Kinematic viscometer
viscometer

Standard Laboratory Viscometers for Liquids
U-shaped viscometers
These devices are also known as glass capillary viscometers or Ostwald viscometers , named after Wilhelm Ostwald . Another version is the Ubbelohde viscometer , which consists of a U-shaped glass tube installed vertically in a temperature-controlled bath. In one of the arms of the U there is a vertical section of a narrow fine lumen (capillaries)
. Above is a bulb, with another bulb under the other arm. During use, the fluid is drawn into the upper bulb by suction, and then allowed to flow down through the capillaries to the lower bulb. Two marks (one above the top lamp and one below the top lamp) indicate a known volume. The time it takes for the fluid level to pass between these marks is proportional to the kinematic viscosity. Calibration can be done with a liquid of known properties. Most commercial units are provided with a conversion factor.
The time required for the test fluid to flow through a capillary tube of a known diameter for a given factor is measured between two specified points. By multiplying the time taken by the viscometer factor, the kinematic viscosity is obtained.
These viscometers can be classified as direct flow or reverse flow. Reverse flow viscometers have the tank above the marks, and direct flow ones have the tank below the marks. Such ratings exist so that the level can be determined even when measuring opaque or smeared liquids, otherwise the liquid will cover the marks and make it impossible to measure when the level exceeds the mark.
This also allows the viscometer to have more than one set of marks to allow immediate timing of the time it takes to reach the third mark [ clarification ] , thus yielding two timings and allowing subsequent computation of determination to ensure accurate results. Two timings in one viscometer can only be used in one cycle if the sample being measured has Newtonian properties. Otherwise a change in the driving head, which in turn changes the shear rate, will result in a different viscosity for the two lamps.
Falling field viscometers
Stokes’ law is the basis of the incident field viscometer, in which a liquid is stationary in a vertical glass tube. A ball of known size and density is allowed to descend through the liquid. If chosen correctly, it reaches a terminal velocity , which can be measured by the time it takes to pass two marks on the tube.
Electronic sensing of opaque liquids can be used. Knowing the final velocity, ball size, density, and fluid density , Stokes’ law can be used to calculate the fluid’s viscosity . In the classical experiment, a series of steel ball bearings of different diameters is usually used to improve the accuracy of the calculation.
The school experiment uses glycerol as a liquid, and the technique is used industrially to check the viscosity of fluids used in operations. It contains many oils and polymeric fluidsdifferent types as solutions [ disambiguation ] .
In 1851, George Gabriel Stokes derived an expression for the frictional force (also called the drag force ) exerted on spherical bodies with very small Reynolds numbers (for example, very small particles) in a continuous viscous liquid by changing the small liquid’s mass limit from Navier’s equations Stokes is generally insoluble.
where
is the force of friction,
is the radius of a spherical body,
is the viscosity of fluids,
is the velocity of the particle.
If the particles fall into the viscous fluid according to their weight, then the final velocity, also known as the settling velocity, is reached when this force of friction and the buoyant force are exactly balanced with the force of gravity. The resultant settling velocity (or final velocity) is given by
where:
- V s is the particle leveling velocity (m/s), vertically downward if ρ r > ρ f , upward if ρ r < ρ f ,
- r is the Stokes radius of the particle (m),
- g is the gravitational acceleration (m/s 2 ),
- Ρ P isparticle (kg / m density 3 ),
- ρ f is the density of the liquid (kg / m 3 ),
- μ is the viscosity of the (dynamic) fluid (Pa s).
Note that Stokes flow is assumed, so the Reynolds number must be small.
The determining factor for the validity of this result is the roughness of the ball used.
The straight falling ball viscometer adjustment is a rolling ball viscometer, which sets the ball rolling down a slope while submerged in the test liquid. This can be further improved by using the patented V-plate, which increases the number of cycles for mileage, allowing for smaller, more portable devices. The controlled rolling motion of the ball avoids turbulence in the fluid, which would otherwise occur with a falling ball. [2] This type of device is also suitable for use on board ships. [ Why? ]
viscosity ball fall
Piston falling viscometer
oscillating piston viscometer
vibratory viscometers
- Measure the power input required to maintain the vibration of an oscillator at a constant amplitude. The higher the viscosity, the more energy is required to maintain the amplitude of oscillation.
- Measure the oscillation decay time once the excitation is stopped. The higher the viscosity, the faster the signal wears out.
- Measurement of resonant frequency as a function of the phase angle between excitation and response waveforms. The higher the viscosity, the greater the frequency change for a given phase change.
The vibrating instrument also suffers from a lack of a specific shear field, which makes it unsuitable for measuring the viscosity of a fluid whose flow behavior is not known in advance.
Vibrational viscometers are durable industrial systems used to measure process condition viscosities. The active part of the sensor is a vibrating rod. The amplitude of the vibration varies according to the viscosity of the fluid in which the rod is immersed.
These viscometers are suitable for measuring blockage of fluids and highly viscous fluids, including those containing fibers (up to 1000 Pa). Currently, many industries around the world consider these viscometers to be the most efficient system for measuring the viscosity of a wide range of liquids; In contrast, rotational viscometers require more maintenance, are unable to measure clogging fluid, and require frequent calibration after extensive use. Vibrational viscometers have no moving parts and no weak parts and the sensitive part is usually small
. Even very basic or acidic liquids can be measured by adding a protective layer, such as enamel, or by changing the sensor material to a material such as 316L stainless steel.
quartz viscometer
Quartz viscometer is a special type of vibratory viscometer. Here, an oscillating quartz crystal is immersed in a liquid and the specific effect on the viscous oscillation behavior determines. The principle of viscosity measurement in quartz is based on the idea of W.B. Mason.
The basic concept is the application of a piezoelectric crystal to determine its viscosity. The high-frequency electric field applied to the oscillator causes the sensor to move and causes the fluid to shear. The motion of the sensor is then affected by external forces (shear stress) of the fluid, which affects the electrical response of the sensor.
The calibration procedure as a prerequisite for determining viscosity by means of a quartz crystal is due to B. Bode, who facilitated detailed analysis of the electrical and mechanical transmission behavior of the oscillating system. On the basis of this calibration, a quartz viscometer has been developed that allows continuous viscosity determination in flowing and resting liquids.]
The exact amount of quartz crystal
The quartz micro-crystal microbalance acts as a vibratory viscometer through the inherent piezoelectric properties of quartz to make measurements of the conduction spectra of liquids and thin films exposed to the surface of the crystal.
From these spectra, frequency shifts and broadening peaks for the resonant and predominant frequencies of the quartz crystal are tracked and used to determine changes in mass as well as viscosity, shear modulus, and other viscoelastic properties of the liquid or thin.
One benefit of using quartz microbalance for viscosity measurement is the small amount of sample required to obtain an accurate measurement. However, as the viscoelastic properties depend on the sample preparation techniques and the thickness of the film or bulk liquid, there can be errors of up to 10% in viscosity measurements between samples.
An interesting technique for measuring the viscosity of a liquid using a quartz crystal microbalance which improves the consistency of measurements using the drop method. [7] [8] Instead of forming a thin film or immersing the quartz crystal in a liquid, one drop of the liquid in question is dropped onto the surface of the crystal. The viscosity is extracted from the shift in frequency data using the following equation.
rotational viscometers
Calculation of shear rate and shear stress factors
Viscometers and torque gauges work with torque and angular velocity. Since viscosity is usually considered in terms of shear stresses and shear rates, a method is needed to convert from “hardware numbers” to “rheology numbers”. Each measurement system used in an instrument has its associated “form factors” to convert torque to shear stress and convert angular velocity to shear rate.
We will call the shear stress form factor C 1 and the shear rate factor C 2 .
- Shear stress = torque ÷ C 1 .
- Shear rate = C 2 x angular velocity.
- For some metering systems such as parallel panels, the user can adjust the gap between the metering systems. In this case the equation used is
- Shear rate = C 2 x angular velocity/gap.
- For some metering systems such as parallel panels, the user can adjust the gap between the metering systems. In this case the equation used is
- Viscosity = shear stress / shear rate.
The following sections explain how to calculate the form factors for each measurement system.
Cone Wolf
where
- r is the radius of the cone,
- θ is the angle of the cone in radians.
parallel panels
where r is the radius of the plate.
Note: The shear stress varies across the radius of the parallel plate. The above formula indicates the position of the radius 3/4 if the test sample is Newtonian.
Coaxial cylinders
where:
- r a = ( r i + r o ) / 2 is the mean radius,
- r i is the inner radius,
- y x is the outer radius,
- H is the height of the cylinder.
Note: C 1 takes the shear stress as that which occurs at the mean radius r a .
Electromagnetic electromagnetic viscometer (EMS viscometer)
Measurement of the principle of viscometer in the electromagnetic field
The EMS viscometer measures the viscosity of fluids by observing the rotation of a ball driven by electromagnetic interaction: two magnets attached to a rotor create a rotating magnetic field. The sample ③ to be measured is contained in a small test tube ②. Inside the tube is an aluminum ball. The tube is located in a temperature controlled chamber ① and is set so that the ball is at the center of the two magnets.
The rotating magnetic field induces eddy currents in the ball. The resulting Lorentz interaction between the magnetic field and these eddy currents results in a torque rotating the ball. The rotation speed of the ball depends on the speed of rotation of the magnetic field, the size of the magnetic field, and the viscosity of the sample around the ball.
The movement of the ball is monitored by a video camera located under the cell. They applied torque on a scale commensurate with the difference in the corner speed of the magnetic field Ω B one of the area Ω S . Thus, there is a linear relationship between ( Ω B – Ω S ) / Ω S and the viscosity of the fluid.
This new measurement principle was developed by Sakai et al. at the University of Tokyo. The EMS viscometer distinguishes itself from other rotational viscometers with three main characteristics:
- All parts of the viscometer that are in direct contact with the sample are disposable and inexpensive.
- Measurements are made in a sealed sample container.
- The EMS viscometer only requires very small samples (0.3 ml).
Stabinger viscometer
By modifying the classic Couette type rotational viscometer, it is possible to combine the accuracy of determining the kinematic viscosity with a wide measurement range.
The external cylinder of a Stabinger viscometer is a sample-filled tube rotating at a constant speed in a temperature-controlled copper sheath. The hollow inner cylinder – shaped like a conical rotor – is centered inside the sample by the effects of hydrodynamic lubrication and centrifugal forces.
In this way the friction of the bearings is completely avoided, which is an inevitable factor in most rotating devices. The shear forces of the fluid drive the rotor rotor, while the magnet inside the rotor forms a vortex current brake with the surrounding copper housing. The equilibrium rotational speed is created between the driving and damping forces, which is an unambiguous measure of dynamic viscosity.
The speed and torque measurement is performed without direct contact by a Hall-effect sensor counting the frequency of the rotating magnetic field. This allows for a high torque accuracy of 50 pN m and a wide measurement range from 0.2 to 30,000 MPa with a single measurement system. Built-in density measurement based on the oscillating U tube principle allows determination of the kinematic viscosity from the measured dynamic viscosity using the relationship
where:
- ν is the kinetic viscosity (mm 2 / s),
- η is the dynamic viscosity (mPa s),
- ρ is the density (g / cm 3 ).
bubble viscometer
Rectangular slit viscometer
where
is the apparent shear rate (s −1 ),
- σ is the shear stress (Pa),
- η a is the apparent viscosity (Pa s),
- ∆ P is the pressure difference between the main pressure sensor and the last pressure sensor (Pa),
- Q is the flow rate (ml/s),
- w is the width of the flow channel (mm),
- h is the depth of the flow channel (mm),
- l is the distance between the main pressure sensor and the last pressure sensor (mm).
To determine the viscosity of a fluid, the fluid sample is pumped through the slit channel at a constant flow rate, and the pressure drop is measured. After these equations, the apparent viscosity of the apparent shear rate is calculated. For the Newtonian fluid, the apparent viscosity is the same as the true viscosity, and a single shear rate measurement is sufficient.
For non-Newtonian fluids, the apparent viscosity is not true viscosity. In order to obtain true viscosity, apparent viscosity is measured at multiple apparent shear rates. The true viscosity η is then calculated at different shear rates using the Weissenberg- Rabinowitsch -Mooney correction factor:
The calculated real viscosity is the same as the cone and plate values for the same shear rate.
A modified version of the rectangular slit viscometer/rheometer can also be used to determine the apparent stretch viscosity.
Krebs Viscometer
Various types of viscometers

Capillary Viscometers
Kinematic viscosity is determined by passing the sample through a narrow tube and measuring how long it takes the sample to pass through. Viscosity can be calculated using the density of the sample (kinematic viscosity = viscosity ÷ density). Capillary tube viscometers can measure Newtonian fluids with a relatively high level of accuracy.
These viscometers are low cost and have been in use for a long time, but the narrow tube makes cleaning a complicated process, which inconveniences the user. ►Uses: petroleum products and pharmaceutical industries.
Falling Ball Viscometers
Viscosity is determined by dropping a ball into the sample and measuring how long it takes for it to fall to the bottom. Since the ball is met by the fluid resistance of the sample, the velocity of the ball’s fall varies with the viscosity of the sample. These viscometers can accurately measure the viscosity of Newtonian fluids, from low viscosity fluids such as water to highly viscous fluids such as oil.
Vibrational Viscometers
Viscosity is determined by inserting a vibrating instrument into the sample and measuring the viscosity resistance when vibrating at a constant frequency. Vibrational viscometers have a fast response time and can perform sequential measurements. They are used as on-line process viscometers for their ability to measure liquid as it flows.
viscosity cup
Viscosity is determined by filling a metal cup with a sample, draining the sample through an opening (hole) in the base of the cup, and measuring how long it takes for the sample to flow out. The time it takes for a sample to flow out of the hole is generally measured using a stopwatch. ►Uses: industrial coatings and oils (rarely used in the food industry).
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