Page 12 - Instrumentation and Measurement

2

MEASUREMENT OF PROCESS VALUE

2.1 Introduction

This chapter discusses the sensors, signal processors, and data presentation elements commonly used
in engineering. The term sensor is used for an element which produces a signal relating to the quantity
being measured. The term signal processor is used for the element that takes the output from the
sensor and converts it into a form which is suitable for data presentation. Data presentation is where
the data is displayed, recorded, or transmitted to some control system.

2.2 Pressure Measurement

When a fluid is in contact with a boundary it exerts a force at right angles to that boundary. This is
the pressure exerted by the fluid.

Force
Pressure = (N/m2)
The fundamental unit of pressure in the ‘S.I. System’ is the “Newton per square metre” (N/m2), this
has been given the special name of the ‘Pascal’ (Pa). Since the Pascal is a very small unit [Standard
Atmospheric Pressure is internationally defined as 101,325 Pa or 101.325 kPa] the kilopascal (kPa)
or megapascal (MPa) is more often used.
At one stage in the development of the S.I. System, the “bar” was considered as a standard unit of
pressure [1 bar = 100 kPa].
There three categories of pressure:
1. Absolute pressure: Measurement of pressure related to a datum of absolute (complete)
vacuum.
2. Gauge pressure: Measurement of pressure related to a datum of local atmospheric pressure.
3. Differential pressure: Measurement of the difference between two unknown pressure
neither of which is atmospheric nor complete vacuum.

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