Introduction to

Measurement System

What is a Measurement ?

◼ The process of comparing an unknown quantity with

an accepted standard quantity

◼ Convert physical parameters to meaningful numbers

◼ Numerical values for physical variables are

measurands

Significance of Measurements

◼ Major functions of all branch of engineering

◼ Design of equipment and processes

◼ Proper operation and maintenance of equipment

and processes

◼ These require measurements

Methods of Measurements

◼ Direct Methods

◼ Measurand is directly compared against a standard

◼ Example : Length, Mass, Time…

◼ Involve Human factors – not possible to make very

accurate measurements

Length

Weight

A

BDAB

Methods of Measurements

◼ Indirect Methods

◼ quantity desired is determined from its mathematical

relationship to direct measurements

◼ Angles and distance are measured between points directly and

used to compute the coordinate of the point

◼ Accurate method

◼ Does not involve human factors

Instruments

◼ Measurements involve the use of instruments.

◼ Determine quantities or variables.

◼ Consists of single unit which gives an output reading

◼ In complex measurement situations, may consist of

several separate elements.

◼ Three phases of instruments

◼ Mechanical instruments

◼ Electrical instruments

◼ Electronic instruments

Instruments

◼ Mechanical Instruments

◼ Reliable for static and stable conditions

◼ Unable to respond rapidly to measurements of dynamic

and transient conditions.

◼ Moving parts involved are rigid, heavy and bulky.

◼ Source of noise

◼ Electrical Instruments

◼ More rapid than mechanical methods

◼ Depends on mechanical meter movement as indicating

device

◼ Mechanical movement has inertia and limits time

response

Instruments

◼ Electronic Instruments

◼ Uses semiconductor devices

◼ Movement involved is that of electrons – fast response

on account of very small inertia of electrons

◼ Weak signals can be detected by using pre-amplifiers

and amplifiers

◼ High sensitivity hence used in Bio-instrumentation

◼ Remote monitoring

◼ Used to measure non-electrical quantities as well.

◼ Light compact, high reliability & low power consumption

Classification of Instruments

◼ Absolute Instruments

◼ give the magnitude of the quantity under measurement in terms of physical constants of the instrument.

◼ Example – Tangent galvanometer

◼ Working with absolute instruments is time consuming

◼ Secondary Instruments

◼ Quantity is measured by observing the output indicated by the instrument.

◼ Calibrated by comparison with an absolute instrument or another secondary instrument.

◼ Example – thermometer, voltmeter, pressure gauge

Modes of operation

◼ Secondary instruments work on two modes

◼ Analog mode

◼ Signals that vary continuously and take infinite values

◼ Device which produce these signals are analog devices

◼ Digital mode

◼ Signals that vary in discrete steps and take finite values

◼ Device which produce these signals are digital devices

Measurement System

◼ A means for making the desired measurement.

What we are measuring

Measured or

quantified output

The measurement method

Usually an instrument or

a sensing element

Measurement System

Sensing

element

Signal Conditioning

Measurand

Human

Interface

Measurement System