What Is a Kinematic Viscometer?
A kinematic viscometer is an essential measuring instrument used to determine the kinematic viscosity of a fluid.
Kinematic viscosity, often referred to as “kinematic viscosity count,” quantifies how resistant a fluid is to flow. It’s important to distinguish kinematic viscosity from “viscosity,” which measures the stickiness or thickness of a fluid. While viscosity assesses how objects move within a fluid, kinematic viscosity assesses the fluid’s inherent resistance to movement. Kinematic viscosity is defined as the viscosity divided by the density of the fluid in the same viscosity state, and a kinematic viscometer is the instrument used to measure it.
Both viscosity and kinematic viscosity are measured using six types of viscometers.
Applications of Kinematic Viscometer
Kinematic viscometers find extensive use in the inspection and development of fluid products because kinematic viscosity is a critical parameter that affects material handling and properties. For example, in the food industry, variations in viscosity impact the drinkability of beverages, and creamy food products are influenced by their ease of handling and melt-in-your-mouth characteristics.
Additionally, kinematic viscometers play a crucial role in quality control, inspection, and product development across various industries, including petrochemicals, pharmaceuticals, paints, and industrial oils.
Principle of Kinematic Viscometer
Kinematic viscometers measure kinematic viscosity using various methods. These methods include the capillary viscometer, falling-ball viscometer, concentric double-cylinder rotational viscometer, single-cylinder rotational viscometer, flat-plate rotational viscometer, and oscillating viscometer.
All these methods evaluate kinematic viscosity by deforming a fluid, such as rotating it, passing it through a narrow tube, immersing a ball, or vibrating it, and then assessing the changes induced by these actions.
Types of Kinematic Viscometers
There are four typical types of kinematic viscometers, except for the capillary viscometer, which directly measures viscosity. To obtain kinematic viscosity, one must divide viscosity by the density of the fluid.
1. Capillary Viscometer
The capillary viscometer calculates kinematic viscosity by passing a fluid sample through a small tube and measuring the time it takes to flow through. Unlike other methods, the capillary viscometer directly determines kinematic viscosity. It is a cost-effective and long-established technique.
2. Falling-Ball Viscometer
Falling-ball viscometers determine viscosity by dropping a ball into a fluid and measuring the time it takes for the ball to descend. This method is suitable for a wide range of viscosities in Newtonian fluids, which maintain constant viscosity regardless of shear rate.
3. Rotational Viscometer
Rotational viscometers gauge viscosity by measuring the torque exerted on a cylindrical surface when a cylindrical rotor rotates within a fluid sample at a consistent speed. There are several types of rotational viscometers, including the coaxial double-cylinder type, single-cylinder type, and cone-plate type. The cone-plate type is particularly useful for investigating the fluid properties of non-Newtonian fluids by varying the rotation speed.
4. Vibro Viscometer
Vibro viscometers evaluate the viscous resistance of a vibrating component immersed in a sample subjected to specific vibration frequencies. An advantage of vibro viscometers is their ability to measure viscosity even when the fluid is in motion.
Additional Information on Kinematic Viscometer
1. Kinematic Viscosity Units
The SI unit for kinematic viscosity is m²/s. In the past, the unit St (Stokes) = cm²/s was used, with 1 m²/s equating to 1 cSt (centistokes). Viscosity is measured in Pa-s in SI units and P (Poise) or cP (centipoise) in the old units.
2. Difference from Viscosity
It’s crucial to recognize that kinematic viscosity and viscosity exhibit a significant difference in their relationship with density. When comparing kinematic viscosity and viscosity of different fluids, their relationship changes significantly. For example, comparing water and dry air, dry air has a kinematic viscosity about 15 times that of water, while its viscosity is only approximately 1/55th that of water. These differences are largely influenced by the substantial difference in density between water and dry air. In the case of viscous fluids like gear oil, both kinematic viscosity and viscosity are significantly higher than those of water.