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.