Every fluid material has a basic physical property, called its viscosity, which is a measure of its resistance to flow. The “thicker” a fluid is, the higher is its viscosity, because of its greater resistance to flow. One might now assume that the question is simply a matter of assigning every fluid a characteristic viscosity value. Unfortunately, there are different types of viscosity, which complicate matters. A simple example will demonstrate this fact. Which product has a higher viscosity, honey or mayonnaise? One could argue that mayonnaise is higher, because it will not run out of the inverted jar, while honey will. On the other hand, one could argue that honey does, because it is more difficult to stir a jar of honey with a spoon than it is to stir a jar of mayonnaise. This is not a paradox; it is simply evidence that there is more to the discussion of viscosity than simply assigning a number to each fluid. The different types of viscosity relate to the manner in which the various fluids respond to shear forces.

Now that the basics of the subject have been explained, a list of the viscosities of some common fluids will prove helpful. This list will enable one to relate the viscosity of a newly encountered product to a more common product and, thus, develop an intuitive feeling for what the viscosity really means. The values in the attached listing are in the most commonly used units, centipoise (cP), and they are specified as effective viscosities; that is, they represent the viscosity of the fluid at the shear levels normally encountered in a typical pump. This term does not have a precise technical definition, but it is intended to allow one to estimate the apparent viscosity of a non-Newtonian fluid at the shear levels. (The viscosities listed are at room temperature unless otherwise noted.)

Occasionally, one will encounter viscosities expressed in units other than centipoise. Many of these other units have arisen because of the numerous instruments and techniques available for measuring viscosities. Since these various methods employ different levels of shear in the measuring technique, there will not be a direct numerical agreement among the results for any non-Newtonian fluids. Fortunately, tables have been prepared which allow for the conversion from most common viscosity units to the equivalent viscosity in centipoise.

Hopefully, this information on the topic of fluid viscosity has created a clearer understanding of this important subject. While it is, admittedly, a complex topic,it is a subject with which one must be familiar in order to effectively select the proper project material.

via: www.APV.com