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TRANSDUCER ENGINEERING
(III SEMESTER)
PREPARED BY:
Dr. Gunaselvi Manohar
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UNIT 1

SCIENCE OF MEASUREMENTS
AND CLASSIFICATION OF
TRANSDUCER
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MEASUREMENT
• Lord kelvin stressed the importance of measurement, by saying:
“When you can measure what you are speaking about, and express it
in numbers, you know something about it”.
• The measurement is usually undertaken to ascertain and present the
state, condition or characteristic of a system in quantitative terms.
• To reveal the performance of a physical or chemical system, the first
operation carried out on it is measurement.
• The process or the act of measurement consists of obtaining a
quantitative comparison between a predefined standard and a
measurand.
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Measurements are generally made

• To understand an event or an operation.

• To monitor an event or an operation.
• To control an event or an operation.
• To collect data for future analysis.
• To validate an engineering design.
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FUNDAMENTAL METHODS OF MEASUREMENT
There are two methods of measurement

1. Direct comparison with either a primary or a secondary

standard.
2. Indirect comparison through the use of a calibrated system.
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Direct comparison
• To measure the length of a bar, we compare the length of a bar with a
standard and find that the bar is so many inches long because that
many inches units on the standard has the same length as the bar.
• Thus we have determined the length by direct comparison.
• The standard used is called a secondary standard
Indirect comparison
• Indirect comparison makes use of some form of transducing device.
• This device converts the basic form of input into an analogous form,
which it then processes and presents at the output as a known
function of the input.
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FUNCTIONAL ELEMENTS OF AN INSTRUMENT

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PRESSURE THERMOMETER
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UNITS AND STANDARD
Units
• The “unit” is a basis for quantification of the entity.
• The result of the measurement of the physical quantity must be defines
both in kind and magnitude.
• The standard measure of each kind of physical quantity is called a unit.
• Magnitude of a physical quantity = (Numerical ratio) x (unit)
• The Numerical ratio is the number of times the unit occurs in any given
amount of the same quantity and hence called the number of measures
or numerical multiplier.
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Types of Units
• Fundamental units
• Derived units
Fundamental units
• Units which are fundamental to most other physical quantities are called
fundamental units.
• Fundamental units are measures of length, mass and time. Since they are
fundamental to most other physical quantities they are called “Primary
Fundamental Units”.
• Measures of certain physical quantities in the thermal, electrical,
illumination fields are also represented by fundamental units.
• These units are used only where these particular disciplines are involved
and hence called “Auxiliary Fundamental Units”.
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Derived Units
• All the other units which can be expressed in terms of fundamental
units with the help of physical equations are called derived units.
• Every derived unit originates from some physical law or equations
which defines that unit.
• E.g) the area A, of a room is equal to product of its length l, and
breadth, b. Therefore, A=l x b.
Dimensions
• Every quantity has a quality which distinguishes it from all other
quantities. This unique quality is called Dimension.
• The dimension is written in a characteristics notation, e.g) [L] for
length, [T] for time etc.
• A derived unit is always recognised by its dimensions, which can be
defined as the complete algebraic formula for the derived unit.
• Area, A = a constant x l xb
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Standards
• Standards have been developed for all the fundamental units as well
as some of the derived mechanical and electrical units.
• They are classified as follows:
1. International standards
2. Primary standards
3. Secondary standards
4. Working standards
International Standards
• These standards are those defined and agreed upon internationally.
• They are maintained at the International Bureau of Weights and
Measures and not accessible outside for calibration of instruments.
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Primary standards
• These standards are those maintained by national standards laboratories in
different parts of the world and they are also not accessible outside for
calibration.
• These are used for verification and calibration of the secondary standards.
Secondary standards
• These standards are usually fixed standards for use in industrial
laboratories.
• The accuracy is maintained by periodic comparison with primary standards.
Working standards
• These standards are for day to day use in measurement laboratories
• These may be lower in accuracy in comparison to secondary standards.
• The accuracy is maintained by comparison with secondary standards.
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CALIBRATION
• Calibration is an essential process to be undertaken for each
instrument and measuring system frequently.
• It is the process where the test instrument is compared with the
standard instrument.
• It consists of reading the standard and test instruments
simultaneously when the input quantity is held constant at several
values over the range of the test instrument.
• The calibration is better carried out under the stipulated
environmental conditions.
• Certification of an instrument manufactured by an industry is
undertaken by the National Physical Laboratory and other
laboratories having secondary and working standards.
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ERRORS IN MEASUREMENT
• A measurement cannot be made without errors.
• These errors can only be minimized but not eliminated completely.
Classification of errors
1. Gross errors
2. Systematic errors
3. Random errors
Gross errors
• This type of errors mainly covers human mistakes in reading the
instruments, making adjustments and application of instruements
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Systematic errors
• Systematic errors are due to shortcomings of the instrument and
changes in external conditions affecting the measurement
• These types of errors are divided into three categories
1. Instrumental errors
2. Environmental errors
3. Observational errors
1) Instrumental errors
These errors arise due to the following:
• Due to inherent shortcomings of the instrument
• Due to misuse of the instruments
• Due to loading effects of instruments
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Inherent shortcomings of instruments
• These errors can inherent in instruments because of their mechanical
structure.
• They may be due to construction, calibration or operation of the
instruments or measuring devices.
Misuse of instruments
• Often, the errors caused in measurements are due to fault of the
operator than that of the instrument.
• A good instrument misused can cause errors.
Loading effects
• Errors occur when we use the instrument in an improper manner
• e.g) a well calibrated voltmeter may give incorrect readings when
connected across a high resistance circuit.
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2) Environmental errors
• Environmental errors are due to changes in the environmental
conditions such as temperature, humidity, pressure, electrostatic and
magnetic fields.
• e.g) the resistance of a strain gauge changes with variation in
temperature.
3) Observational errors
• The observational error may be caused due to parallax.
• e.g) the pointer of a voltmeter rests slightly above the surface of the
scale. Thus an error account of parallax will occur unless the line of
vision of the observer is exactly above the pointer.
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Random (Residual) errors
• These errors are unpredictable errors and occur even when all
systematic errors are accounted for, although the instrument is used
under controlled environment and accurately pre-calibrated before
measurement.
• Over a period of observation, the readings may vary slightly.
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Error analysis
• The analysis of the measurement data is necessary to obtain the
probable true value of the measured quantity.
• Any measurement is associated with a certain amount of uncertainity.
• The best method of analysis is the statistical method
• The statistical method will give the most probable true value of
temperature.
• For statistical methods the terms like arithmetic mean, deviation,
mode and median are to be determined.
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1.Arithmetic Mean
• The most probable value of measured variable is the arithmetic mean
of the number of readings taken.
• The mean is the average of all numbers and is sometimes called the
arithmetic mean.
• For example, in a data center rack, five servers consume 100 watts,
98 watts, 105 watts, 90 watts and 102 watts of power, respectively.
The mean power use of that rack is calculated as (100 + 98 + 105 + 90
+ 102 W)/5 servers = a calculated mean of 99 W per server.
2.Deviation
• Deviation is departure of the observed reading from the arithmetic
mean of the group of readings. Let deviation of readings x1 and d1 and
that of reading x2 and d2 etc
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then
• d1 = x1 - X
• d2 = x2 - X
•.
•.
• dn = xn – X
• Average deviation is defined as the average of the modulus of the
individual deviations.
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Median
• The statistical median is the middle number in a sequence of
numbers. To find the median, organize each number in order by size;
the number in the middle is the median.
• FOR EXAMPLE:
• For the five servers in the rack, arrange the power consumption
figures from lowest to highest: 90 W, 98 W, 100 W, 102 W and 105 W
• The median power consumption of the rack is 100 W. If there is an
even set of numbers, average the two middle numbers.
Mode
• The mode is the number that occurs most often within a set of
numbers.
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FOR EXAMPLE:
• There is a set of numbers is 90 W,
• 104 W, 98 W, 98 W, 105 W, 92 W,
• 102 W, 100 W, 110 W, 98 W, 210 W and 115 W. The mode is 98 W
since that power consumption measurement occurs most often
amongst the 12 servers.
Range
• The range is the difference between the highest and lowest values
within a set of numbers.
• FOR EXAMPLE:
• If a six-server rack includes 90 W,98 W, 100 W, 102 W, 105 W and
110 W, the power consumption range is 110 W - 90 W = 20 W
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TRANSDUCER
• A device which converts a physical quantity into the proportional
electrical signal is called a transducer.
• The electrical signal produced may be a voltage, current or frequency.
• The process of transforming signal from one form to other is called
transduction.
• The transduction element transforms the output of the sensor to an
electrical output.
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A transducer will have basically two main components. They are
Sensing Element
• The physical quantity or its rate of change is sensed and responded to
by this part of the transistor.
Transduction Element
• The output of the sensing element is passed on to the transduction
element.
• This element is responsible for converting the non-electrical signal
into its proportional electrical signal.
• There may be cases when the transduction element performs the
action of both transduction and sensing.
• The best example of such a transducer is a thermocouple. A
thermocouple is used to generate a voltage corresponding to the heat
that is generated at the junction of two dissimilar metals.
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Classification of Transducer
• The transducers can be classified broadly
• On the basis of transduction form used
• As primary and secondary transducers
• As active and passive transducers
• As transducers and inverse transducers.
Based upon principle of Transduction
• Resistive transducer
The resistance of a length of metallic wire is given by
R = pl
a
Where, “ R” is the resistance in ohm
“ p” is the specific resistance of the material in ohm-m
“ l “ is the length of wire in m
“ a “is the area of cross section in m2
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Some of the resistive transducers are
• Potentiometer Type – The change in resistance of a potentiometer
reading due to the movement of the slider as a part of an external
force applied is known by its corresponding pressure or displacement.
• Photoconductive Cell – The change in resistance of a cell due to a
corresponding change in light flux is known by its corresponding light
intensity.
• Resistance Strain Gauge – The change in value of resistance of metal
semi-conductor due to elongation or compression is known by the
measurement of torque, displacement or force.
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• Inductive transducers
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Some of the capacitive transducers are
Magnetic circuit transducer
• Principle of operation: Self inductance or mutual inductance of ac-
excited coil is varied by changes in the magnetic circuit. Applications:
Pressure, displacement
Reluctance pickup
• Principle of operation: Reluctance of the magnetic circuit is varied by
changing the position of the iron core of a coil. Applications: Pressure,
displacement, vibration, position.
Eddy current gage
• Principle of operation: Inductance of a coil is varied by the proximity
of an eddy current plate. Applications: Displacement, thickness
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• Capacitive transducers

Where A – overlapping area of plates in m2

d – the distance between two plates in meter
ε – permittivity of the medium in F/m
εr – relative permittivity
ε0 – the permittivity of free space

Variable capacitance pressure gauge

• Principle of operation: Distance between two parallel plates is varied by an
externally applied force Applications: Measurement of Displacement,
pressure
Capacitor microphone
• Principle of operation: Sound pressure varies the capacitance between a
fixed plate and a movable diaphragm. Applications: Speech, music, noise
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Primary and Secondary transducers
• Basically one form of energy converted into another form is Primary
transducer.
• The output signal from the primary transducer is converted
subsequently into a usable output is called Secondary transducer.
• E.g) Bourden tube acting as a primary detector senses the pressure
and converts the pressure into a displacement of its free end.
• The displacement of the free end moves the core of a linear variable
differential transformer(LVDT) which produces an output voltage.
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Passive and Active transducers
• A component whose output energy is supplied entirely by its input
signal (physical quantity under measurement) is commonly called a
passive transducer.
• In other words the passive transducers derive the power required for
transduction from an auxiliary source.
• Active transducers are those which do not require an auxiliary power
source to produce their output.
• They are also known as self generating type since they produce their
own voltage or current output.
• E.g for active transducers: Thermocouple and thermopile
Principle of operation: An emf is generated across the junction of two
dissimilar metals or semiconductors when that junction is heated.
Applications: Temperature, heat flow, radiation.
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Analog and Digital Transducers
• Analog Transducers-These transducers convert the input quantity into an
analog output which is a continuous function of time.
• Ex) Strain Gauge ◦ LVDT ◦ Thermocouple ◦ Thermistor
• Digital Transducers-These transducers convert the input quantity into an
electrical output which is in the form of pulses.
• Ex) Glass Scale can be read optically by means of a light source,an optical
system and photocells
Transducers and Inverse Transducers
• Transducers- A Transducer can be broadly defined as a device which
converts a non-electrical quantity into an electrical quantity. Ex:-Resistive,
inductive and capacitive transducers
• Inverse Transducers- It is defined as a device which converts an electrical
quantity into a non-electrical quantity. Ex:-Piezoelectric crystals
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Selection of Transducers
Factor to be considered while selecting transducer:
• It should have high input impedance and low output impedance, to avoid
loading effect.
• It should have good resolution over is entire selected range.
• It must be highly sensitive to desired signal and insensitive to unwanted
signal.
• Preferably small in size.
• It should be able to work in corrosive environment.
• It should be able to withstand pressure, shocks, vibrations etc..
• It must have high degree of accuracy and repeatability.
• Selected transducer must be free from errors.
• The transducer circuit should have overload protection so that it will
withstand overloads.
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Requirements of a good transducers

• Smaller in size and weight.

• High sensitivity.
• Ability to withstand environmental conditions.
• Low cost.