Standardization - international standardization, the Bureau of Indian Standards (BIS) - legal metrology - definition

STANDARDS OF MEASUREMENTS

In metrology, length is the important parameter even angles can be measured by combination of linear measurements.

It is essential that some standards be prescribed for the length.

For Linear Measurements,

i) LINE STANDARD :

When the length being measured is expressed as the distance between two lines, then it is called as “ Line standard”.

It does not provide high accuracy as that of end standard.

ii) END STANDARD

When the length being measured expressed as the distance between the two parallel end faces” then it is called “End Standard”.

End standards can be made to a very high degree of accuracy.

Ex: Slip gauge, end gauge, end of micrometer anvils.

iii) WAVE LENGTH STANDARD:

Material standards are liable to damage and their dimensions change slightly with time.
But with the monochromatic light, we have the advantage of constant wavelength.
Since the wavelength is not a physical one, it need not be preserved. This is reproducible one and the error of reproduction is negligible.
The international standards measures the metre in terms of wavelength of krypton Kr86, cadmium 114 & Mercury 198 but Kr 86 is the most suitable element.

SUBDIVISIONS OF STANDARDS:

Depending upon the importance of accuracy required the standards are divided into four grades.

i) Primary standards

ii) Secondary standards

iii) Teritary standards

iv) Working standards

i) PRIMARY STANDARDS:

To define a unit most precisely, there is only one material standard which is preserved under very specifically created conditions such type of material standard is known as primary standard.

Ex: International yard and the International prototype meter. This has no direct application to a measuring problem.

They are use only at rare intervals of 10 to 20 yards solely for comparison with secondary standards.

ii) SECONDARY STANDARDS:

Secondary standards should be exactly alike the primary standard with respect to design, material and length.
Initially they are compared with primary standards after long intervals and records of deviation are noted.
These standards should be kept at a number of places in custody for occasional comparison with teritary standards.
This also acts as safeguard against the damages of primary standards.

iii) TERITARY STANDARDS:

Teritary standards are the first standard to be used for reference in laboratories and workshops.
They are also made a true copies of secondary standards.
They are kept as reference for comparison with working standards.

iv) WORKING STANDARDS:

Working standards developed for laboratories and workshops.
These standards are similar in design to primary, secondary and teritary Standards.
They are less in cost and also made of low grade materials.

Standards are classified as

Reference standards – used for reference purpose

Calibration standard – used fro calibration of inspection and working standard

Inspection standard – used by inspectors

Working standard – used by operators

DEPARTMENT OF MECHANICAL ENGINEERING