, and \rho_Q is the density of the quartz. An extension of this technique corrects the shift in the resonant frequency by the size of the drop deposited on the quartz crystal.


Rotational viscometers

Rotational viscometers use the idea that the torque required to turn an object in a fluid is a function of the viscosity of that fluid. They measure the torque required to rotate a disk or bob in a fluid at a known speed. "Cup and bob" viscometers work by defining the exact volume of a sample 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 at very high shear rates, but the rotating bob is more commonly used, as the instrument design can be more flexible for other geometries as well. "Cone and plate" viscometers use a narrow-angled cone in close proximity to a flat plate. With this system, the shear rate between the geometries is constant at any given rotational speed. The viscosity can easily be calculated from shear stress (from the torque) and shear rate (from the angular velocity). If a test with any geometries runs through a table of several shear rates or stresses, the data can be used to plot a flow curve, that is a graph of viscosity vs shear rate. If the above test is carried out slowly enough for the measured value (shear stress if rate is being controlled, or conversely) to reach a steady value at each step, the data is said to be at "equilibrium", and the graph is then an "equilibrium flow curve". This is preferable over non-equilibrium measurements, as the data can usually be replicated across multiple other instruments or with other geometries.


Calculation of shear rate and shear stress form factors

Rheometers and viscometers work with torque and angular velocity. Since viscosity is normally considered in terms of shear stress and shear rates, a method is needed to convert from "instrument numbers" to "rheology numbers". Each measuring system used in an instrument has its associated "form factors" to convert torque to shear stress and to convert angular velocity to shear rate. We will call the shear stress form factor and the shear rate factor . : shear stress = torque ÷ . : shear rate = × angular velocity. :: For some measuring systems such as parallel plates, the user can set the gap between the measuring systems. In this case the equation used is ::: shear rate = × angular velocity / gap. : viscosity = shear stress / shear rate. The following sections show how the form factors are calculated for each measuring system.


Cone and plate

: \begin C_1 &= \frac r^3, \\ C_2 &= \frac, \end where : is the radius of the cone, : is the cone angle in radians.


Parallel plates

: \begin C_1 &= \frac r^3, \\ C_2 &= \frac r, \end where is the radius of the plate. Note: The shear stress varies across the radius for a parallel plate. The above formula refers to the 3/4 radius position if the test sample is Newtonian.


Coaxial cylinders

: \begin C_1 &= \frac r_\text^2 H, \\ C_2 &= \frac, \end where: : is the average radius, : is the inner radius, : is the outer radius, : is the height of cylinder. Note: takes the shear stress as that occurring at an average radius .


Electromagnetically spinning-sphere viscometer (EMS viscometer)

The EMS viscometer measures the viscosity of liquids through observation of the rotation of a sphere driven by electromagnetic interaction: Two magnets attached to a rotor create a rotating magnetic field. The sample ③ to be measured is in a small test tube ②. Inside the tube is an aluminium sphere ④. The tube is located in a temperature-controlled chamber ① and set such that the sphere is situated in the centre of the two magnets. The rotating magnetic field induces eddy currents in the sphere. The resulting Lorentz interaction between the magnetic field and these eddy currents generate torque that rotates the sphere. The rotational speed of the sphere depends on the rotational velocity of the magnetic field, the magnitude of the magnetic field and the viscosity of the sample around the sphere. The motion of the sphere is monitored by a video camera ⑤ located below the cell. The torque applied to the sphere is proportional to the difference in the angular velocity of the magnetic field and the one of the sphere . There is thus a linear relationship between and the viscosity of the liquid. This new measuring principle was developed by Sakai et al. at the University of Tokyo. The EMS viscometer distinguishes itself from other rotational viscometers by three main characteristics: * All parts of the viscometer that come in direct contact with the sample are disposable and inexpensive. * The measurements are performed in a sealed sample vessel. * The EMS viscometer requires only very small sample quantities (0.3 mL).


Stabinger viscometer

By modifying the classic Couette-type rotational viscometer, it is possible to combine the accuracy of kinematic viscosity determination with a wide measuring range. The outer cylinder of the Stabinger viscometer is a sample-filled tube that rotates at constant speed in a temperature-controlled copper housing. The hollow internal cylinder – shaped as a conical rotor – is centered within the sample by hydrodynamic lubricationBeitz, W. and Küttner, K.-H., English edition by Davies, B. J., translation by Shields, M. J. (1994). Dubbel Handbook of Mechanical Engineering. London: Springer-Verlag Ltd., p. F89. effects and
centrifugal force In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is paralle ...
s. In this way all bearing
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of ...
, an inevitable factor in most rotational devices, is fully avoided. The rotating fluid's shear forces drive the rotor, while a magnet inside the rotor forms an
eddy current brake An eddy current brake, also known as an induction brake, electric brake or electric retarder, is a device used to slow or stop a moving object by generating eddy currents and thus dissipating its kinetic energy as heat. Unlike friction brakes, wh ...
with the surrounding copper housing. An equilibrium rotor speed is established between driving and retarding forces, which is an unambiguous measure of the dynamic viscosity. The
speed In everyday use and in kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a scalar quant ...
and
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
measurement is implemented without direct contact by a Hall-effect sensor counting the frequency of the rotating
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
. This allows a highly precise
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
resolution of 50  pN·m and a wide measuring range from 0.2 to 30,000 mPa·s with a single measuring system. A built-in
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
measurement based on the oscillating U-tube principle allows the determination of kinematic
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
from the measured dynamic viscosity employing the relation : \nu = \frac, where: : is the kinematic viscosity (mm2/s), : is the dynamic viscosity (mPa·s), : is the density (g/cm3).


Bubble viscometer

Bubble viscometers are used to quickly determine kinematic viscosity of known liquids such as resins and varnishes. The time required for an air bubble to rise is directly proportional to the viscosity of the liquid, so the faster the bubble rises, the lower the viscosity. The alphabetical-comparison method uses 4 sets of lettered reference tubes, A5 through Z10, of known viscosity to cover a viscosity range from 0.005 to 1,000 stokes. The direct-time method uses a single 3-line times tube for determining the "bubble seconds", which may then be converted to stokes.ASTM Paint and Coatings Manual 0-8031-2060-5. This method is considerably accurate, but the measurements can vary due to variances in buoyancy because of the changing in shape of the bubble in the tube. However, this does not cause any sort of serious miscalculation.


Rectangular-slit viscometer

The basic design of a rectangular-slit viscometer/rheometer consists of a rectangular-slit channel with uniform cross-sectional area. A test liquid is pumped at a constant flow rate through this channel. Multiple pressure sensors flush-mounted at linear distances along the stream-wise direction measure pressure drop as depicted in the figure: Measuring principle: The slit viscometer/rheometer is based on the fundamental principle that a viscous liquid resists flow, exhibiting a decreasing pressure along the length of the slit. The pressure decrease or drop () is correlated with the shear stress at the wall boundary. The apparent shear rate is directly related to the flow rate and the dimension of the slit. The apparent shear rate, the shear stress, and the
apparent viscosity In fluid mechanics, apparent viscosity (sometimes denoted ) is the shear stress applied to a fluid divided by the shear rate: :\eta = \frac For a Newtonian fluid, the apparent viscosity is constant, and equal to the Newtonian viscosity of ...
are calculated: : \begin \dot_\text &= \frac, \\ \sigma &= \frac \frac, \\ \eta_\text &= \frac, \end where : \dot is the apparent shear rate (s−1), : is the shear stress (Pa), : is the apparent viscosity (Pa·s), : is the pressure difference between the leading pressure sensor and the last pressure sensor (Pa), : is the flow rate (ml/s), : is the width of the flow channel (mm), : is the depth of the flow channel (mm), : is the distance between the leading pressure sensor and the last pressure sensor (mm). To determine the viscosity of a liquid, the liquid sample is pumped through the slit channel at a constant flow rate, and the pressure drop is measured. Following these equations, the apparent viscosity is calculated for the apparent shear rate. For a Newtonian liquid, the apparent viscosity is the same as the true viscosity, and the single shear-rate measurement is sufficient. For non-Newtonian liquids, the apparent viscosity is not true viscosity. In order to obtain true viscosity, the apparent viscosities are measured at multiple apparent shear rates. Then true viscosities at various shear rates are calculated using Weissenberg–Rabinowitsch–Mooney correction factor: : \frac = \frac\left(2 + \frac\right). The calculated true viscosity is the same as the cone and plate values at the same shear rate. A modified version of the rectangular-slit viscometer/rheometer can also be used to determine apparent extensional viscosity.


Krebs Viscometer

The Krebs Viscometer uses a digital graph and a small sidearm spindle to measure the viscosity of a fluid. It is mostly used in the paint industry.


Miscellaneous viscometer types

Other viscometer types use balls or other objects. Viscometers that can characterize
non-Newtonian fluid A non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity, i.e., constant viscosity independent of stress. In non-Newtonian fluids, viscosity can change when under force to either more liquid or more solid. Ketchup, for ex ...
s are usually called ''
rheometer A rheometer is a laboratory device used to measure the way in which a dense fluid (a liquid, suspension or slurry) flows in response to applied forces. It is used for those fluids which cannot be defined by a single value of viscosity and t ...
s'' or '' plastometers''. 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. The
Marsh funnel The Marsh funnel is a simple device for measuring viscosity by observing the time it takes a known volume of liquid to flow from a cone through a short tube. It is standardized for use by mud engineers to check the quality of drilling mud. Other ...
viscometer measures viscosity from the time (''efflux time'') it takes a known volume of liquid to flow from the base of a cone through a short tube. This is similar in principle to the flow cups (efflux cups) like the
Ford Ford commonly refers to: * Ford Motor Company, an automobile manufacturer founded by Henry Ford * Ford (crossing), a shallow crossing on a river Ford may also refer to: Ford Motor Company * Henry Ford, founder of the Ford Motor Company * Ford F ...
, Zahn and
Shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses ** Thin-shell structure Science Biology * Seashell, a hard o ...
cups which use different shapes to the cone and various nozzle sizes. The measurements can be done according to ISO 2431,
ASTM ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
D1200 - 10 or
DIN DIN or Din or din may refer to: People and language * Din (name), people with the name * Dīn, an Arabic word with three general senses: judgment, custom, and religion from which the name originates * Dinka language (ISO 639 code: din), spoken by ...
53411. Th
flexible-blade rheometer
improves the accuracy of measurements for the lower-viscosity liquids utilizing the subtle changes in the flow field due to the flexibility of the moving or stationary blade (sometimes called wing or single-side-clamped cantilever).


See also

*
Flow measurement Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways. The common types of flowmeters with industrial applications are listed below: * a) Obstruction type (differential pressure or variable area ...
*
Poiseuille equation The poiseuille (symbol Pl) has been proposed as a derived SI unit of dynamic viscosity, named after the French physicist Jean Léonard Marie Poiseuille (1797–1869). In practice the unit has never been widely accepted and most international s ...
*
Viscotherm Viscotherms is a general name used to describe equipment for control of viscosity and temperature of a fluid, in particular of fuel oil in fuel viscosity control systems. The term originated from a brand name Viscotherm registered by VAF Instrumen ...


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


External links


RHEOTEST Medingen GmbH
- History and Collection of rheological instruments from the time of Fritz Höppler
ASTM International
(ASTM D7042)



- Alpha Technologies (formerly Monsanto Instruments and Equipment) - Akron, Ohio USA
Viscopedia , A free knowledge base for viscosity
{{Laboratory equipment Polymers Viscosity meters Fluid dynamics