All instruments, whether electrical or electronic, are calibrated at the time of manufacture against a measurement standard.
1. INTERNATIONAL STANDARDS
These are defined by international agreement and are maintained at the international Bureau of Weights and Measurements in Paris.
2. PRIMARY STANDARDS
These are maintained at national standards laboratories in each country. They are not available for use outside these laboratories. Their principal function is to calibrate and verify the secondary standards used in industry.
3. SECONDARY STANDARDS
These are the basic reference standards used by industrial laboratories and are maintained by the particular industry to which they belong. They are periodically sent to national laboratory for calibration and verification against primary standards.
4. WORKING STANDARDS
These are the main tools of a measurement laboratory and are used to check and calibrate the instrument used in the laboratory.
