To understand the importance of using a capillary viscometer in different testing and analysis applications, it is first important to back to the basics. By understanding the basic considerations in measuring viscosity, it is easy to see why this is so important in the development of different types of materials.
An Overview of Viscosity
For most people, the idea of viscosity is the thickness of a liquid or how easy it is to pour. The thicker the liquid, the higher the viscosity. A good example of this would be the difference between molasses and water with water having a much lower viscosity.
In scientific terms, viscosity is a way to measure how a fluid will resist deformation by shear stress or tensile stress. Shear stress is stress applied in a parallel fashion to the cross section. Tensile stress is stress pulling along the fluid, similar to stretching an elastic band to make it longer.
This is essential to understand how stress will impact a particular material. When the material remains constant in flow a viscometer can be used for the measurement. If the fluid changes in its viscosity or flow with differences in pressure, a rheometer will be the appropriate measuring device.
The Viscometer
A capillary viscometer is one way to measure the viscosity of the fluid. In these types of devices, a capillary of a known bore is used under a continually held temperature.
The liquid or fluid that is being tested is kept at a steady temperature, gravity, which is a steady factor, pulls the fluid being tested from one point in the system to another point that is in a lower position.
The time is then measured to move the specific volume of the liquid or fluid through the capillary. Since gravity is a constant, the calculation can be made to determine what is known as the kinematic viscosity.
Every liquid or fluid has a unique kinematic viscosity. In this way, it is possible to compare the results of a specific test to a known standard.
The precision of the capillary viscometer is a central factor. Every aspect of the system including temperature control and bore of the capillary has to be absolutely accurate to provide the correct readings and test parameters. Additionally, some equipment is also to allow two distinct tests to be run, but this is only possible with specific liquids having Newtonian characteristics.
