Introduction to

Measurement System

03-08-2020 & 05-08-2020

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

Characteristics of Instruments and

Measurement systems

Measurement System Performance

◼ Static characteristics

◼ Measurement of quantities remain constant or vary

slowly with time

◼ Defines a set of criteria that gives meaningful

description of measured quantity

◼ Dynamic characteristics

◼ Measurements concerned with rapidly varying quantities

◼ Use differential equations

◼ Performance criteria based on dynamic relations

Static Characteristics

◼ Static error

◼ Reproducibility

◼ Drift

◼ Sensitivity

◼ Accuracy

◼ Dead Zone

Static Error

◼ No measurement is free from errors

◼ True value – average of an infinite number of measured

values

◼ Such situation is impossible to realise in practice and not

possible to determine “true value”

◼ In practice, true value is measured by “Examplar Method” –

method agreed by experts as being sufficiently accurate for

the purpose used

◼ Accuracy of an instrument is measured in terms of its error

Reproducibility and Drift

◼ Degree of closeness with which a given value may be

repeatedly measured

◼ Perfect reproducibility means the instrument has no drift

◼ Drift – with a given input the measured values do not vary

with time

◼ Drift classified into three categories

◼ Zero drift

◼ Span drift or sensitivity drift

◼ Zonal drift

Drift

◼ Zero drift

◼ Whole calibration shifts due to slippage or warming up of

electronic circuits, zero drift sets in.

◼ Prevented by zero setting

Output

Input

Nominal

characteristics

Characteristics

with zero drift

Zero

drift

Drift

◼ Span drift or sensitivity drift

◼ Proportional change in the indication all along the upward

scale

◼ Zonal drift – drift occurs only over a portion of span of an

instrument

Output

Input

Nominal

characteristics

Characteristics

with span driftOutput

Input

Nominal

characteristics

Characteristics with zero

drift and span drift

Static Sensitivity

◼ Ratio of the magnitude of the output signal or response to the

magnitude of input signal or the quantity being measured

◼ Slope of the calibration curve

◼ Ratio of the magnitude of the measured quantity to the

magnitude of the response – reciprocal of sensitivity –

deflection factor or inverse sensitivity

Output, q0

Input, qi

Sensitivity = ∆q0/ ∆qi

Linearity

◼ Output is linearly proportional to the input

◼ Independent linearity is done with reference to a straight

line showing the relationship between output and input

Maximum deviation

Actual calibration

curve

Idealized straight line

Output

Input

Threshold and Dead Time

◼ Threshold

◼ minimum value below which no output change is detected

◼ Due to input hysteresis

◼ Dead Time

◼ Time required by measurement system to begin to

respond to change in measurand

◼ Dead zone

◼ Largest change in input quantity for which there is no

output of the instrument

◼ Resolution

◼ Smallest increment in input detected by an instrument

Accuracy and Precision

◼ Accuracy – closeness to the true value

◼ Accuracy is the ability of an instrument to show the

exact reading

◼ Precision – reproducibility or agreement with each

other for multiple trials

◼ Defined as the capability of an instrument to show the

same reading when used each time (reproducibility of

the instrument).

Noise

◼ A signal that does not convey any useful information

◼ Unwanted signal superimposed upon the signal cause

deviation of output from its expected value

◼ Ratio of desired signal to unwanted noise – Signal to

Noise Ratio

◼ Sources of Noise

◼ Generated noise

◼ Conducted noise

◼ Radiated noise