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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.
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.
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.

Contents

Standard Laboratory Viscometers for Liquids

U-tube viscometers

Falling Sphere Viscometers

Vibrating viscometers

Rotation viscometers

Stormer viscometer

Miscellaneous viscometer types

References

External links

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. Multiplying the kinematic viscosity by the density gives the viscosity.
Common variations for particular sorts of liquid are the Cannon-Fenske and reverse flow viscometers for opaque liquids. In these it is the rise of liquid level up through a lower bulb which is measured.

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.

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 such as 316L, SUS316, Hastelloy, enamel etc...
Some companies have devices based on vibrating blades or other vessels, following the original designs patented by SOFRASER.

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) yields the viscosity in a straightforward manner.

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.
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''.
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

★ http://www.pra.org.uk/viscosityoils/notes-units.htm

External links

★ Examples of process vibrating viscometers

★ Rotaional Viscometers: from RHEOTEC