TABLE OF CONTENTS Pressure is probably the second most commonly used and important measurement in process control. The most familiar pressure measuring devices are manometers and dial gauges, but these require a manual operator.
For use in process control, a pressure measuring device needs a pressure transmitter that will produce an output signal for transmission, e.g. an electric current proportional to the pressure being measured. A transmitter typically that produces an output of a 4 20 mA signal is rugged and can be used in flammable or hazardous service.
Pressure is defined as force per unit area and may be expressed in units of newtons per square meter, millimeters of mercury, atmospheres, bars or torrs. There are three common references against which it can be measured:
If measured against a vacuum, the measured pressure is called absolute pressure
Against local ambient pressure it is gauge pressure
If the reference pressure is user supplied, differential pressure is measured.
There are seven principle methods of electronically measuring pressure for use in process control and each of these is listed and described under its numeric heading, in principle detail below:
Strain gauge (bonded or unbonded wire or foil, bonded or diffused semiconductor)
Capacitive
Potentiometric
Resonant wire
Piezoelectric
Magnetic (inductive and reluctive)
Optical.
In process control applications, one of the most common ways to measure pressure is using a strain gauge sensor. There are two basic types of strain gauge, bonded and unbonded, each utilizing wire or...
