6 Ways to Measure Fluid Viscosity
It's important to understand the difference between dynamic and kinematic viscosity and to take appropriate test mechanisms for the sample at hand.
Essentially, viscosity indicates a fluid’s resistance to deformation by shear or tensile stresses. In other words, this property describes the friction between the fluid molecules causing opposition relative motion between fluid layers moving at different velocities. Viscosity can be a clue about how a fluid will behave under an applied force or its own weight.
The more viscous a fluid is, the “thicker” it appears to be. For instance, oil or grease have higher viscosities than water and therefore appear thicker.
Oil, coatings, paint and adhesive manufacturers are often tasked with determining the optimum viscosity of their products for specific applications. (For more on this topic, see: Service Requirements & Environmental Factors for Coatings.)
Low viscosity fluids tend to flow more easily. Therefore, having a coating with a viscosity that is too low can cause running and sagging. On the other hand, a coating with a viscosity that is too high can be “stiff” and difficult to apply.
Dynamic viscosity, also known as absolute viscosity, is a fluid’s resistance to shear flow due to an applied external force. It describes the amount of internal resistance offered when one layer of the fluid moves over another layer in a horizontal plane.
Dynamic viscosity is especially useful when describing non-Newtonian fluids.
Mathematically, dynamic viscosity can be expressed as:
μ = τ dy / dc = τ/γ
- τ = shearing stress in fluid (N/m2).
- μ = dynamic viscosity of fluid (N s/m2).
- dc = unit velocity (m/s).
- dy = unit distance between layers (m).
- γ = dc / dy = shear rate (s-1).
The SI unit for dynamic viscosity is N s/m2 or the Pascal-second (Pa s). Another unit of measurement for dynamic viscosity is poise (p), where one poise equals one-teneth N s/m2 or 1/10 Pa s.
The poise unit can sometimes be too large for practical purposes. For this reason, centipoise (cP) unit is often used in its place. In the centipoise unit, one cP equals 0.01P, 0.001 N s/m2 or 0.001 Pa s.
Kinematic viscosity is simply the ratio of the dynamic viscosity to the fluid's density. It reflects a fluid’s resistance to shear flow under the influence of gravity, i.e., shear flow due to the fluid’s own weight.
This viscosity is especially useful in describing Newtonian fluids. Mathematically, kinematic viscosity can be expressed as:
ν = μ / ρ
- ν = kinematic viscosity (m2/s).
- μ = absolute or dynamic viscosity (N s/m2).
- ρ = density (kg/m3).
The SI unit for dynamic viscosity is m2/s. Another unit of measurement for this property is Stoke (St), where one St equals 10-4 m2/s equals 1 cm2/s.
Where the viscosity value in Stoke is too large, the smaller unit centistoke (cSt) is often used in its place. In the centistoke, one cSt equals 10-6 m2/s = 1 mm2/s.
How Is Viscosity Measured?
There are several different methods to measure both dynamic and kinematic viscosity. Some of the most common methods are as follows:
1. Viscosity Cups
Viscosity cups are used to determine a fluid’s kinematic viscosity and are typically made of anodized aluminium with a stainless steel orifice. (For more on this topic, see: Understanding Aluminum Corrosion.)
This relatively simple test involves placing the fluid in a container with a small opening at the bottom. The fluid is allowed to flow through the opening in a precise amount. The time it takes for the fluid to pass through the opening is measured and correlated to viscosity through the use of charts supplied for the given cup.
Viscosity cups are typically used for measuring the consistency of paints, varnishes and similar products. A table is then used to convert efflux time (in seconds) to viscosity in centistokes (cSt).
Ford and Zahn cups are some of the most commonly used viscosity cup varieties. Each cup design is unique; thus care must be taken when comparing viscosity values between different cup types. The values a viscosity cup provides is an absolute value and does not include the allowed tolerances—as these differ considerably between each of the standards.
2. Vibrational Viscometers
Vibrational viscometers operate by immersing an oscillating electromechanical resonator in the test fluid and measuring the degree of damping the fluid offers. The resonator generally oscillates either torsionally or transversely and the damping may be determined by:
- Recording the power input required to keep the apparatus vibrating at a constant amplitude.
- Measuring the time decay of the oscillation after vibration is switched off.
- Measuring the frequency of the resonator with respect to varying phase angles.
The quartz viscometer is one example of a vibrational viscometer. With this method, an oscillating quartz crystal is immersed into a fluid and the specific influence on the oscillating behavior defines the viscosity. An electrical field applied to the oscillator causes the sensor to move and results in the fluid shearing. (For more on this topic, see: Corrosion and Electrical Interference in Buried Metallic Structures.)
The sensor's movement is then influenced by the external forces (the shear stress) of the fluid, which affects the electrical response of the sensor.
3. Rotational Viscometers
Rotational viscometers work by measuring the torque required to rotate an object in the test fluid. Here's how the process pans out:
- One of the surfaces is stationary.
- The mating surface is rotated by an external drive.
- Fluid fills the space in between the surfaces. The torque required to rotate a disk or bob at a predetermined speed is measured and recorded.
The torque maintaining the set speed is directly proportional to the viscosity; therefore, the apparatus is capable of outputting viscosity, shear stress and shear rate values. Because an external shear force is applied to the liquid, rotational viscometers measure a fluid’s dynamic viscosity.
Cups, bobs, cones and plates are all types of rotational viscometers. Cup and bob viscometers consist of coaxial cylinders of different diameters. A volume of a sample to be sheared is stored within a test cell; the torque required to achieve a certain rotational speed is measured and plotted.
Cone and plate viscometers have a precise torque meter which is driven as discrete rotational speed. It uses narrow-angled cone in close proximity to a flat plate. The viscosity is calculated from shear stress and shear rate. (For more on this topic, see: The Effects of Corrosion on the Shear Behavior of Materials.)
4. Capillary Viscometers
The capillary viscometer is one of the earliest known methods to determine fluid viscosity.
This method measures the time taken for a defined volume of fluid to flow through a U-shaped capillary tube of known diameter and length. The tube usually has two marks—an upper and lower mark—which are used as a measurement reference. The time it takes for the fluid to flow past these marks is proportional to the kinematic viscosity; hence the viscosity can be determined using standard formulas.
Capillary viscometers include the Ostwald and Ubbelohde viscometers. Both are U-shaped instruments, have two glass bulbs and use capillary tubes. (For more on how glass can prevent corrosion, see: A Look at Corrosion Barrier Linings for Internal Corrosion Protection.)
One major advantage of Ubbelohde viscometer, however, is that the values it obtains are independent of the total volume of the liquid used. The key difference between Ostwald and Ubbelohde viscometers is that the Ostwald viscometer is suitable for measuring low- to moderate-viscosity liquids, whereas the Ubbelohde viscometer is suitable for measuring high-viscosity liquids.
5. Falling Sphere Viscometers
The falling sphere viscometer is used to determine transparent Newtonian fluid's dynamic viscosity.
The concept involves measuring the time it takes for a sphere of known density to fall through a sample-filled tube under gravity. The tube is usually mounted on an apparatus that can quickly rotate 180 degrees to allow repeat testing. The average time of three tests is recorded and used in a conversion formula to determine the viscosity of the sample.
Falling sphere viscometers are used for quality control in various industries as well as in academic institutions to illustrate scientific method. The ease of use and straightforward method for recording time measurements ensure meaningful test results.
A consistometer is an apparatus comprised of a metal trough with a small section barred behind a spring-loaded gate. Here's how it works:
- The sample to be tested is placed behind the spring-loaded gate.
- The gate is lifted, allowing the sample to flow freely under its own weight.
- The distance the liquid flows in a specific time is measured via the apparatus' gradations.
The consistometer itself does not measure viscosity values directly; it instead allows users to develop their own standards specific to the products being tested. This method is more popular in the food industry and is typically used to measure the viscosity of products such as ketchup, mayonnaise, preserves, fillings, soups, baby foods and salad dressings. (For more on the food industry, see: The Corrosion Properties of Aluminum and Its Alloys.)
Factors Affecting Viscosity
There are various factors on which fluid viscosity depends. These are:
- Fluid Temperature. Usually, a liquid's viscosity decreases with an increase in temperature. However, a gas' viscosity usually increases with an increase in temperature.
- Flow Conditions. For laminar flow, a liquid's viscosity remains constant; whereas for turbulent flow viscosity changes.
- Pressure. When pressure increases, a gas' viscosity will usually increase. For liquids, because they are incompressible, pressure does not have much impact.
- Multiphase Flow. The viscosity of multiphase flow is changed by the volume of each phase.
- Suspended Particles.Suspended materials result in increase in the viscosity.
Newton’s Law of Viscosity
The relationship between a fluid's shear stress and shear rate under mechanical stress is governed by Newton’s law of viscosity.
Newton’s viscosity law states that, for a given temperature and pressure, the shear stress between two adjacent layers in a fluid is proportional to the velocity gradients between those layers. Put differently, the ratio of shear stress to shear rate in a fluid is a constant and is viscosity's coefficient.
However, Newton’s law of viscosity applies only to Newtonian fluids. Non-Newtonian fluids do not follow Newton’s law of viscosity; and therefore their viscosity changes and depends on the shear rate.
Viscosity is an important fluid property that is essential for a number of different products in various industries.
Dynamic and kinematic viscosities describe different properties and can produce very different results when testing fluids. It is therefore important to understand the difference between viscosity types and to take appropriate test mechanisms for the sample at hand.