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GENERALIZED CONFIGURATION

AND FUNCTIONAL DESCRIPTIONS

OF MEASUREMENT SYSTEMS

Lecture 2Instructor : Dr Alivelu M Parimi


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OUTLINE

Functional elements of an instrument

Active and passive transducers

Analog and digital modes of operation

Null and deflection methods

Input output configuration of measurement system

Methods of correction of modifying and interfering inputs

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Introduction

Rapid development in instrumentation techniques is certain

because of the increasing demand for the measurement and

control of physical variables in the wide variety of applications.

To ensure precise control of processes to deliver consistentquality.

To meet this demand the engineer must be familiar with

constituent elements of instruments and their functioning.

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Functional elements of an

instrument Operation of any instrument can be described in generalized

way in terms of functional elements (not physical)

Figure represents possible arrangement of functional

elements in an instrument.

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instrument Primary Sensing Element (PSE)

PSE first receives energy from the measured medium and

produces an output depending on some way on the measured

quantity (measurand) e.g. the bulb of a mercury thermometer,

temperature sensor in air-conditioners. Variable Conversion Element (VCE)

VCE converts the signal from one form to another without

changing the information content of the signal. Bourdon tube,

bellows, proving ring and other elastic members convert forceto displacement.

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instrument Variable Manipulation Element (VME)

The element that operates on the signal according to some

mathematical rule without changing the physical form of

signal is VME. For example: amplifier which accepts small level

of signal at input and outputs the signal which is a constantmultiplied by input.

Data transmission element (DTE)

When the functional elements are physically separated, it

becomes necessary to transmit data from one element toanother. Data transmission element transmits the signal from

one location to another without changing its information

content. Ex. Shaft, gears, fiber optic cables, coaxial cables,

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instrument Data Presentation Element (DPE)

This element provides a display/record of the output by

moving a pointer over the graduated scale, or displays

waveform on CRO, gives digital readouts or records the signal

in analog /digital fashion.

A given instrument may involve the basic functions in any

number and combination. Also there may be certain

instruments which lack some of these functional elements butthe Primary Sensing Element is always the first element and

the last element is always the Data Presentation Element. A

given hardware component may serve as several of the basic

functional element.7


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instrument

For the pressure gage shown in Figure, describe the operationof the instrument in terms of functional elements and

arrange them from measured medium to observer.

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instrument

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