Standards and types of standards
Standards and types of standards
Standards
The
term standard is used to denote universally accepted specifications for
devices. Components or processes which ensure conformity and interchangeability
throughout a particular industry. A standard provides a reference for assigning
a numerical value to a measured quantity. Each basic measurable quantity has
associated with it an ultimate standard. Working standards, those used in conjunction
with the various measurement making instruments.
The
national institute of standards and technology (NIST) formerly called National
Bureau of Standards (NBS), it was established by an act of congress in 1901,
and the need for such body had been noted by the founders of the constitution.
In order to maintain accuracy, standards in a vast industrial complex must be
traceable to a single source, which may be national standards.
Subdivision of standards
The
international standard yard and the international prototype meter cannot be
used for general purposes. For practical measurement there is a hierarchy of
working standards. Thus depending upon their importance of accuracy required,
for the work the standards are subdivided into four grades;
1. Primary standards
2. Secondary standards
3. Territory standards
4. Working standards.
1.
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 yard and International meter are the
examples of primary standards. Primary standard is used only at rare intervals
(say after 10 to 20 years) solely for comparison with secondary standards. It
has no direct application to a measuring problem encountered in engineering.
2.
Secondary Standards
Secondary
standards are made as nearly as possible exactly similar to primary standards
as regards design, material and length. They are compared with primary
standards after long intervals and the records of deviation are noted. These
standards are kept at number of places for safe custody. They are used for
occasional comparison with tertiary standards whenever required.
3.
Tertiary Standards
The
primary and secondary standards are applicable only as ultimate control.
Tertiary standards are the first standard to be used for reference purposes in
laboratories and workshops. They are made as true copy of the secondary
standards. They are used for comparison at intervals with working standards.
4.
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 standards are
made from H-cross-sectional form.
Sometimes
the standards are also classified as:
1.
Reference standards- Used for reference purposes.
2.
Calibration standards - Used for calibration of inspection and working
standards.
3.
Inspection standards - Used by inspectors.
4.
Working standards - Used by operators, during working.
