Introduction to metrology

Introduction to Metrology
PREPARED BY: RATNADEEPSINH M JADEJA

What is Metrology?
 Metrology comes from the Greek word “metron” and “logos” which literally
means the study of measurement.
 This study covers both the experimental and theoretical aspects of
measurement and the determination of the levels of uncertainty of these
aspects.
 The study of measurement is a basic requirement in any field of science
and technology, most importantly in engineering and manufacturing.
 Metrology is divided into three subfields.
1. Scientific or fundamental metrology
2. Applied or industrial metrology
3. Legal metrology
Prepared by: Ratnadeepsinh M Jadeja

Scientific or Fundamental Metrology
 This subfield deals with the establishment of units of measurement, unit
systems, and quantity systems. The units of measurement sets standards
adopted conventionally and by law, of the definite magnitude of a physical
quantity.
 On the other hand the units systems are composed of the traditional
systems, metric systems, and the natural systems.
 There are also some unit systems that are derived from a set of
fundamental units.
 Moreover, scientific metrology goes beyond than just the establishment of
units, and includes the realization of these standards to the users in the
society; and the development of new methods in measurement.

Applied or Industrial Metrology
 Applied metrology is rather specific in its applications, which are primarily
various industrial processes including manufacturing among others.
 This metrology subfield intends to establish the importance of
measurement in the industry. Moreover, it also ensures the
appropriateness of measurement instruments including the maintenance,
quality control, and proper calibration of these instruments.

Legal Metrology
 For the protection of life, the environment, health, and public safety,
regulatory requirements of measurement and measurement instruments
have to be looked after. These are the concerns of legal metrology.
 With the objective of regulating appropriate rules and regulations
pertaining to measurement, and measurement instruments as well; the
consumers are definitely protected and guaranteed that fair trade is
observed.

 Thus, in a broader sense metrology is not limited to length and angle
measurement but also concerned with numerous problems theoretical as
well as practical related with measurement such as:
1. Units of measurement and their standards, which is concerned with the
establishment, reproduction, conservation and transfer of units of measurement
and their standards.
2. Methods of measurement based on agreed units and standards.
3. Errors of measurement.
4. Measuring instruments and devices.
5. Accuracy of measuring instruments and their care.
6. Industrial inspection and its various techniques.
7. Design, manufacturing and testing of gauges of all kinds.

Need of Inspection
 Where and When to Inspect?
Inspection can be performed at any of several places in production:
1. Receiving inspection, when raw materials and parts are received from
suppliers.
2. At various stages of manufacturing, and
3. Before shipment to the customer.

Need of Inspection
 To determine the true dimensions of a part.
 To convert physical parameters into meaningful numbers.
 To test if the elements that constitute the system function as per the design.
 For evaluating the performance of a system.
 To ensure interchangeability with a view to promoting mass production.
 To establish the validity of design and for finding new data and new designs.
 To ensure that the part conforms to established standard.
 To meet interchangeability of manufacture.
 To maintain customer relations.
 To find shortcomings in manufacture.
 Helps to purchase good quality of raw materials.
 Helps co ordination of different departments.
 To take decision on defective parts.

 Measurement is an essential part of the
development of technology.
 When you can measure what you are speaking
about and express it in numbers, you know
something about it, but when you cannot
measure it, when you cannot express it in
numbers, your knowledge is a meager and
unsatisfactory kind
Principles of Measurements

Process of Measurements
Process of
Comparison
(Measurement)
Measurand
(Quantity to be
measured)
Result
(Read out)
Standard
(Known quantity)

 The methods of measurement may be broadly classified into two
categories:
1. Direct Measurement
2. Indirect Measurement
 Following are the three modes of measurement:
1. Primary Measurement
2. Secondary Measurement
3. Tertiary Measurement.

 The unknown quantity (also called the measurand) is directly compared against a
standard.
 The result is expressed as a numerical number and a unit.
 The standard, in fact, is a tangible form of a unit.
 Direct methods are quite common for the measurement of physical quantities like
length, mass and time.
Direct Method of Measurement

 Measurements by direct methods are not always possible, feasible and
practicable. These methods in most cases, are inaccurate because they
involve human factors. They are also less sensitive. Hence direct methods
are not preferred and are less commonly used.
 In engineering applications Measurement Systems are used. These
measurement systems use indirect methods for measurement purposes.
 A measurement system consists of a transducing element which converts
the quantity to be measured into an analogous signal. The analogous
signal is then processed by some intermediate means and is then fed to
the end devices which present the results of the measurement.
Indirect Methods of Measurement

 A primary measurement is one that can be made by direct
observation without involving any conversion (translation) of the
measured quantity into length.
 Typical example of primary measurements are 1. the matching of
two lengths, such as when determining the length of an object
with a metre rod, (2) the matching of two colors, such as when
judging the colour of red hot metals and (3) the counting of
strokes of a clock chime to measure the time.
Primary Measurement

 A secondary measurement involves only one translation (conversion) to be done
on the quantity under measurement to convert into a change of length.
 The measured quantity may be pressure of gas, and therefore, may not be
observable. Therefore, secondary measurement requires
1. An instrument which translates pressure changes into length changes, and
2. A length scale or a standard which is calibrated in length units equivalent to known
change in pressure.
 Therefore, in a pressure gauge, the primary signal (pressure) is transmitted to a
translator and the secondary signal (length) is transmitted to observer’s eye.
Secondary Measurements

 A tertiary measurements involves two translations (conversions). A typical examples
of such a measurement is the measurement of temperature of an object by
thermocouple.
Tertiary Measurements
Object
Translation
(Thermocouple)
Translation
(voltmeter)
Observer’s
Eye
Temp. Voltage Length
Primary
Signal
Secondary
Signal
Tertiary
Signal

Instruments
 The instrument serves as an extension of human faculties and enables the man to
determine the value of unknown quantity or variable which unaided human faculties can
not measure.
 A measuring instruments provides information about the physical value of some variable
being measured.
 The instrument would sense a physical parameter (e.g., velocity, pressure, temperature, etc.
), process and translate it into a format and range which can be interpreted by the
observer.
 The instrument must also provide the controls by which the operator can obtain,
manipulate and respond to the information.

 The instruments may be classified as follows:
1. Absolute and secondary instruments.
2. Analog and Digital instruments.
3. Mechanical, Electrical and electronic instruments.
4. Manual and automatic instruments.
5. Self-contained and remote indicating instruments.
6. Self-operated and power-operated instruments.
7. Deflection and null output instruments.
Classification of Measuring Instruments

1. Absolute instruments:
 These instruments give magnitude of quantity under measurement in terms
of physical constants of the instrument.
 Examples: Tangent galvanometer and Rayleigh’s current balance.
 These instruments are seldom used except in standard institutions.
2. Secondary Instruments:
 These instruments are so constructed that the quantity being measured can
only be measured by observing the output indicated by the instrument.
 These instruments are calibrated by comparison with an absolute
instrument or another secondary instrument which has already been
calibrated against an absolute instrument.
 Examples: Voltmeter, Glass Tube Thermometer, Pressure Gauge etc…
 The secondary instruments find wide use in every sphere of measurement.
Absolute and Secondary Instruments

1. Analog Instruments:
 The signal of an analog unit in a continuous fashion and can take
on infinite number of values in a given range.
 Examples: Fuel gauge, ammeters and voltmeters, wrist watch,
speedometer of an automobile etc.…
2. Digital Instruments:
 Signals varying in discrete steps and taking on a finite number of
different values in a given range are digital signals and the
corresponding instruments are of digital type.
 Examples: Odometer of an automobile, calibrated balance of a
platform scale, timer on a scoreboard, etc.
Analog and Digital Instruments

 These instruments are very reliable for static and
stable conditions.
 They are unable to respond to the
measurements of dynamic and transient
conditions due to the fact that they have
moving parts that are rigid, heavy and bulky and
consequently have a large mass. Mass presents
inertia problems and hence these instruments
cannot faithfully follow the rapid changes which
are involved in dynamic instruments.
 Most of the mechanical instruments cause noise
pollution.
Mechanical Instruments

 The electrical methods of indicating the
output of detectors are more rapid than that
of mechanical methods.
 Unfortunately an electrical system normally
depends upon a mechanical meter as an
indicating device. This mechanical
movement has some inertia due to which
the frequency response of these instruments
is poor.
Electrical Instruments

 Most of the scientific and industrial instruments require
very fast responses. Such requirements can not be met
with by mechanical and electrical instruments.
 These instruments use semiconductor devices. In electronic
devices, since the only movement involved is that of
electrons, the response time is extremely small owing to
very small inertia of electrons.
 With the use of electronic devices very weak signals can be
detected by using pre-amplifiers and amplifiers.
Electronic Instruments

 In case of manual instruments services of an operator are
required.
 Example: Measurement of temperature by resistance thermometer
incorporating a Wheatstone bridge in its circuit.
 In an automatic instruments an operator is not required.
 Example: Measurement of temperature by mercury in glass tube
thermometer.
Manual and Automatic Instruments

 A self contained instrument has all
its different elements in one physical
assembly.
 In a remote indicating instrument,
the primary sensing element may be
located at an adequate long
distance from the secondary
indicating element. Such types of
instruments are finding wide use in
the modern instrumentation
technology.
Self-contained and remote indicating instruments

 A self operated instrument does not require any outside
power for its operation; the out put energy is supplied
wholly or almost wholly by the input signal.
 Examples: Mercury in glass thermometer, Dial Indicator
 The power operated Instruments are those instruments
which requires some auxiliary source of power such as
electricity, compressed air, hydraulic supply etc. for their
operation. In such cases, the input signal supplies only an
insignificant portion of the output power.
 Example: Electro-mechanical measurement system
Self-operated and power-operated instruments

 In a “deflection type instrument”, the deflection of the instrument
provides a basis for determining the quantity under measurement. The
measured quantity produces some physical effect which deflects or
produces a mechanical displacement of the moving system of the
instrument.
 An opposing effect is built in the instrument which tries to oppose the
deflection or the mechanical displacement of the moving system. The
balance is achieved when opposing effect equals to cause producing
the deflection or mechanical movement. The deflection or mechanical
displacement at the point of balance then gives the value of measured
quantity.
 Examples: Permanent magnet moving coil (PMMC) ammeter, spring
scale for weight measurement, etc.
 These instruments are more suited for measurements under dynamic
conditions than null type of instruments whose intrinsic response is
slower.
Deflection and null output instruments

 In a “null type” of instrument, a zero or null indication leads to determination of
the magnitude of measured quantity. The null condition is dependent upon some
other known conditions.
 Example: D. C. Potentiometer.
 Advantages:
more accurate than deflection type
Highly sensitive as compared with deflection type.
Deflection and null output instruments

 The selection of measuring instruments depends on the measurement to be
performed.
1. The range and magnitude of the parameter to be measured and the accuracy of
the measurement (the instrument should have the range to cover effectively the
range of the parameter).
2. The resolution of the measuring instrument should be smaller than the minimum
unit of the measurement of the parameter.
3. Third important criterion for the selection of a measuring instrument is the
accuracy of measurement.
Selection of Measuring Instruments

 For example, if a process temperature of 100°C is being measured, the range of the
temperature indicator should be such that it can measure not only 100°C, but also
temperature above and below that value.
 Suppose the following thermometers are available:
(a) 0-99 °C (b) 0-199 °C (c) 0-99.9 °C (d) 0-199.9 °C
 From the range specification it is clear that the thermometers at (a) and (b) have a
resolution of 1 °C, while those at (c) and (d) have a resolution of 0.1 °C. For
measurement of the above parameter, i.e 100 °C, the thermometers at (a) and (C)
above are not suitable, since these do not have the required range. The choice is
therefore between (b) and (d).
Selection of Measuring Instruments

 Following are the three main functions of
instruments:
1. Indicating Function
2. Recording Function
3. Controlling Function
Functions of Instruments and Measurement
System

 The instruments and measurement systems are employed for the following
applications:
1. Monitoring of processes and operations.
2. Control of processes and operations.
3. Experimental engineering analysis.
Application of Measurement systems

1. Monitoring of processes and applications.
 In these type of applications the measuring instruments simply indicate the value
or condition of parameter under study and do not serve any control function.
 Examples :
1. Water and electric energy meters installed in homes.
2. An Ammeter or a voltmeter indicates the value of current or voltage being
monitored at a particular instant.

2. Control of processes and operations:
 There is a strong association between measurements and control. The instruments
find a very useful application in automatic control systems.
 A common example is the typical refrigeration system which employs a
thermostatic control. A temperature measuring device senses the room
temperature, thus providing the information necessary for proper functioning of
the control system.

3. Experimental engineering analysis:
 The engineering problems can be solved by theoretical as well as experimental
methods; several applications require the use of both methods.
 Following are the uses of experimental engineering analysis:
1. To determine system parameters, variable and performance indices.
2. To formulate the generalized empirical relationships in cases where there is no
proper theoretical backing.
3. To test validity of theoretical predictions.
4. To solve mathematical relationships with the help of analogies.

 Accuracy: It may be defined as conformity with or closeness to an accepted
standard value (true value).
 Accuracy of an instrument is influenced by factors like static error, dynamic error,
reproducibility, dead zone.
 Precision: It refers to the degree of agreement within a group measurements.
 It is usually expressed in terms of the deviation in measurement.
Accuracy and Precision…

 Error in Measurement = Measured Value – True Value.
Types of Errors
 Static errors, which includes
a. Reading errors
b. Characteristics errors.
c. Environmental errors.
 Instrument loading errors
 Dynamic errors
a. Systematic Errors
b. Random Errors
Errors in Measurement

Introduction to metrology

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