- Viscosity measurement
- Newton's theory
- Cambridge Viscosity: The Leader in Viscosity Measurement Technology
- U-tube viscometers
- Falling sphere viscometers
-Falling Piston Viscometer
- Vibrational viscometers
- Rotational viscometers
- Q-Tube™ Pressure Tube Reactors
- Stormer viscometer
- Bubble viscometer
- Miscellaneous viscometer types
- Portable Viscometer
- Bohlin Visco 88 Viscometer
- DV-E Low Cost Digital Viscometer
- VISCOpro 1600 Viscometer
-Dial Reading Viscometer
- High Shear CAP-1000+ Viscometer
- Viscometer Types
- CANNON® INSTRUMENT COMPANY GLASS CAPILLARY VISCOMETERS
Viscosity measurement
Dynamic viscosity is measured with various types of rheometer. Close temperature control of the fluid is essential to accurate measurements, particularly in materials like lubricants, whose viscosity can double with a change of only 5 °C. For some fluids, it is a constant over a wide range of shear rates. These are Newtonian fluids.
The fluids without a constant viscosity are called non-Newtonian fluids. Their viscosity cannot be described by a single number. Non-Newtonian fluids exhibit a variety of different correlations between shear stress and shear rate.
One of the most common instruments for measuring kinematic viscosity is the glass capillary viscometer.
In paint industries, viscosity is commonly measured with a Zahn cup, in which the efflux time is determined and given to customers. The efflux time can also be converted to kinematic viscosities (cSt) through the conversion equations.
A Ford viscosity cup measures the rate of flow of a liquid. This, under ideal conditions, is proportional to the kinematic viscosity.
Also used in paint, a Stormer viscometer uses load-based rotation in order to determine viscosity. The viscosity is reported in Krebs units (KU), which are unique to Stormer viscometers.
Vibrating viscometers can also be used to measure viscosity. These models such as the Dynatrol use vibration rather than rotation to measure viscosity.
In general, in any flow, layers move at different velocities and the fluid's viscosity arises from the shear stress between the layers that ultimately opposes any applied force.
Isaac Newton postulated that, for straight, parallel and uniform flow, the shear stress, τ, between layers is proportional to the velocity gradient, ∂u/∂y, in the direction perpendicular to the layers.
Here, the constant μ is known as the coefficient of viscosity, the viscosity, the dynamic viscosity, or the Newtonian viscosity. Many fluids, such as water and most gases, satisfy
The relationship between the shear stress and the velocity gradient can also be obtained by considering two plates closely spaced apart at a distance y, and separated by a homogeneous substance. Assuming that the plates are very large, with a large area A, such that edge effects may be ignored, and that the lower plate is fixed, let a force F be applied to the upper plate. If this force causes the substance between the plates to undergo shear flow (as opposed to just shearing elastically until the shear stress in the substance balances the applied force), the substance is called a fluid. The applied force is proportional to the area and velocity of the plate and inversely proportional to the distance between the plates. Combining these three relations results in the equation F = μ (Au/y), where μ is the proportionality factor called the dynamic viscosity (also called absolute viscosity, or simply viscosity). The equation can be expressed in terms of shear stress; τ = F/A = μ (u / y). The rate of shear deformation is u / y and can be also written as a shear velocity, du/dy. Hence, through this method, the relation between the shear stress and the velocity gradient can be obtained.
Viscosity Measurement Technology
Viscosity Measurement Technology
Cambridge Viscosity is the global leader in fluid viscosity measurement,
including oil and gas; petroleum and biofuels; coatings, paints,
and inks; pharmaceuticals, and other substances. Where accurate
viscosity measurement is mission critical in both process and laboratory
environments, Cambridge Viscosity technologies are chosen time and time
again.
Our leadership is based on a quarter-century of experience, the extraordinary
range of our products – including both in-line and in-tank applications – and
our innovative proprietary technology. Our customers include leading
organizations in oil and gas exploration, refining, automotive, research,
industrial materials, and other high-stakes fields.
Cambridge Viscosity has set the standard for extraordinarily precise, reliable, and virtually maintenance-free viscosity measurement systems. Our sensors and viscometer systems conform to ASTM, DIN, JIS and ISO standards, with a range of models designed to meet specific industry and application needs. At least one of them is right for you.
U-tube viscometers
These are also known as Ostwald viscometers named after Wilhelm Ostwald or glass capillary viscometers. Another type is the Ubbelohde viscometer. 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.
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
Stokes' law 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, 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 of the fluid. A series of steel ball bearings of different diameter is normally used in the classic experiment to improve the accuracy of the calculation. The school experiment uses glycerine 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 liquids such as solutions.
In 1851, George Gabriel Stokes derived an expression for the frictional force (also called drag force) exerted on spherical objects with very small Reynolds numbers (e.g., very small particles) in a continuous viscous fluid by solving the small fluid-mass limit of the generally unsolvable Navier-Stokes equations:
where:
F is the frictional force,
· r is the radius of the spherical object,
· η is the fluid viscosity, and
· v 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:
· Vs is the particles' settling velocity (m/s) (vertically downwards if ρp > ρf, upwards if ρp < ρf),
· r is the Stokes radius of the particle (m),
· g is the gravitational acceleration (m/s2),
· ρp is the density of the particles (kg/m3),
· ρf is the density of the fluid (kg/m3), and
· μ 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
Falling Piston Viscometer
Also known as Norcross viscometer due to inventor, Austin Norcross. Principle of viscosity measurement in this rugged and sensitive industrial device is based on piston and cylinder assembly. 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 which is formed below the piston as it is raised. The assembly is then held up for typically for few seconds. The assembly is then allowed to fall by gravity, expelling the sample out through the same path as it entered, creating a shearing effect on 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 measure of viscosity) and displays the resulting viscosity value. Controller can calibrate time-of-fall value to cup seconds(known efflux cup), SSU 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. Principle of operation can be adopted for many different conditions, making it ideal for process control environment
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 before hand.
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 1,000 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 very small. Even very basic or acidic fluids can be measured by adding a protective coating or by changing the material of the sensor to a material such as 316L, SUS316, Hastelloy, or enamel
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 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 measure 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) yields the viscosity in a straightforward manner.
Q-Tube™ Pressure Tube Reactors
Q-Tube pressure reactors are affordable alternatives to a microwave synthesizer and feature a safe pressure-release system (patent pending) that prevents accidental explosions due to over -pressurization. The starter kits contain all items needed for immediate use. Optional higher pressure-release threshold adapters are available separately for 12mL and 35mL Q-Tubes
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 per 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.
This method is intended for paints applied by brush or roller.
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 inversely proportional to the visosity 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.[1]
Miscellaneous viscometer types
Other viscometer types use balls or other objects. Viscometers that can characterize non-Newtonian fluids are usually called rheometers 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.
Portable Viscometer
These small, battery-operated rotational viscometers are suitable for quick and reliable tests and comparative measurements for quality control applications.
The hand-held instruments can also be operated on a stand. The operation of the instrument is especially easy due to the one button operation.
Contrary to the traditional Viscotester models where the viscosity value is read from an analog dial, the Viscotester 1 plus and 2 plus show the viscosity value on a digital display. Therefore, errors caused by misreading dials belong in the past.
Possible handling errors as well as service information are also shown on the display.
Bohlin Visco 88 Viscometer
The Bohlin Visco 88 Viscometer is an easy to use viscometer which provides accurate shear viscosity measurements at single or multiple shear rates.
The Visco 88's flexibility and construction make it appropriate for use in a variety of industries and facilitates a wide measuring range: low viscosity fluids through to viscous pastes. The Visco 88 is designed for routine testing in QC laboratories, although its large measuring range also makes it suitable for basic development applications. The Visco 88 is supplied with a rechargeable battery unit making it ideal for completely portable measurements in the field or on the production floor
DV-E Low Cost Digital Viscometer
The DV-E is
The DV-E combines economy and ease of operation with traditional
VISCOpro 1600 Viscometer
In process environments, ensuring proper viscosity is a key success factor. You need an accurate, reliable, and durable in-line viscometer capable of monitoring fluid resistance without requiring a lot of operator involvement or maintenance.
You need the Cambridge Viscosity VISCOpro1600 viscometer. Used alone or in a multi-channel configuration controlled by a touch-screen display, the VISCOpro1600 provides round-the-clock monitoring you can rely on.
The VISCOpro 1600 viscometer is a compact workhorse instrument for applications where minimal operator involvement is desired.
It features built-in optional LCD display with readout in centipoise, cSt, or SSU for monitoring of critical fluid conditions. It also can be connected to a PLC controller for seamless integration into a proprietary data management system. The unit's sensor and electronics are encased in an explosionproof housing for durability and reliability.
Thanks to Cambridge Viscosity's patented technology, only a very small amount of fluid (1 ml) is required to assure an accurate reading. Because of the small sample size and its automatic operation, the VISCOpro 1600 helps to maximize the efficiency of your process line and minimize waste. In addition, it works with any Cambridge Viscosity in-tank or in-line 300 series or 500 series sensor, giving you the flexibility to choose from a range of high-quality, maintenance-free options.
Optional Display The optional explosion-proof display clearly shows viscosity and temperature readings for each line or tank.
Dial Reading Viscometer
The original
standard used around the world. Now it's improved with
a multi-speed electronic drive and ergonomically designed
speed control knob. Quickly select any one of 10 pre-set
speeds (8 speeds on LVT models). This new direct-drive
design means extremely quiet operation and greater versatility.
The new Universal Power Supply facilitates the use of worldwide power sources.
High Shear CAP-1000+ Viscometer
The CAP 1000+ is designed for many applications including paints, coatings, adhesives, sealants, polymers and resins. For example, it meets ASTM D4287, ISO2884 and BS3900 industry standards which require high shear rate testing to formulate brushing or roller action.
The Brookfield CAP 1000+ Viscometer can be custom configured for the specific viscosity range which meets your requirement. With assistance from
The compact size of the CAP 1000+ fits easily on any test bench. This rugged design instrument is equally at home near the production line or in the QC Lab. The open-access work area makes sample prep and clean up simple and quick.
Viscometer Types
A viscometer (also called viscosimeter) is an instrument used to measure the viscosity and flow parameters of a fluid.
Glass viscometers
The classical method of measurement due to Stokes, consisted of measuring the time for a fluid to flow through a capillary tube. Refined by Cannon, Ubbelohde and others, the glass tube viscometer is still the master method for the standard determination of the viscosity of water. The viscosity of water is 0.890 mPa·s at 25 degrees Celsius, and 1.002 mPa·s at 20 degrees Celsius.
Rotational viscometers
Rotational viscometers use the idea that the force required to turn an object in a fluid, can indicate the viscosity of that fluid. The viscometer determines the required force 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. 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. 'Cone and Plate' viscometers use a cone of very shallow angle in theoretical contact with a flat plate. With this system the shear rate beneath the plate is constant to a modest degree of precision, a graph of shear stress (torque) against shear rate (angular velocity) yields the viscosity.
Rotational viscometers fall into two main types:
1. Synchronous (Stepper) Motor / Spring
2. Servo Motor / Digital encoder
The first type uses a stepper motor to drive the main shaft. A spring & pivot assembly rotate on the shaft. The spindle or rotor hangs from this assembly. As the spindle rotates the spring is deflected by the viscosity of the sample under test.
The second type uses a precision servo motor to drive the shaft. The Spindle or rotor is attached directly to the shaft. High speed microprocessors measure the speed from a digital encoder and calculate the current required to drive the rotor at the test speed. The current required is proportional to the viscosity of the sample under test.
CANNON® INSTRUMENT COMPANY GLASS CAPILLARY VISCOMETERS
ALL VISCOMETERS are SUPPLIED WITH CANNON® INSTRUMENT company’s A2LA Why does CANNON® offer so many different types of glass capillary viscometers? Primarily because no single capillary viscometer is ideally suited for all kinematic viscosity determinations. Individual analysts also have their own preferences. The brief descriptions on the following page may assist you in determining which viscometer is most appropriate for your particular application.
Expanded Uncertainty
Instead of listing the precision of Cannon® glass capillary viscometers, we now provide the expanded uncertainty. This is the universally accepted statistic when dealing with calibration data. In order to obtain A2LA accreditation, it is necessary to determine and specify the expanded uncertainty.
ISO publications define expanded uncertainty as “the interval about the result of a measurement within which the values that could reasonably be attributed to the measurand may be expected to lie with a high level of confidence.”
The use of expanded uncertainty instead of precision does not reflect any change in the accuracy or quality of Cannon viscometers. It is a statistical term that is more comprehensive than precision. When comparing the accuracy of viscometers it is essential that the expanded uncertainty be specified.
A certificate specifying the expanded uncertainty is provided with every calibrated viscometer. We also supply such a certificate with every bottle of viscosity standard.
CANNON® GLASS CAPILLARY VISCOMETERS - TYPES
Follow the blue links below for full information, photos and ordering information.
Cannon-Fenske Routine The Cannon-Fenske Routine viscometer is a rugged and inexpensive viscometer that works well if the liquid to be measured is transparent or translucent. In general, if the meniscus (the curvature at the top of the liquid column) can be readily observed through a column of liquid 3-mm in diameter, the Cannon-Fenske Routine viscometer and other transparent-type viscometers (such as the Zeitfuchs Transparent and BS/U-Tube viscometers) can be used.
Ubbelohde The Ubbelohde viscometer and other supended level viscometers are also used to measure transparent liquids. Unlike the Cannon-Fenske Routine viscometer, suspended level viscometers possess the same viscometer constant at all temperatures. This property is advantageous when measurements are to be made at a number of different temperatures. CANNON has improved the design of the Ubbelohde viscometer to make a more rugged instrument called the Cannon-Ubbelohde viscometer. Other suspended level viscometers in this bulletin include the BS/IP/SL, BS/IP/SL(S), and BS/IP/MSL viscometers.
Reverse Flow Viscometers Special viscometers have been designed for testing opaque liquids. These reverse flow type viscometers wet the timing section of the viscometer capillary only during the actual measurement. The Cannon-Fenske Opaque, Zeitfuchs Cross-Arm, and BS/IP/RFU-Tube viscometers available from CANNON Instrument Company are all reverse flow types. Reverse flow viscometers must be cleaned, dried, and refilled before a repeat measurement can be made. By contrast, other viscometer types commonly used to measure transparent liquids allow the sample to be repeatedly drawn up into the capillary, permitting duplicate measurements.
Small Volume Viscometers
In some situations, such as in a clinical laboratory, the amount of liquid available for measurement is quite small. Several viscometers have been designed which require one milliliter or less of liquid. These are refered to as semi-micro or micro viscometers. The Cannon-Manning Semi-Micro is a U-tube viscometer that has been modified to measure the kinematic viscosity of samples as small as 1.0 mL. The Cannon-Ubbelohde Semi-Micro viscometer is a modification of the standard size Cannon-Ubbelohde viscometer requiring a sample volume of only one milliliter. The Cannon-Manning Semi-Micro Extra Low Charge viscometer will permit kinematic viscosity determination with as little as 0.5 milliliters of sample.
Dilution Viscometers Estimates of the molecular size and shape of large polymer molecules can be obtained from kinematic viscosity measurements of dilute solutions of the polymers. The Cannon-Ubbelohde Dilution viscometer has an extra-large reservoir which allows polymer solutions to be diluted several times. Dilute polymer solutions frequently appear to exhibit changes in kinematic viscosity when the shear rate is changed. By using the Cannon-Ubbelohde Four-Bulb Shear Dilution Viscometer, measurements can be made at four different shear rates.
Vacuum Viscometers In most glass capillary viscometers, the samples flow under gravity. When liquids are too viscous to flow readily under gravity, vacuum viscometers may be used to measure viscosity (in mPa•s or cP). In these instruments a vacuum is applied to one end of the vicscometer to pull the liquid through the capillary into the timing bulb(s). CANNON offers several type of vacuum viscometers, including the Cannon-Manning Vacuum, the Asphalt Institute Vacuum, and the Modified Koppers Vacuum. Like the Cannon-Fenske Opaque viscometer, these are all “reverse flow” viscometers. Vacuum viscometers require a vacuum that is very accurately controlled.
Refrences
http;//en.wikipedia.or/wiki/viscosity#dynamic-viscosity
http;//www.globalspec.com/featuredproducts/detail/brookfieldenineering
http;//www.prorheo.de
http;//www.lemis-balti.com
http;//www.sofraser.com
http;//hydramotion.com
http;//www.cambridgeviscosity.com
http;//www.reologicainstruments.com
No comments:
Post a Comment