Understanding the SI Units of Viscosity: Dynamic Kinematic Viscosity

Viscosity, a key physical property of fluids, quantifies a fluid's resistance to flow or deformation. The standard international (SI) units of viscosity are the pascal-second (Pa·s) or kilogram-meter per square second (kg·m-1·s-1). However, in practical applications, it can also be measured in poise (P), where 1 poise is equivalent to 0.1 Pa·s. This article will elaborate on the SI units of both dynamic and kinematic viscosities, their definitions, and their practical applications.

SI Units of Viscosity

The SI unit of dynamic viscosity is the pascal second (Pa·s) or kilogram-meter per square second (kg·m-1·s-1). This unit is widely used in scientific and engineering contexts due to its consistency and comprehensibility.

The SI units of kinematic viscosity are square meters per second (m2/s) or stokes (St).

Viscosity Coefficient Formula

The dynamic viscosity coefficient is given by the formula:

η F / (A · dv/dy)

where η is the dynamic viscosity, F is the force required to cause the deformation, A is the area of the parallel plates, and dv/dy is the velocity gradient.

Practical Measurement Techniques

In order to measure kinematic viscosity, one can use a simple method involving a paper with a circle drawn on it. By pouring a fluid at the center of the circle and measuring the time it takes to cover the entire area, one can determine the kinematic viscosity.

For dynamic viscosity, the method involves determining the force needed to provide a flow rate of 1 m2/s. This is done by spreading the fluid on a paper and applying a force to cover the entire area of the circle, which quantifies the dynamic viscosity.

It's important to note that the units of the remaining measurements (m and s) are consistent across both types of viscosity, allowing for a clear understanding of their physical meanings.

CGS Units of Viscosity

The centimeter-gram-second (CGS) units of viscosity are poise (P), named after the French physiologist Jean Léonard Marie Poiseuille, and its sub-units are centipoise (cP) and stoke (St) or its sub-unit centistokes (cSt).

Kinematic viscosity in CGS units is measured in stokes (St), named after Sir George Gabriel Stokes, an Irish mathematician and physicist.

Conclusion

In summary, the SI unit of viscosity is the pascal-second (Pa·s) for dynamic viscosity, and square meters per second (m2/s) for kinematic viscosity. Understanding and applying these units is crucial for accurate measurements in scientific and engineering fields. Whether you're using SI or CGS units, being familiar with the fundamental concepts of dynamic and kinematic viscosity can greatly enhance your ability to analyze and understand fluid behavior in various applications.

Further Reading

I recently uploaded a video covering essential information about viscosity of fluids. You can check it out by clicking the link in the comments section. Here is the video here.