6 Ways to Measure Fluid Viscosity
Viscosity can be accurately measured using a number of tests. However, some tests measure dynamic viscosity, while others measure kinematic viscosity. It is important that the difference between these properties two be known to ensure that the desired results are obtained.
One of the most essential physical properties of industrial fluids, such as coatings, paints and adhesives is its viscosity. Viscosity is defined as a fluid’s resistance to deformation by shear or tensile stresses. In other words, this property describes the friction between the molecules of a fluid that causes opposition relative motion between fluid layers moving at different velocities. Viscosity gives an indication as to how a fluid will behave under an applied force or its own self-weight. The more viscous a fluid is, the “thicker” it appears to be. For instance, oil or grease has a higher viscosity than water and therefore appears to be thicker.
Manufacturers of oil, coatings, paints, and adhesives are often tasked with determining the optimum viscosity of their products for specific applications. (For more on this subject, see Defining Service Requirements & Environmental Factors for Coating Specification.) 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.
In this article, we will look at the different types of viscosities (dynamic and kinematic) as well as the various methods in which they are measured.
Dynamic viscosity, also known as absolute viscosity, is defined as 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 Pa.s. Another unit of measurement for dynamic viscosity is poise (p), where:
1 poise = 1/10 N s/m2 or 1/10 Pa.s
The poise unit can sometimes be too large for practical purposes, therefore, the unit centipoise (cP) is often used, where:
1cP = 0.01P, 0.001 N s/m2 or 0.001 Pa.s
Kinematic viscosity is simply the ratio of the dynamic viscosity to the density of the fluid. It reflects a fluid’s resistance to shear flow under the influence of gravity, i.e., the applied force is the fluid’s self-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:
1 St = 10-4 m2/s = 1 cm2/s
Where the viscosity value in Stoke is too large, the smaller unit centistoke (cSt) is often used, where:
1 cSt (centiStoke) = 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:
Viscosity cups are used to determine a fluid’s kinematic viscosity. 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. Some of the most commonly used cups are the Ford and Zahn cups.
Vibrational viscometers operate by immersing an oscillating electromechanical resonator in the test fluid and measuring the degree of damping offered by the fluid. 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, or
- Measuring the frequency of the resonator with respect to varying phase angles
Rotational viscometers work by measuring the torque required to rotate an object in the test fluid. 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 values of viscosity, shear stress and shear rate. Because an external shear force is being applied to the liquid, rotational viscometers measure a fluid’s dynamic viscosity.
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) that 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 value of viscosity can be determined using standard formulas.
Falling Sphere Viscometer
The falling sphere viscometer is used to determine the dynamic viscosity of a transparent Newtonian fluid. 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.
A consistometer is an apparatus that is comprised of a metal trough with a small section barred behind a spring-loaded gate. The sample to be tested is first placed behind the spring-loaded gate. Next, the gate is lifted, allowing the sample to flow freely under its own weight. The distance that the liquid flows in a specific time is measured via gradations of the apparatus. 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 and mayonnaise.
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 that the difference between the two types of viscosities be understood and the appropriate test selected for the given sample.