Standards of Measurement are classified into the following categories:
- International Standards
- Primary Standards
- Secondary Standards
- Working standards
These international standards are defined on the basis of international agreement. These standards mainly represent the units of measurements. Whose values are close to the possible accuracy. This then done with help of present day technological and scientific methods. These standards are also in regular check and evaluation against the absolute measurement.
The International Bureau of Weights and Measures maintains the international standards. They are not provided for the use of ordinary users for measuring instruments and calibrating them.
Primary standards:
Primary standards are of very high accuracy. We can also use them as ultimate reference standards. The national standards laboratory in different parts of world make them. One of the main functions of primary standards is the verification and calibration of secondary standards.
The basic considerations which are necessary for building primary standards are:
(i) The stability of the material should last longer.
(ii) The temperature coefficient of the material should also be as small as possible.
(iii) It is necessary to remove the deterioration of the material by moisture and other environmental conditions.
(iv)There should be accuracy in machining of parts.
(v) The measurement of physical dimension should be performed with special techniques because the accuracy of the standards depends on it.
(vi) There must be warranty of rigidity of the construction.
Secondary Standard:
The secondary standard is the basic reference standard. Then the industry involved is only responsible for maintenance and calibration of these standards. These standards are also checked locally against reference standards available in the area.
The national standards laboratories will then do the periodic calibration and comparison of these standards. The national laboratory certifies the secondary standards with regards to their values in terms of primary values.
Working Standards
The working standards are the major tools of the measurement laboratory. The working standards are also very useful for calibrating general laboratory instruments. This not only increases the instrument’s performance but its accuracy as well. They are also useful for quality control department. Therefore, this department uses them for checking the values of resistors.
The difference of measured value of any quantity and its actual value is called measurement error. This error is an observational error.
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