However, it also depends on the density of the fluid since the more denser the fluid, the faster it will flow through the orifice. With this type of measurement device, a specific volume of fluid passes through the orifice and the time it takes for the volume of fluid to pass through the orifice is proportional to the fluid viscosity. However, kinematic viscosity is the preferred unit when the shear stress and shear rate of the fluid is influenced by the density.Īn example of this is the measurement of viscosity using gravity techniques such as a cup with a small orifice in the bottom. Because density is an intrinsic property in itself, it can be argued that kinematic viscosity is not a precise measure of internal fluid friction. Kinematic viscosity is defined as the dynamic viscosity divided by the density of the fluid. Essentially, the stickier the liquid and the more it resists the tendency to flow, the higher its viscosity.Īnother concept to understand is the relationship between dynamic viscosity (as given in the above definition) and kinematic viscosity. An over-simplified but more intuitive grasp of viscosity can simply be stated as the measure of internal friction that arises whenever a liquid flows. This gives the viscosity in (dyne sec)/cm2 or centipoise. The units of shear stress are given as dynes per square centimeter. The rate of shear is then defined as the relative velocity (V) between the upper and lower faces divided by the length between them (L). If we were able to isolate a cubic volume of fluid (right), we could visually see the shear stress as the relative force (F) between the upper and lower faces of the cube per unit area (A). Viscosity (n) is defined as the ratio of shear stress (t) to shear rate (y) (Figure 1). For purposes of this article, the terms "liquid" and "fluid" will be used interchangeably.įirst, let's define viscosity (more correctly, absolute or dynamic viscosity) first in a rigorous manner and then in a more intuitive style. The second half will discuss the oval gear technology as applied to viscous fluids and provide successful application examples and some common fluids that have been used with oval gear flow meters. The first half of this article will define some of the basic terms relating to viscosity, viscous fluids and the reason why viscous liquids can cause large errors in flow measurement. Using a flow meter calibrated for water can cause very large errors when that same flow meter is used for higher viscosity liquids. When a flow meter application involves the use of high viscosity liquid, the end-user must be very careful in choosing an appropriate meter.
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