transmitter

Technical Data Sheet

Selection Guide

Pressure
Fundamentals and
Transmitter Selection

Reference-25

Reference-25

Pressure Fundamentals and Transmitter Selection

Gage
Absolute

Gage
Compound
Barometric
Range

Pressure
Total Vacuum
(Zero or Absolute)

FIGURE 1. Definition of Pressures.

INTRODUCTION

Selection Guide

This document defines pressure and fundamentals of pressure measurement, as they relate to industry, and factors that should be considered in selecting a pressure transmitter. Included are definitions of functional specifications, performance specifications, and material selection, and their relationships to functional performance requirements.

PRESSURE DEFINITION
This section defines pressure as it is pertinent and as it is measured in industry.

General
Pressure is the action of one force against another force. Pressure is force applied to, or distributed over, a surface. The pressure P of a force F distributed over an area A is defined as
F
P = --A

Glossary of Measurable Pressures
Absolute Pressure
Measured above total vacuum or zero absolute.
Zero absolute represents total lack of pressure.
Atmospheric Pressure
The pressure exerted by the earth’s atmosphere.
Atmospheric pressure at sea level is 14.696 psia.
The value of atmospheric pressure decreases with increasing altitude.
Barometric Pressure
Same as atmospheric pressure.

Reference-26

Differential Pressure
The difference in magnitude between some pressure value and some reference pressure.
In a sense, absolute pressure could be considered as a differential pressure with total vacuum or zero absolute as the reference. Likewise, gage pressure (defined below) could be considered similarly with atmospheric pressure as the reference. Gage Pressure
The pressure above atmospheric. Represents positive difference between measured pressure and existing atmospheric pressure. Can be converted to absolute by adding actual