Kinematic, dynamic, relative, apparent, extraordinary — these are the words you may come across when using a viscometer for viscosity measurements. They refer to different types of viscosities (or coefficient values) that can be measured in fluids. The type of viscosity that a viscometer's measurements give depends on the type of viscometer you have, but is usually dynamic or moving.
Viscosity definition:
Dynamic (Absolute) Viscosity: Dynamic viscosity, also known as absolute viscosity, is usually associated with non-Newtonian fluids. It refers to the internal flow resistance of a fluid when a force is applied.
Kinematic Viscosity: Kinematic viscosity is a measure of the viscosity of a fluid in motion (usually Newtonian). 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 does not change, but in non-Newtonian fluids the apparent viscosity is directly affected by the shear rate. It can be calculated by dividing the shear stress by the shear rate.
Relative Viscosity: Relative viscosity is important for non-Newtonian fluids, especially polymers. It refers to the relationship between molar mass (the mass of the compound divided by the total amount) and viscosity - higher molar mass means higher viscosity in the polymer. Calculated by dividing the polymer viscosity by that 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 common types:
Orifice Viscometers: Orifice viscometers include different varieties of cup viscometers. Since they act under the force of gravity, the measured viscosity is kinematic.
Capillary Viscometer: A capillary viscometer uses gravity to measure the time it takes a fluid sample to travel through a pipe. They also measure kinematic viscosity.
Falling Piston Viscometer: A falling piston viscometer uses the force created by a falling piston to measure viscosity. They measure dynamic viscosity as stress is applied to the fluid.
Rotational Viscometer: A Rotational Viscometer measures the torque required to rotate a spindle immersed in a fluid. The spindle applies stress to the fluid, thereby measuring dynamic viscosity.
Falling Ball Viscometer: A falling ball viscometer measures the force required for a ball to fall into a liquid. The ball applies stress to the fluid, allowing dynamic viscosity to be measured. The falling ball viscometer, patented by Fritz Höppler in 1932, was actually the first viscometer to measure dynamic viscosity.
Oscillating Viscometer: A vibrating viscometer measures the resistance of a fluid to vibration. These viscometers measure dynamic viscosity because vibrations constitute the force applied to the fluid.
