High Viscosity Flow meters: Solution to a Sticky Problem

When a flow meter application involves the use of high viscosity liquid, the end-user must be very careful in choosing an appropriate meter. Using a flow meter calibrated for water can cause very large errors when that same flow meter is used for higher viscosity liquids. 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. 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. For purposes of this article, the terms "liquid" and "fluid" will be used interchangeably.

Definition of Viscosity

First, let's define viscosity (more correctly, absolute or dynamic viscosity) first in a rigorous manner and then in a more intuitive style. Viscosity (n) is defined as the ratio of shear stress (t) to shear rate (y) (Figure 1). 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). 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). The units of shear stress are given as dynes per square centimeter. For shear rate, the units are s-1. This gives the viscosity in (dyne sec)/cm2 or centipoise. 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. Essentially, the stickier the liquid and the more it resists the tendency to flow, the higher its viscosity.