Imagine that the fluid you want to test is sandwiched between two plates separated by a known distance. Understanding a defined “shear rate” and how it can affect the viscosity of the fluid is important. This method was one of the earliest quality control (QC) tests that checked viscosity in a quantifiable way.īut the cup method could not always discriminate successfully between materials that proved acceptable and those that were marginal or even poor performers because of the varying shear rate. This type of measurement is referred to as kinematic viscosity. As the level in the cup goes down, the shear rate at the orifice decreases because the weight of the fluid remaining in the cup is lower. The shearing action on the fluid takes place at the orifice on the bottom of the cup. This type of test uses the force of gravity to drain the fluid out of the cup. The cups (for example efflux cups) are relatively inexpensive and easy to use. This type of practical “measurement” of a material’s characteristic was eventually developed into a somewhat more scientific approach by using cups with holes in the bottom and a stopwatch to measure how much time it would take to drain the fluid.
Plant personnel may have an indication of the viscosity or “consistency” of a material by looking at it, rubbing it between their fingers, or having it drip off a stick or shovel. Engineers and quality-control personnel need ways to measure viscosity so that they can quantify whether a material will flow the way it needs to for the process or for the application. Scientifically, viscosity is the property of a fluid that causes it to resist flow.įor materials that flow, either while being processed (for pumping, spraying or coating) or in an end-use (like shampoo, detergent or paint), it is important to think about the material’s flow characteristics or viscosity. Most engineers know what viscosity is, but may have trouble explaining it or even understanding the full implications of the measured number. It is generally not a subject that is covered in much detail in many engineering curricula. Viscosity is a property that is often considered by process engineers, but seldom completely understood. Let’s first discuss the subject of viscosity. This article presents the applications for inline viscosity measurement and the means by which they are achieved. While it is difficult to control all factors in the process that can affect a fluid’s viscosity (such as temperature, air bubbles, shear history, turbulence and so on), if these factors are kept relatively constant, then good control can be achieved. Inline viscosity measurements can give continuous, realtime readings of a fluid’s viscosity during processing and consequently, can provide a means to automate the viscosity control of process fluids.
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