Kinematic, dynamic, relative, apparent, absolute — when dealing with viscosity measurements using a viscometer, you will likely come across these words. They refer to the different types (or coefficient values) of viscosity that can be measured in a fluid.The type of viscosity yielded by your viscometer’s measurements depends on the kind of viscometer you’re using, but it’s usually either dynamic or kinematic.
Viscosity demonstration. The fluid on the left has a lower viscosity than the fluid on the right.
Image used with permission (CC SA-BY 4.0; Synapticrelay).
Let’s define the different types of viscosity:
Dynamic (absolute) viscosity: Dynamic viscosity is also known as absolute viscosity and most often relates to non-Newtonian fluids. It refers to the fluid’s internal resistance to flow when force is applied.
Kinematic viscosity: Kinematic viscosity is a measure of the viscosity of a (usually Newtonian) fluid in motion. It can be defined as the ratio of dynamic viscosity to density. Any viscometer that uses gravity in its measurement design is measuring kinematic viscosity.
Apparent (shear) viscosity: Apparent, or shear, viscosity refers to the relationship between viscosity and shear rate. In Newtonian fluids, this value doesn’t change, but with non-Newtonian fluids, apparent viscosity is directly affected by the shear rate. It can be calculated by dividing shear stress by shear rate.
Relative viscosity: Relative viscosity is important for non-Newtonian fluids, specifically polymers. It refers to the relationship between molar mass (the mass of a chemical compound divided by total amount) and viscosity — higher molar mass means higher viscosity in the polymer. It’s calculated by dividing the polymer viscosity by the viscosity of the pure solvent.
What type of viscosity do different viscometers measure?
Not every viscometer is the same — the type of viscosity it measures depends on its design. Let’s review the most common types:
Orifice viscometers: Orifice viscometers include the different varieties of cup viscometers. As they function using gravity, the viscosity measured is kinematic viscosity.
Capillary viscometers: Capillary viscometers use gravity to measure how long it takes a fluid sample to travel the length of a tube. They also measure kinematic viscosity.
Falling piston viscometers: Falling piston viscometers use the force created by a falling piston to measure viscosity. They measure dynamic viscosity, because stress is applied to the fluid.
Rotational viscometers: A rotational viscometer measures how much torque is required to turn a spindle immersed in a fluid. The spindle applies stress to the fluid, resulting in a measurement of dynamic viscosity.
Falling ball viscometers: A falling ball viscometer measures the force required for a ball to fall through a fluid. The ball applies stress to the fluid, giving a measurement of dynamic viscosity. Patented in 1932 by Fritz Höppler, the falling ball viscometer was actually the first type of viscometer to measure dynamic viscosity.
Vibrational viscometers: Vibrational viscometers measure the resistance of a fluid to vibration. Since the vibration constitutes a force being applied to the fluid, these viscometers measure dynamic viscosity.
If you’d like more information on viscosity or how to measure it using viscometers, contact Saint Clair Systems.