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Why Measuring Flow is a Difficult Task: Introduction

By David W. Spitzer and Walt Boyes

E-Zine April 2014

It is a widely held perception that sensors producing traditional measurements are easily selected, purchased, installed and operated. This perception seems to drive the current trend of shifting this work from instrumentation engineers to technicians who often have little or no supervision, and sometimes little or no training in understanding sensors and measurements.

Logically, there is some truth to the argument of shifting the work to technicians. After all, anybody can put in a pressure gauge -- right? Perhaps in most cases, but what about the application where an important pressure gauge was checked by technicians every month and replaced every one to two months? Installation of a remote pressure gauge with proper materials of construction not only paid for itself in about six months, but also increased operator safety. Similar examples can be produced for any of the four most common measurement variables: flow, level, pressure and temperature. Each of these measurement variables has its idiosyncrasies.

Despite level, pressure and temperature measurements being more prevalent, the measurement of flow often presents the greatest challenges -- and can result in some of the greatest rewards.

Measuring flow is not easy. The barriers to achieving accurate and reliable flow measurements are many and varied, and even worse, these problems and issues are often interrelated.

Steady-state flow is a dynamic process variable. Steady-state flow describes the dynamic movement of fluid, where, in contrast, steady-state level, pressure and temperature fundamentally describe conditions that are static.

By their nature, steady-state static process variable conditions tend to be relatively free of noise. An example of this is a water level measurement without turbulence or other “noise” producers. In contrast, because of the dynamic nature of steady-state flow, the existence of “flow noise” in the flow measurement signal is such a common phenomenon that we often do not recognize it as “noise.” But when similar noise occurs in other process variable measurements, it is unusual enough that the signal is quickly termed “noisy.”

Click here to read "Damping the Signal"

From Flow Control (August 2002)

ISSN 1538-5280

Spitzer and Boyes, LLC
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