Measurement and Control Basics Fourth Edition

Chapter 5 - Pressure Measurement

Introduction

This chapter discusses the principles of pressure measurement and the
pressure sensors commonly found in industrial control applications.
Many methods are used to measure pressure. Which method your plant
chooses depends on several factors, including the specific requirements of
the measuring system, the accuracy required, and the static, dynamic, and
physical properties of the process being measured or controlled.

A pressure sensor is always used to convert pressure into a mechanical or
electrical signal. This signal is then used for process indication or to generate
a scaled transmitted signal that a controller can use to control a process.
Before discussing pressure elements or sensor types, we need to
define pressure and other terms used in pressure measurement.

Definition of Pressure

A major concern in process control applications is the measurement of
fluid pressure. The term fluid means a substance that can flow; hence, the
term applies to both liquids and gases. Both will occupy the container in
which they are placed. However, if a liquid does not completely fill the
container it will have a free liquid surface, whereas a gas will always fill
the volume of a container. If a gas is confined in a container, its molecules
strike the container walls. The collision of molecules against the walls
results in a force against the surface area of the container. The pressure is
equal to the force applied to the walls divided by the area that is perpendicular
to the force. For a liquid at rest, the pressure exerted by the fluid at any
point is perpendicular to the boundary of the liquid.

 

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