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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
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