Definition:
A standard is defined as
something that is set up and established by authority as a rule for the
measurement of quantity, weight, extent, value or quality etc.
Any system of measurement must be related
to a known standard otherwise the measurement has no meaning. Industry
commerce, international trade and in fact modern civilization itself would be
impossible without a good system of standards. The role of standards is to
support the system which makes uniform measurement throughout the world an
helps to maintain interchangeability in mass production.
Measurement: Measurement is the
act , or the result of a quantitative comparison between a predetermined
standard and an unknown magnitude. Measurement provides us with a means of
describing various phenomena in quantitative terms. The quantities such as
pressure, displacement, accoustics, temperature, fluid flow and related
parameters, mass, length, time etc; can be done by mechanical measurement.
These quantities can also be measured by electrical means by transducing them
into an analogous electrical quantity.
System of
measurement: A measuring system is based on few fundamental units
e.g., length, mass, time, temperature, etc. All the physical quantities can be
expressed in terms of these fundamental units. the following systems of
measurement are in use in different countries such as FPS, Metric and SI
system. Our country follows SI system which is an extension of metric system.
SI base unit
definitions:
The metre is the length of the path travelled by light in a vacuum
during a time interval of 1/299 792 458 of a second.
The kilogram is equal to the mass of the international prototype of
the kilogram.
The second is the duration of 9 192 631 770 periods of the radiation
corresponding to the
transition between the two hyperfine levels of the ground state of the
caesium-133 atom.
The ampere is that constant current which, if maintained in two
straight parallel
conductors of infinite length, of negligible circular cross-section,
and placed 1 metre apart in vacuum, would produce between these conductors a
force equal to 2 x 10-7 newton per metre of length.
The kelvin is the fraction 1/273.16 of the thermodynamic temperature
of the triple point of water.
The mole is the amount of substance of a system that contains as
many elementary entities as there are atoms in 0,012 kg of carbon-12. When the
mole is used, the elementary entities must be specified and may be atoms, molecules,
ions, electrons, other particles, or specified groups of such particles.
The candela is the luminous intensity in a given direction of a
source that emits monochromatic radiation of frequency 540 x 1012 hertz and has
a radiant intensity in that direction of 1/683 watts per steradian.
Subdivision of
standard:
Depending upon the importance of accuracy required for the work the
standards are subdivided into four grades:-
Primary standards:
For precise definition of the unit,
there shall be one, and only one material standard, which is to be preserved
under most careful conditions. It is called as primary standard. International meter
is an example of primary standard. Primary standard is used only at rare
intervals solely for comparison with secondary standard. It has no direct
application to a measuring problem encountered in engineering.
Secondary standard:
Secondary standards are made as
nearly as possible exactly similar to primary standards as regard design,
material and length. They are compared with primary standards after ling
intervals and the records of deviation are noted. These stands are kept at
number of places for safe custody. They are used for occasional comparison with
tertiary standards whenever required.
Tertiary standards: The primary and secondary
standards are applicable only as ultimate control. Tertiary standards are the
first standard to be used for reference purpose in laboratories and workshops.
They are made as true copy of the secondary standards. They are used for
comparison at intervals with working standards.
Working standards:
Working standards are used more
frequently in laboratories and workshops. They are usually made of low grade of
material as compared to primary, secondary and tertiary standards, for the sake
of economy. They are derived from fundamental standards. Both line and end
working standards are used.
Line
and end standards
Line Standards – When length is
measured as the distance between centers of two engraved lines, it is called
Line Standards. Material Standards, yard and meter are line standards E.g.
Scale, Rulers, Imperial Standard Yard.
Characteristics of Line Standards :
(i) Scale can be accurately emblemed, but the engraved lines posses
thickness and it is not possible to accurately measure
(ii) Scale is used over a wide range
(iii) Scale markings are subjected to wear. However the ends are subjected
to wear and this leads to undersized measurements
(iv) Scale does not posses built in datum. Therefore it is not possible
to align the scale with the axis of measurement
(v) Scales are subjected to parallax errors
(vi) Assistance of magnifying glass or
microscope is required.
End Standards –
When length is
expressed as the distance between centers of two flat parallel faces, it is
called End Standards. Slip Gauges, End Bars, Ends of micrometer Anvils.
Characteristics of End Standards:
(i) Highly accurate and used for measurement of closed tolerances in
precision engineering as well as standard laboratories, tool rooms, inspection
departments.
(ii) They require more time for measurement and measure only one
dimension.
(iii) They wear at their measuring faces.
(iv) They are not subjected to parallax error.
Comparison
between line and end standard
S.No.
|
Characteristics
|
Line
standard
|
End
standard
|
1
|
Principle
|
Length is expressed
as distance between 2 lines
|
Length is expressed as distance between 2 ends
|
2
|
Accuracy
|
Ltd.
To ± 0.2mm.
|
Highly
accurate of closed tolerances to ±0.001mm
|
3
|
Ease
|
Quick and easy
|
Time consuming and
requires skill
|
4
|
Effect of wear
|
Wear at only the
ends
|
Wear at measuring surfaces
|
5
|
Alignment
|
Cannot be easily
aligned
|
Easily aligned
|
6
|
Cost
|
low cost
|
High cost
|
7
|
Parallax Effect
|
Subjected to
parallax effect
|
Not subjected to parallax effect
|
8
|
Examples
|
Scale, meter
|
Slip gauges, end bars, vernier caliper, micrometer etc.
|
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