Dr. B.

ANAND RONALD
Assoc. Prof.
Dept. of Mech. Engg.,
SSNCE
 Metrology

Metre + ology (from Greek metron ‘measure’ + logy)

The Science of Measurements

Development of Metrics

Meteorology is a branch of the atmospheric sciences which includes

atmospheric chemistry and atmospheric physics, with a major focus on
weather forecasting.
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Concept of measurement

What is Not measured cannot be improved

All activities need to be improved
Therefore, need to be measured

For everyone to understand and mutually participate,

there needs to be some scientific approach

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Concept of Unit and Standard
For any measurement, it is essential to have a unit to
which reference can be made, compared and quantified.

Basic principles of measurement are

Unit
Standard

A set of items that need to be measured by all have to

be identified and units and standards developed for all
these- for universal acceptance.

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Role of metrology
The practice of metrology involves precise
measurements requiring the use of apparatus and
equipments to permit the degree of accuracy required to
be obtained.

A Metrologist has to understand the underlying

principles of what is to be measured, so that he can
design and develop new instruments and also use the
available instruments in a better way.

He also has to measure the inaccuracy and error of

the instrument, find its cause and continuously strive to
improve the instrument.

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 Application based metrology
In mass production, it is not possible to measure all
parameters.
Then we develop comparators.
It is also not possible to measure every component. Then we
take concepts from statistics to understand the nature of
deviation in measurements -leading to Statistical process
controls (SPC) and Statistical Quality Control (SQC)

When different parts are made at different locations and

then assembled later, we need to have a plus minus
tolerance over a base size, to enable correct fit when
assembled. This leads to “Fits and Tolerances”

We need the component back after testing- leading to Non

destructive testing (NDT) methods.
If we need to measure without contacting- we get Non
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Legal Metrology
That part of metrology that deals with units of measurement,
methods of measurements and the measuring instruments, in
relation to the statutory, technical and legal requirements.
It assures security and appropriate accuracy of the measurements.

Lack of Legislation will lead to great uncertainty

To maintain uniformity of measurements throughout the

world, there are two organisations

1.The International Organisation of Weights and

Measures

2.National service of Legal Metrology- resolves

problem of Legal Metrology in a particular country. Conserve the
national standards and guarantee their accuracy by comparison
with International standards.

Lays down procedure to be followed for measurement related

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activities and levies penalty for non conformance.
22K916 for 22 Karat gold,
18K750 for 18 Karat gold &
14K585 for 14 Karat gold. Ag, Cu, Zn, NI 8
Deterministic Metrology
New philosophy where part measurement is replaced
by process measurement.
Full advantage is taken of the deterministic nature of
the automated machines.
All manufacturing subsystems are optimised to maintain
deterministic performance within acceptable quality
levels.
The system processes are monitored by measuring and
adjusting temperature, pressure, flow etc, in a non
intrusive way.

Measurements are in built as part of the process (in-

situ) aimed at a defined output.

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Objectives

of Metrology
To determine the measuring instrument capabilities
(to ensure their adequacy for required measurements)
(selection)
To measure the product dimension (QC)
To determine the process capability (to be better than
the product tolerances)(PC)

To minimise the cost of inspection (by proper selection

of statistical techniques) (COQ)
To standardise measuring methods (Standardisation)
To arbitrate and resolve issues in shopfloor regarding
methods of measurement.
To maintain accuracy of measuring equipment (by
calibration)
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Standardisation-need
For overall efficiency and productivity in a country, it is
essential that diversity be minimised and
interchangeability among parts encouraged.
This is possible only through standardisation.

Standardisation is done at various levels - Department,

Company, National and International

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Standardisation- organisations
In India, Bureau of Indian standards (BIS) is
responsible for evolving standards for use in India, in
common reference terms with other similar units across
the world.
Mechanical Engg Division Council of BIS has a
separate Engineering Metrology Sectional
Committee.

International Organisation for Standardisation

(ISO)-global organisation
The National Standards Organisation of each country
are members of the ISO.

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Standardisation-NPL

Role of National Physical Laboratory

Provides basic backbone of organizational structure for
metrology in India.
It is the custodian for national measurement standards
for physical measurements in India.
It has the obligation to establish, maintain and
update the national standards and calibration facilities
for different parameters.

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Units
International System of Units (SI) (before, we were following
metric system)
Based on decimal arithmetic
For each physical quantity, units of different sizes are formed by
multiplying or dividing a single base value by powers of 10.
Coherent system-if unit of length is metre, then area is square
metre (Not hectare, acre etc)
There are seven basic units established by the General
Conference on Weights and Measures.
There are also special units (like newton, joule, coulomb etc)

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Basic Units
Physical quantity-Unit-symbol
Length -metre- m
Time-second-s
Mass-kilogram-kg
Temperature-kelvin-K
Electric current-ampere-A
Luminous Intensity-candela-cd
Quantity of substance-mole-mol

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Units-definition

Ampere is the constant current which, if maintained in

two parallel , rectilinear conductors of infinite length of
negligible circular cross- section, and placed at a distance
of one meter apart in vacuum, would produce between
these length conductors a force equal to 2 x 10-7 N/m
length .

The metre is defined as the length of the path travelled

by light in a vacuum in 1/299 792 458 of a second

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Units-definition
Other physical quantities are derived from these
basic units
Volume is cubic meter m3
Speed is metre per second m/s

Force is mass metre per square second (kgm/s2)

Since this is complex expression of base units, it is
given special name as Newton.

Pressure is newton per metre squared = Pascal

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Units- Supplementary units
Only authorised supplementary units is for angular measurements.
Plane angles are measured by radians and solid angles by
steradians.

Decimal multiples are designated by suitable prefixes

10 power 3-kilo-k
 6-mega-M
 9-giga-G
 12-tera-T
 Minus 6-micro-µ
 Minus 9-nano-n
 Minus 12-pico-p

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Aspects of Measurement
Measurement involves

Standards (Reference masters for setting standards)-

These are used to reproduce one or several definite
values of a given quantity

Fixed gauges-These are used to check the dimensions,

form and position of product features

Measuring Instruments -These are used to determine

the values of the measured quantity.

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Measurement

Measurement is a series of operations carried out by

means of measuring instruments to determine the
numerical value of the size which describes the object of
the measurement.

The act of deriving quantitative information about a

physical object or action by comparison with a
reference.

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Measuring aspects
The measurand-the physical property like length, angle
etc, being measured

The comparator- the means of comparing measurand

with some reference to render a judgement

Reference- the physical quantity or property , to which

quantitative comparisons are made.

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Measuring aspects
A mechanic wants to measure the length of a table.

Table length is the measurand.

The tape he uses is the reference.

The eye is the comparator.

This is a direct measurement using a calibrated, fixed

reference.

Similar examples- length measurement using vernier

caliper, measurement of angle using a bevel protractor. 24
Error and correction
Error of measurement: difference between the true
value of the size being measured and the value found by
measurement.

Error pertains to a measurement and not to an

instrument.

Correction: it is the amount which should be

algebraically added to the indicated value to obtain the
actual value of the size being measured. The correction is
numerically equal to the error but opposite in sign.

Correctness of measurement: Quantitative

characteristic showing how close to zero are the systemic
errors of measurement results.
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Measuring

instruments
Measuring instruments are measuring devices that
transform the measured quantity or a related quantity
into an indication or information.

Can either indicate directly or as a comparison to a

known reference value.

They utilize a measuring sequence in which the

measured quantity is transformed into a quantity
perceptible to the observer.

Can be used in conjunction with separate material

measures (like a balance that compares with a
standard weight). Or they may contain internal parts to
reproduce the unit (graduated scales)
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 STATIC
Characteristics of Measurement Systems
Static characteristics:
The set of criteria defined for the instruments, which are used to
measure the quantities which are slowly varying with time or mostly
constant, i.e., do not vary with time, is called ‘static characteristics’.
The various static characteristics are:
i) Accuracy
ii) Precision
iii) Sensitivity
iv) Linearity
v) Reproducibility
vi) Repeatability
vii) Resolution
viii) Threshold
ix) Drift
x) Stability
xi) Tolerance
xii) Range or span
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Precision and Accuracy

Both terms are associated with the measuring process.

Precision is defined as the repeatability of a measuring process.

Accuracy is the agreement (Closeness) of the measured value

with the true value.

In most measurements, precision is considered more

important.

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Precision and Accuracy

Example:
If a carpenter wants to cut a board to fit a shelf into two projections
on the wall, it does not matter whether his scale is accurate or not.

As long as he uses the same scale to measure the wall and the
board.

 Here, the precision with which he measures the wall and the
board is important.

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Precision and Accuracy

Supposing he has to order the board from market for direct fitting.

Then it is necessary that the scale used by him and the one in
market are in agreement with each other.

One way to achieve this is that both use the accurate scales in
accordance with the standard scales.

In this case, accuracy is more important.

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 Accuracy
The accuracy of an instrument is its ability to give correct results.

Factors that affect accuracy

1.Design of the equipment (Parallax error and mirror below the

pointer)
2.Skill of the operator
3.Method adopted for the measurement

If a measurement is desired to an accuracy of 0.01 mm, then an

instrument with an accuracy of 0.001 mm must be used. (general
rule is to use an instrument which can be read to the next
decimal place beyond that required in the measurement).

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Precision and Accuracy
Factors that are affected by Accuracy:

Higher accuracy can be achieved by incorporating magnifying

devices in the instrument.

These magnifying devices carry their own inaccuracies.

Magnification involves optical systems. In mechanical systems,

the errors are introduced by bending of levers, backlash at the
pivots, inertia of the moving parts and error of the screw
threads etc.

Thus, the greater the accuracy is aimed at , greater is the

number of sources of errors to be controlled.

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 Precision and Accuracy

Accuracy and cost:

SWIPE - Standard, Workpiece, Instrument, Person,

Environment.

Higher accuracy can be achieved only if all the sources of errors due
to the above five elements in the measuring system be analysed and
steps taken to eliminate them.

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 Precision and Accuracy
dimension
Precise but not
x average accurate
x
x

error

True value

frequency

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 Precision and Accuracy
dimension
Accurate but not
precise

x
x average
x
x error

x
x True value
x

frequency

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 Precision and Accuracy
dimension
Precise and accurate

average
x x
x True value

frequency

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38
Sensitivity
Sensitivity:

Quotient of increase in observed variable (indicated by pointer

and scale) and the corresponding increase in the measured quantity.

 It is expressed as length of scale division, divided by value of that

division .

Sensitivity can be constant along the scale (linear transmission) or

variable along the scale (non-linear transmission).

Eg:. Type J (iron–constantan) has a more restricted range (−40 °C to +750 °C)
than type K but higher sensitivity of about 50 µV/°C.

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Sensitivity and Readability
Sensitivity refers to the ability of a measuring device to detect
small differences in a quantity being measured. It may so happen
that high sensitivity instrument may lead to drifts due to thermal or
other effects, so that its indications may be less repeatable or less
precise than those of the instrument of lower sensitivity.

Readability refers to the susceptibility of a measuring device to

having its indications converted to a meaningful number. A
micrometer can be made more readable by using verniers. Very finely
spaced lines may make a scale more readable when a microscope is
used, but for the unaided eye, the readability is poor.

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Measuring Range or Span
Measuring range:

Range of value of the measured quantity for which the

error obtained from a single measurement under normal
conditions of use, does not exceed the maximum
permissible error.

Limited by maximum and minimum capacity

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Repeatability
Repeatability:

Ability of measuring system to give the same value every time the
measurement of a given quantity is repeated.

Sources of variation:
environmental changes
Variability in operator performance
Instrument parameters

Repeatability is characterized by the dispersion in values when same

quantity is measured again and again.
The dispersion is represented by two limiting values or by a standard
deviation.
Conditions under which repeatability was tested, must be
specified.

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a) Zero drift: If the whole calibration gradually shifts due to slippage, permanent
set, or due to undue warming up of electronic tube circuits, zero drift sets in.
b) Span drift or Sensitivity drift: If there is proportional change in the indication
all along the upward scale, the drifts is called span drift or sensitivity drift.
c) Zonal drift: In case the drift occurs only a portion of span of an instrument , it
is called Zonal Drift.

Zero Drift Span Drift

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Resolution
If the input is slowly increased from some arbitrary input value, it will again be found that
output does not change at all until a certain increment is exceeded. This increment is
called resolution.

Threshold:
If the instrument input is increased very gradually from zero there will be some minimum
value below which no output change can be detected. This minimum value defines the
threshold of the instrument.

Stability:
It is the ability of an instrument to retain its performance throughout is specified
operating life

Tolerance:
The maximum allowable error in the measurement is specified in terms of some value
which is called tolerance.

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 DYNAMIC
Characteristics of Measurement Systems

The set of criteria defined for the instruments, which changes rapidly with
time, is called ‘dynamic characteristics’.
The various Dynamic characteristics are:
i)Speed of response
ii)Measuring lag
iii)Fidelity
iv)Dynamic error

Speed of response: It is defined as the rapidity with which a measurement system

responds to changes in the measured quantity.

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Response Time
Response Time: It is the time which elapses after a sudden change in
the measured quantity until the instrument gives an indication differing
from the true value by an amount less than a given permissible error.

If inertia elements are small, we get first order response-exponential

curve
(Y axis indication, x-axis time)

If inertia elements are NOT negligible, we get second order response.
(Three possibilities)

Overdamped system -final indication is approached exponentially

from one side.
Under damped system- pointer approaches the position
corresponding to the final reading , passes it and makes a number of
oscillations around it, before it stops.
Critically damped system- pointer is aperiodic but quicker than in
the case of overdamped system.
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Measuring lag:
It is the retardation or delay in the response of a measurement system to changes in the
measured quantity. The measuring lags are of two types:
a)Retardation type: In this case the response of the measurement system begins
immediately after the change in measured quantity has occurred
b)Time delay lag: In this case the response of the measurement system begins after a
dead time after the application of the input.

Fidelity of a system is defined as the ability of the system to reproduce the output in the
same form as the input. It is the degree to which a measurement system indicates
changes in the measured quantity without any dynamic error.

Dynamic error:
It is the difference between the true value of the quantity changing with time & the value
indicated by the measurement system if no static error is assumed. It is also called
measurement error.

Both static and dynamic characteristics of a Transducer determine its

performance and indicate how effectively it can accept desired input signals
and reject unwanted inputs.
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Bias: Characteristic of a measuring instrument to give indications

Bias
of the value of a measured quantity whose average differs from the
true value of that quantity.

Sources of bias are maladjustment of the instrument,

Permanent set, non linear errors and errors of material measures.

Instrument bias (imperfections in the instrument or method used

to collect your data. )
Researcher bias (Researcher bias can be introduced when the
researcher's judgment is involved in the measurement process.)
Respondent bias (Respondent bias can result when the
respondent is motivated to answer in any way other than the truth)
Testing bias - (Hawthorne Effect) (Testing bias is a common
concern in pretest/post-test studies, where taking the pretest may
alter a subject's interest in the topic, altering their subsequent
results on the post-test)
Recall bias (past events)
Ref: http://my.ilstu.edu/~gjin/hsc204-hed/module-9-bias/module-9-bias5.html

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Scale Interval / Hysteresis
Scale Interval: difference between two successive scale marks in
unit of the measured quantity. The scale interval determines the
ability of the instrument to give accurate indication of the measured
quantity.

Hysteresis: difference between the indications of a measuring

instrument when the same value of the measured quantity is reached
by increasing or by decreasing that quantity.

Hysteresis is the dependence of the state of a system on its history.

magnetic hysteresis, ferroelectric hysteresis, mechanical hysteresis,
superconducting hysteresis, adsorption hysteresis, optical hysteresis, electron beam
hysteresis, economic hysteresis
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Hysteresis
Elastic hysteresis of an idealized rubber band. The area in
the centre of the hysteresis loop is the energy dissipated due
to internal friction.
The phenomenon of hysteresis is due to the presence of dry friction as well
as to the properties of elastic elements.

 It results in loading and unloading curves of the instrument being

separated by a difference called the hysteresis error.

 It also results in the pointer not returning to zero, when the load is
completely removed.

Hysteresis is noted in instruments having elastic elements.

 The phenomenon of hysteresis in elements is due to the presence of

internal stresses.

 It can be reduced considerably by proper heat treatment.

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Inaccuracy (Uncertainities in measurement)
Inaccuracy:
Total error of a measure or measuring instrument under
specified conditions of use (including bias and repeatability errors).

Inaccuracy is specified by two limiting values obtained by adding

and subtracting to the bias error , the limiting value of the
repeatability error.

If the known systematic errors are corrected, the remaining

inaccuracy is due to the random errors and the residual
systematic errors that also have a random character. This
inaccuracy is called the “Uncertainty of measurement”.

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Calibration
Calibration is the process of checking the dimensions and
tolerances of a gauge, or the accuracy of a measurement
instrument by comparing it to a like instrument / gauge that
has been certified as a standard of known accuracy.

Calibration is done over a period of time , according to the

usage of the instrument and the material of its parts.

Calibration is done by detecting and adjusting any

discrepancies in the instrument's accuracy to bring it within
acceptable limits.

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54
Methods of Measurement:
 Method of direct measurement: The value of the quantity to be
measured is obtained directly without the necessity of carrying out
supplementary calculations based on a functional dependence of
the quantity to be measured in relation to the quantities actually
measured.
Eg: Weight of a substance is measured directly using a physical
balance.
 Method of indirect measurement: The value of the quantity is
obtained from measurements carried out by direct method of
measurement of other quantities, connected with the quantity to be
measured by a known relationship.
Eg: Weight/ Volume of a substance is measured by measuring the
length, breadth & height of the substance
Ex: Diameter measurement by using three wires

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 Method of measurement without contact: The sensor is not placed
in contact with the object whose characteristics are being measured.
(IR Thermometer)

 Method of combination measurement closed series: The results of

direct or indirect measurement or different combinations of those
values are made use of & the corresponding system of equations is
solved.

 Method of fundamental measurement: Based on the

measurements of base quantities entering into the definition of the
quantity.

 Method of measurement by comparison: Based on the comparison

of the value of a quantity to be measured with a known value of the
same quantity (direct comparison), or a known value of another
quantity which is a function of the quantity to be measured (indirect
comparison). Eg. Surface finish
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surface finish comparison plates

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 Method of measurement by substitution: The value of a quantity to
be measured is replaced by a known value of the same quantity, so
selected that the effects produced in the indicating device by these
two values are the same (a type of direct comparison)
 Measurement of Medium Resistance by Substitution
Method
 In Substitution Method, the Resistance whose value is to be
measured is compared with the Standard Resistance by
some technique which is described in this section. The
connection diagram for Substitution Method is given below.
 R is the unknown Resistance, S the Standard variable
Resistance, A is Ammeter and r is Regulating Resistance.
 (When we put the Switch at position 1 then R is connected in the circuit. The Regulating
Resistance r is adjusted till the reading of Ammeter is at a chosen scale mark. Now the Switch
is thrown to position 2 putting the Standard variable Resistance S in the circuit. Now the
variable Resistor S is adjusted till the reading of Ammeter is same as when R was in the
circuit. The setting of dial of S is read. Since the substitution of one resistance for another has
left current unaltered, and provided that EMF of battery and position of Regulating Resistance
r remain unaltered, the two Resistance R and S must be equal. Thus the value of unknown
Resistance R is equal to the dial setting of Standard Resistance S.)

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 Method of measurement by transposition : The value of the quantity
to be measured is in the beginning, balanced by a first known value A of
the same quantity, then the value of the quantity to be measured is put
in place of this known value and is again balanced by another known
value B. If the position of the element indicating equilibrium is the same
in both the cases, the value of the quantity measured is equal to A & B.
 Ex: Determination of mass by balancing methods.

 Method of differential measurement/ Coincidence method : It is a

differential method of measurement in which a very small difference
between the value of the quantity to be measured and the reference
is determined by the observation of the coincidence of certain lines
or signals.
 Eg., measurement by vernier calliper micrometer.

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 Method of measurement by complement: In this method the
value of the quantity to be measured is combined with a
known value of the same quantity. The combination is so
adjusted that the sum of these two values is equal to
predetermined comparison value.
 Eg., determination of the volume of a solid by liquid
displacement.

 Method of measurement by interpolation : It consists of determining

value of the quantity measured on the basis of the law of
correspondence & known values of the same quantity, the value to be
determined lying between two known values. (mohs scale of
hardness)

 Method of measurement by extrapolation : It consists of

determining the value of the quantity measured on the basis of the law
of correspondence & known values of the same quantity, the value to
be determined lying outside the known values.
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Types of errors
A) Error of Measurement:
 Systematic error: It is the error which during several measurements, made
under the same conditions, of the same value of a certain quantity, remains
constant in absolute value and sign or varies in a predictable way in
accordance with a specified law when the conditions change. The causes of
these errors may be known or unknown. The errors may be constant or
variable. Systematic errors are regularly repetitive in nature. (A/C room &
without)
 Eg., this could happen with blood pressure measurements if, just
before the measurements were to be made, something always or
often caused the blood pressure to go up.

https://www.webassign.net/question_assets/unccolphysmechl1/measurements/
manual.html 63
 Random error: This error varies in an unpredictable manner in absolute
value & in sign when a large number of measurements of the same value of
a quantity are made under practically identical conditions. Random errors
are non-consistent. Random errors are normally of limited time duration.

 Parasitic error: It is the error, often gross, which results from incorrect
execution of measurement.

https://slideplayer.com/slide/13889116/
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B) Instrumental error:
 Error of a physical measure: It is the difference between the
nominal value and the conventional true value reproduced by the
physical measure.
 Error of a measuring mechanism: It is the difference between the
value indicated by the measuring mechanism and the conventional
true value of the measured quantity.
 Zero error: It is the indication of a measuring instrument for the zero
value of the quantity measured.
 Calibration error of a physical measure: It is the difference
between the conventional true value reproduced by the physical
measure and the nominal value of that measure.
 Complementary error of a measuring instrument: It is the error of
a measuring instrument arising from the fact that the values of the
influence quantities are different from those corresponding to the
reference conditions.

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 Error of indication of a measuring instrument: It is the difference
between the measured values of a quantity, when an influence
quantity takes successively two specified values, without changing
the quantity measured.

 Error due to temperature: It is the error arising from the fact that the
temperature of instrument does not maintain its reference value.

 Error due to friction: It is the error due to the friction between the
moving parts of the measuring instruments.

 Error due to inertia: It is the error due to the inertia (mechanical,

thermal or otherwise) of the parts of the measuring instrument.

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C) Error of observation:
 Reading error: It is the error of observation resulting from incorrect
reading of the indication of a measuring instrument by the observer.

 Parallax error: It is the reading error which is produced, when, with

the index at a certain distance from the surface of scale, the reading
is not made in the direction of observation provided for the
instrument used.

 Interpolation error: It is the reading error resulting from the inexact

evaluation of the position of the index with regard to two adjacent
graduation marks between which the index is located.
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D) Based on nature of errors:
 Systematic error: (already discussed)

 Random error: (already discussed)

 Illegitimate error: As the name implies, it should not exist. These

include mistakes and blunders, computational errors and chaotic
errors. Chaotic errors are random errors but unlike the latter, they
create chaos in the final results.

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E) Based on control:
 Controllable errors: The sources of error are known and it is possible to
have a control on these sources. These can be calibration errors,
environmental errors and errors due to non-similarity of condition while
calibrating and measuring.
 Calibration errors: These are caused due to variation in the calibrated
scale from its normal value. The actual length of standards such as slip
gauges will vary from the nominal value by a small amount. This will
cause an error of constant magnitude.
 Environmental (Ambient /Atmospheric Condition) Errors:
International agreement has been reached on ambient condition which is
at 20°C temperature, 760 mm of Hg pressure and 10 mm of Hg
humidity. Instruments are calibrated at these conditions. If there is any
variation in the ambient condition, errors may creep into final results. Of
the three, temperature effect is most considerable.

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 Stylus pressure errors: Though the pressure involved during measurement is
generally small, this is sufficient enough to cause appreciable deformation of
both the stylus and the work piece. This will cause an error in the
measurement.

 Avoidable errors: These errors may occur due to parallax in the reading of
measuring instruments. This occurs when the scale and pointer are separated
relative to one another.
The two common practices to minimise this error are: i) Reduce the separation
between the scale and pointer to minimum. ii) A mirror is placed behind the
pointer to ensure normal reading of the scale in all the cases. These avoidable
errors occur also due to non-alignment of work piece centers, improper location
of measuring instruments, etc.

 Non-controllable errors: These are random errors which are not controllable.

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The person in the picture is trying to measure the length
of a piece of wood. Discuss what he is doing wrong?
How many mistakes you can find?

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 STANDARDS OF MEASUREMENT

 A standard is defined as “something that is set up and

established by an authority as rule of the measure of
quantity, weight, extent, value or quality”.
 For example, a meter is a standard established by an
international organization for measurement of length.
Industry, commerce, international trade in modern
civilization would be impossible without a good system of
standards.

 Role of Standards:
 The role of standards is to achieve uniform, consistent and
repeatable measurements throughout the world. Today our
entire industrial economy is based on the interchangeability
of parts the method of manufacture. To achieve this, a
measuring system adequate to define the features to the
accuracy required & the standards of sufficient accuracy to
support the measuring system are necessary.

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 STANDARDS OF LENGTH

 In practice, the accurate measurement must be made

by comparison with a standard of known dimension
and such a standard is called “Primary Standard”.
 The first accurate standard was made in England and
was known as “Imperial Standard yard” which was
followed by International Prototype meter” made in
France. Since these two standards of length were
made of metal alloys they are called ‘material length
standards’.

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Imperial Standard yard:
An imperial standard yard, shown in fig, is a bronze
(82% Cu, 13% tin, 5% Zinc) bar of 1 inch square
section and 38 inches long. A round recess, 1 inch
away from the two ends is cut at both ends upto the
central or ‘neutral plane’ of the bar.
 Further, a small round recess of (1/10) inch in
diameter is made below the center. Two gold plugs of
(1/10) inch diameter having engravings are inserted
into these holes so that the lines (engravings) are in
neutral plane.

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 Yard is defined as the distance between the two
central transverse lines of the gold plug at 62 deg. F.
 The purpose of keeping the gold plugs in line with the
neutral axis is to ensure that the neutral axis remains
unaffected due to bending, and to protect the gold
plugs from accidental damage.

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International Prototype meter
It is defined as the straight line distance, at 0 degree C, between the
engraved lines of pure platinum-iridium alloy (90% platinum & 10% iridium)
of 1020 mm total length and having a ‘tresca’ cross section as shown in fig.
The graduations are on the upper surface of the web which coincides with the
neutral axis of the section.

The tresca cross section gives greater rigidity for the amount of material involved and is
therefore economic in the use of an expensive metal. The platinum-iridium alloy is used
because it is non oxidizable and retains good polished surface required for engraving
good quality lines.

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Disadvantages of Material length standards:
1. Material length standards vary in length over the
years owing to molecular changes in the alloy.
2. The exact replicas of material length standards were
not available for use somewhere else.
3. If these standards are accidentally damaged or
destroyed then exact copies could not be made.
4. Conversion factors have to be used for changing over
to metric system

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Light (Optical) wave Length Standard

 Because of the problems of variation in length of

material length standards, the possibility of using light
as a basic unit to define primary standard has been
considered.
 The wavelength of a selected radiation of light and is
used as the basic unit of length.
 Since the wavelength is not a physical one, it need not
be preserved & can be easily reproducible without
considerable error.

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 Meter as on Today: In 1983, the 17th general
conference on weights & measures ,proposed the use of
speed of light as a technically feasible & practicable
definition of meter.
 Meter is now defined as the length of path of travelled
by light in vacuum in (1/299792458) second. The
light used is iodine stabilized helium-neon laser.
Advantages of using wave length standards:
 1. Length does not change.
 2. It can be easily reproduced easily if destroyed.
 3. This primary unit is easily accessible to any physical
laboratories.
 4. It can be used for making measurements with much
higher accuracy than material standards.
 5. Wavelength standard can be reproduced consistently at
any time and at any place.
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Subdivision of standards
The imperial standard yard and the international prototype meter
are master standards & cannot be used for ordinary
purposes. Thus based upon the accuracy required, the standards
are subdivided into four grades namely;
1. Primary Standards
2. Secondary standards
3. Teritiary standards
4. Working standards

Primary standards:
 They are material standard preserved under most careful
conditions.
 These are not used for directly for measurements but are used
once in 10 or 20 years for calibrating secondary standards.
Ex: International Prototype meter, Imperial Standard yard.

86
Secondary standards:
 These are close copies of primary standards w.r.t
design, material & length.
 Any error existing in these standards is recorded by
comparison with primary standards after long
intervals.
 They are kept at a number of places under great
supervision and serve as reference for tertiary
standards. This also acts as safeguard against the loss
or destruction of primary standards.

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Tertiary standards:
 The primary or secondary standards exist as the
ultimate controls for reference at rare intervals.
 Tertiary standards are the reference standards
employed by National Physical laboratory
 (N.P.L) and are the first standards to be used for
reference in laboratories & workshops.
 They are made as close copies of secondary
standards & are kept as reference for comparison
with working standards.

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Working standards:
These standards are similar in design to primary,
secondary & tertiary standards.
But being less in cost and are made of low grade
materials, they are used for general applications in
metrology laboratories.
Sometimes, standards are also classified as;
• Reference standards (used as reference purposes)
• Calibration standards (used for calibration of
inspection & working standards)
• Inspection standards (used by inspectors)
• Working standards (used by operators)

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GENERAL MEASUREMENT SYSTEM

Filter

Cables
, wires
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 PRESSURE GAUGE

92
BORDON TUBE
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96
Generalised Measurement Systems
https://www.youtube.com/watch?v=oAdNKL8SgNY
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1. Primary Sensing Element:
The primary sensing element receives signal of the
physical quantity to be measured as input. It converts the
signal to a suitable form (electrical, mechanical or other
form), so that it becomes easier for other elements of the
measurement system, to either convert or manipulate it.

2. Variable Conversion Element:

Variable conversion element converts the output of the
primary sensing element to a more suitable form. It is
used only if necessary.
3. Variable Manipulation Element:
Variable manipulation element manipulates and
amplifies the output of the variable conversion element. It
also removes noise (if present) in the signal.
4. Data Processing Element:
Data processing element is an important element used in
many measurement systems. It processes the data signal
received from the variable manipulation element and
produces suitable output.
Data processing element may also be used to compare the
measured value with a standard value to produce required
output.
5. Data Transmission System:
Data Transmission System is simply used for transmitting
data from one element to another. It acts as a
communication link between different elements of the
measurement system. Some of the data transmission
elements used are cables, wireless antennae,
transducers, telemetry systems etc.
6. Data Presentation Element:
It is used to present the measured physical quantity
in a human readable form to the observer. It
receives processed signal from data processing
element and presents the data in a human readable
form. LED displays are most commonly used as
data presentation elements in many measurement
systems.
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