METROLOGY

METROLOGY

Science of Measurements

For engineering purposes it is

restricted to measurements of length
and angles and other quantities
which are expressed in linear or
angular terms
• Metrology is mainly concerned with:
(i) Establishing the units of measurements, reproducing these units
in the form of standards and ensuring the uniformity of
measurements.
(ii) Developing methods of measurement.
(iii) Analysing the accuracy of methods of measurement, researching
into the causes of measuring errors and eliminating these.

• In the broader sense, metrology is not limited to length

measurement but is also concerned with the industrial inspection
and its various techniques. Inspection is carried out with gauges
and the metrologist is intimately concerned with the design,
manufacturing and testing of gauges of all types.
 LEGAL METROLOGY
“Legal Metrology” is that part of metrology which treats units of
measurements, methods of measurements and the measuring
instruments, in relation to the technical and legal requirements.

The activities of the service of “Legal Metrology” are :

(i) Control (testing, verification, standardisation) of measuring
instruments;
(ii) Testing of prototypes/models of measuring instruments;
(iii) Examination of a measuring instrument to verify its conformity to
the statutory requirements etc.
 DYNAMIC METROLOGY

‘Dynamic metrology' is the technique of measuring small

variations of a continuous nature. The technique has proved very
valuable, and a record of continuous measurement, over a
surface, for instance, has obvious advantages over individual
measurements of an isolated character.
 DETERMINISTIC METROLOGY

Deterministic metrology is a new philosophy in which part

measurement is replaced by process measurement. The new
techniques such as 3D error compensation by CNC (Computer
Numerical Control) systems and experts systems are applied,
leading to fully adaptive control. This technology is used for very high
precision manufacturing machinery and control systems to achieve
microtechnology and nanotechnology accuracies.
 OBJECTIVES OF METROLOGY
Basic objective of a measurement is to provide the required accuracy at a minimum
cost, metrology has further objectives in a modern engineering plant with different
shapes which are:
1. Complete evaluation of newly developed products.
2. Determination of the process capabilities and ensure that these are better than
the relevant component tolerances.
3. Determination of the measuring instrument capabilities and ensure that they are
quite sufficient for their respective measurements.
4. Minimising the cost of inspection by effective and efficient use of available
facilities.
5. Reducing the cost of rejects and rework through application of "Statistical quality
control techniques”.
6. To standardise the measuring methods.
7. To maintain the accuracies of measurement.
8. To prepare designs for all gauges and special inspection fixtures.
NECESSITY AND IMPORTANCE OF METROLOGY
The importance of the science of measurement as a tool for scientific research (by
which accurate and reliable information can be obtained). This is essential for
solving almost all technical problems in the field of engineering in general, and in
production engineering and experimental design in particular.

Higher productivity and accuracy is called for by the present manufacturing

techniques. This cannot be achieved unless the science of metrology is understood,
introduced and applied in industries. Improving the quality of production
necessitates proportional improvement of the measuring accuracy and marking out
of components before machining and the in-process and post process control of the
dimensional and geometrical accuracies of the product. Proper gauges should be
designed and used for rapid and effective inspection. Also automation and
automatic control, which are the modern trends for future developments, are based
on measurement. Means for automatic gauging as well as for position and
displacement measurement with feedback control have to be provided.
 DIMENSIONAL ACCURACY
For any particular feature of an engineering component, the degree of accuracy
necessary, varies according to the function of the feature. Considering components,
generally, those intended for use with aircraft engines or similar units will require making
to a higher degree of accuracy than components for use with such products as
agricultural machinery. On the other hand, any component part (whatever its function
may be) must have its working features correct dimensionally; they must be made to
some geometric form, and be definite in relationship to the working features of one or
more mating features, or adjacent components, or both. Whether or not a feature (e.g. a
surface) works in conjunction with another mating feature, or fits another feature without
relative movement, or is merely free in air, depends upon its mechanical function.
However, either of the first two conditions may be of such vital importance that gauging,
or accurate measurement of the feature in one or more ways, proves essential.
Whatever method of dimensional control is adopted, it can be successful only if used in
conjunction with a fundamental standard of length. In other words, all measurements are
comparative. Working standards of length are such, in fact, only if they have been
determined relative to some ultimate standard.
 PRECISION MEASUREMENT - ITS NEED
The mass production which characterise so many branches of modern engineering
manufacture would be impossible if component parts could not be produced to close
dimensional tolerances (and thus made interchangeable in motor vehicles, refrigerators and
washing machines etc.) It is seldom, however, that the components themselves are
subjected to precision checks. It is therefore essential that accuracy required should be
built into the machine tools, jigs and fixtures which produce them. Precision measurement
is concerned with the precise determination of the linear, angular and non-linear functions
of the machine surfaces of the tools and devices used to produce engineering components.
Precision measurement must be carried our on both the dies and punches of the press
tools used, and provided the dimensions are within the limits laid down the press tool can
be put into production with every confidence in the acceptability of the parts produced. It
has been observed that precision measurements are always required in the manufacture of
machine tools such as lathes, milling machines and drilling machines. The dimensional and
geometric accuracy of the components produced using the above machine tools is
proportional to the inherent accuracy built into the machine tool and thus the operator or
craftsman is able to produce work which is accurate.
 QUALITY CONTROL-METROLOGY AS A MEANS TO ACHIEVE
Whenever parts must be inspected in large numbers, hundred percent inspection of each part
is not only slow and costly, but in addition does not eliminate all of the defective pieces. Mass
inspection Tends to be careless; operators become fatigued; and inspection gauges become
worn or out of adjustment more frequently. The risk of passing defective parts is variable and
of unknown magnitude, whereas in a planned sampling procedure the risk can be calculated.
Many products such as fuses or matches cannot be hundred percent inspected since any
final test made on one results in the destruction of the product. Inspection is costly and
nothing is added to a product that has been produced to specifications.
An inspector to sample the parts being produced in a mathematical manner and to determine
whether or not the entire stream of production is acceptable, provided that the company is
willing to allow a certain known number of defective parts. The following steps must be taken
while using quality control techniques:
(i) Sample the stream of products.
(ii) Measure the desired dimensions in the sample.
(iii) Calculate the deviations of the dimensions from the mean dimension.
(iv) Construct a control chart.
(v) Plot succeeding data on the control chart
 STANDARDS OF MEASUREMENTS

Two systems are generally used:

- Metric System (originated in France)
- Yard System (Imperial or English)

For linear measurements the various standards known are :

l. Line standard
2. End standard
3. Wave length standard
 LINE STANDARD
A yard or metre is defined as the distance between scribed lines on a
bar of metal under certain conditions of temperature and support.
These are legal standards and Act of Parliament authorises their use.

• The Metre is defined as 1650763.73 wavelengths of the orange

radiation in vacuum of krypton-86 isotype.

• The Yard is defined as 0.9144 metre. This is equivalent to

1509458.35 wavelengths of the same radiation
 YARD
A Yard was formerly known as the
Imperial Standard Yard

It consists of a bronze bar made

from an alloy known as Baily's metal,
consisting of 16 parts copper, 2½
parts tin and 1 part zinc. The bar, 1
sq. in. in cross-section has an
overall length of 36". Two counter
bored holes, ½" diameter by ½"
deep, at 36" centres (1" from each
end of the bar) provide sighting
holes for two gold plugs inserted in
the holes at the base of each
counter bore.
 METRE
The length of the metre is
defined as the distance, at 0 C
between the centre portions of
pure platinum- irridium alloy
(10% irridium) of 102 cm total
length-and having a cross-
section as shown figure. The
graduations are on the upper
surface of web which
coincides with the neutral axis
of the section
 SUB-DIVISION OF STANDARDS
Primary These are used only at rare intervals and solely for comparison
standards with secondary standard
Secondary These standards are distributed to a number of places for safe
standards custody and used in their turn for occasional comparison with
tertiary standards. These standards also act as safeguard
against the loss or destruction of primary standards
Tertiary Tertiary standards are the first standards to be used for reference
standards purposes in Laboratories and workshops. These should also be
maintained as a reference for comparison at Intervals with
working standards.
Working These standards are necessary for use in metrology laboratories
standards and similar institutions. These are derived from fundamental
standards.
 SOMETIMES STANDARDS ARE CLASSIFIED

Reference standards used for reference purposes

Calibration standards used for calibration of inspection
and working standards
Inspection standards used by inspectors
Working standards used by operators
 CHARACTERISTICS OF LINE STANDARDS
1. Accurate engraving on the scales can be done but it is difficult to take
full advantage of this accuracy. For example, a steel rule can be read to
about 0.2 mm of true dimension.
2. It is easier and quicker to use a scale over a wide range.
3. The scale markings are not subject to wear although significant wear
on leading end leads to under sizing.
4. There is no 'built in’ datum in a scale which would allow easy scale
alignment with the axis of measurement this again leads to under sizing.
5. Scales are subjected to the parallax effect, a source of both positive
and negative reading errors.
6. For close tolerance length measurement (except in conjunction with
microscopes) scales are not convenient to be used.
 END STANDARD
End standards, in the form of the bars and slip gauges, are in general use in
precision engineering as well as in standard laboratories such as the N.P.L.
(National Physical Laboratory). Except for applications where microscopes
can be used, scales are not generally convenient for the direct measurement
of engineering products, whereas slip gauges are in everyday use in tool-
rooms, workshops, and inspection departments throughout the world.

A modern end standard consists fundamentally of a block or bar of steel

generally hardened whose end faces are lapped flat and parallel to within a
few millionth of a cm. By the process of lapping, its size too can be controlled
very accurately. Although, from time to time, various types of end bar have
been constructed, some having flat and some spherical faces, the flat,
parallel faced bar is firmly established as the most practical method of end
measurement.
 CHARACTERISTICS OF END STANDARDS
1. Highly accurate and well suited to close tolerance measurements.
2. Time – consuming in use.
3. Dimensional tolerance as small as 0.0005 mm can be obtained.
4. Subjected to wear on their measuring faces.
5. To provide a given size, the groups of blocks are "wrung“ together.
Faulty wringing leads to damage,
6. There is a "built-in" datum in end standards, because their measuring
faces are flat and parallel and can be positively located on a datum
surface.
7. As their use depends on "feel" they are not subject to the parallax
effect.
End bars: Primary end standards usually consist of bars of carbon
steel about 20 mm in diameter and made in sizes varying from l0 mm
to 1200 mm. These are hardened at the ends only. They are used for
the measurement of work of larger sizes.

Slip gauges: Slip gauges are used as standards of measurement in

practically every precision engineering works in the world. These were
invented by C.E. Johansom of Sweden early in the present century.
These are made of high-grade cast steel and are hardened
throughout. With the set of slip gauges combination of slip gauge
enables measurements to be made in the range of 0.0025 to 100 mm
but in combinations with end/length bars measurement range upto
1200 mm is possible.
 WAVELENGTH STANDARD
It is obvious from the method given above for comparing and verifying
the sizes of gauges that a considerable difficulty has to be faced in their
use. This is because the working standard to which one can refer is
derived from a physical standard and in establishing the size of a
working standard by above process, successive comparisons must be
made. This leads to errors of unacceptable order of magnitude. Using
wavelength of monochromatic light which is natural and invariable unit
of length, the working standard is no more dependent upon the physical
standard. Rather the definition of a standard of length relative to the
metre is expressed in terms of the wavelength of the red radiation of
cadmium. Thus for all practical purposes the use of phenomenon of the
interference of light waves to provide working standard may be
accepted as ultimate.
For some time light wavelength standard had to be objected because of the
impossibility of producing pure monochromatic light as wavelength depends
upon the amount of isotope impurity in the elements. But now with the rapid
development in atomic energy industry, pure isotopes of natural elements have
been produced. Cadmium 114, Krypton 86, and Mercury 198 are possible
sources of radiation of wavelengths suitable as natural standard of length.

Since wavelength standard is not a physical one, it need not be preserved. This
is reproducible standard of length, and the error of reproduction can be of the
order of 1 part in 100 million.

Finally it was decided that Kr 86 is the most suitable element if used in a hot-
cathode discharge lamp maintained at 68K temperature. The orange radiation
was selected for the measurement. According to this standard, metre is defined
as 1650763.73 x wavelength of the radiation corresponding to the transition
between the level 2p10-5d5 of the Krypton 86 atom in vacuum
 ADVANTAGES OF WAVELENGTH STUNDARDS
The following are the advantages of using wavelength standard as
basic unit to define primary standards:
1. It is not influenced by effects of variation of environmental
temperature, pressure, humidity and ageing because it is not a material
standard.
2. There is no need to store it under security and thus there is no fear
of its being destroyed as in the case of yard and metre.
3. It is easily available to all standardising houses, laboratories and
industries.
4. It can be easily transferred to other standards.
5. This standard can be used for making comparative statement of a
much higher accuracy.
6. It is easily reproducible.