Viscosity is a fundamental property of fluids that describes their resistance to flow. Understanding the difference between absolute viscosity and kinematic viscosity is crucial for various industries, including petroleum, chemical, and food processing. As a leading viscometer supplier, we often encounter questions about these two types of viscosity. In this blog post, we will delve into the details of absolute viscosity and kinematic viscosity, explore how they are measured using a viscometer, and highlight their key differences.
What is Absolute Viscosity?
Absolute viscosity, also known as dynamic viscosity, is a measure of a fluid's internal resistance to flow. It quantifies the force required to move one layer of fluid relative to another at a given velocity. The SI unit of absolute viscosity is the pascal-second (Pa·s), although centipoise (cP) is commonly used in practical applications. One Pa·s is equal to 1000 cP.
The concept of absolute viscosity can be illustrated using Newton's law of viscosity, which states that the shear stress (τ) between adjacent layers of a fluid is directly proportional to the velocity gradient (du/dy) between those layers. Mathematically, it can be expressed as:
τ = μ * (du/dy)
where μ is the absolute viscosity of the fluid. This equation shows that the higher the absolute viscosity, the greater the shear stress required to maintain a given velocity gradient.
What is Kinematic Viscosity?
Kinematic viscosity is defined as the ratio of absolute viscosity (μ) to the density (ρ) of the fluid. It is a measure of a fluid's resistance to flow under the influence of gravity. The SI unit of kinematic viscosity is the square meter per second (m²/s), but the more commonly used unit is the centistoke (cSt). One m²/s is equal to 1,000,000 cSt.
The relationship between kinematic viscosity (ν), absolute viscosity (μ), and density (ρ) is given by the following equation:
ν = μ / ρ
Kinematic viscosity provides information about how easily a fluid can flow under its own weight. It is particularly useful in applications where gravity plays a significant role, such as in the flow of liquids through pipes or in the lubrication of machinery.
Measuring Absolute and Kinematic Viscosity with a Viscometer
Viscometers are instruments used to measure the viscosity of fluids. There are several types of viscometers available, each based on different principles of operation. Some common types include rotational viscometers, capillary viscometers, and falling ball viscometers.
Measuring Absolute Viscosity
Rotational viscometers are commonly used to measure absolute viscosity. These viscometers work by rotating a spindle or bob in the fluid and measuring the torque required to maintain a constant rotational speed. The absolute viscosity of the fluid is then calculated based on the measured torque and the geometry of the spindle or bob.
Capillary viscometers can also be used to measure absolute viscosity. In a capillary viscometer, the fluid is forced to flow through a narrow capillary tube under a known pressure. The time taken for the fluid to flow through a specific length of the tube is measured, and the absolute viscosity is calculated using Poiseuille's law.
Measuring Kinematic Viscosity
Capillary viscometers are the most widely used instruments for measuring kinematic viscosity. The principle behind capillary viscometers is based on the fact that the flow of a fluid through a capillary tube is governed by the balance between the gravitational force and the viscous force.
In a typical capillary viscometer, the fluid is placed in a vertical capillary tube, and the time taken for the fluid to flow between two marked points is measured. The kinematic viscosity of the fluid is then calculated using the following equation:
ν = C * t
where ν is the kinematic viscosity, C is the viscometer constant (which depends on the geometry of the capillary tube), and t is the flow time.
Key Differences between Absolute and Kinematic Viscosity
Physical Meaning
The main difference between absolute and kinematic viscosity lies in their physical meaning. Absolute viscosity measures the internal resistance of a fluid to flow, while kinematic viscosity takes into account the density of the fluid and measures its resistance to flow under the influence of gravity.
Units of Measurement
Absolute viscosity is measured in units of pascal-second (Pa·s) or centipoise (cP), while kinematic viscosity is measured in units of square meter per second (m²/s) or centistoke (cSt).
Applications
Absolute viscosity is more relevant in applications where the internal friction of the fluid is the primary concern, such as in the formulation of lubricants, paints, and adhesives. Kinematic viscosity, on the other hand, is commonly used in applications where the flow of the fluid under gravity is important, such as in the petroleum industry for measuring the viscosity of crude oils and refined products.
Our Viscometer Products
As a viscometer supplier, we offer a wide range of viscometers for measuring both absolute and kinematic viscosity. Our product portfolio includes Semi-automatic Kinematic Viscometer, Intelligent Kinematic Viscometer, and Engler Viscometer.
Our semi-automatic kinematic viscometers are designed for easy and accurate measurement of kinematic viscosity. They feature automatic temperature control and timing functions, making them suitable for use in laboratories and industrial settings.
The intelligent kinematic viscometers offer advanced features such as digital display, automatic calculation of viscosity, and data storage. These viscometers are ideal for high-precision applications and research laboratories.
The Engler viscometer is a traditional type of viscometer that is commonly used in the petroleum industry. It measures the viscosity of a fluid relative to the viscosity of water at a specified temperature.
Conclusion
In conclusion, absolute viscosity and kinematic viscosity are two important properties of fluids that provide valuable information about their flow behavior. Absolute viscosity measures the internal resistance of a fluid to flow, while kinematic viscosity takes into account the density of the fluid and measures its resistance to flow under the influence of gravity.
Viscometers are essential tools for measuring both absolute and kinematic viscosity. By understanding the differences between these two types of viscosity and choosing the appropriate viscometer for your application, you can ensure accurate and reliable viscosity measurements.
If you are in need of a viscometer for your industry or research needs, we invite you to contact us for more information. Our team of experts will be happy to assist you in selecting the right viscometer for your specific requirements and guide you through the procurement process.
References
- ASTM D445 - Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)
- ASTM D2196 - Standard Test Methods for Rheological Properties of Non - Newtonian Materials by Rotational Viscometer
- Barnes, H. A., Hutton, J. F., & Walters, K. (1989). An Introduction to Rheology. Elsevier Science Publishers.
