Standards of measurement

1. Primary standard
2. Secondary standard
3. Working standard 

1. Primary standard: The highest standard of either a base unit or a derived unit is called as primary standard.These standard are copies of international prototypes and are kept by national standard laboratory. These standardized units are quite stable independent invariant.The primary standards constitute the ultimate bases of the references and are used for the reference verification and calibration of secondary standards.The primary standard are not available for the use outside the national laboratories
   
   
2. Secondary standard: These are the reference calibrated standard designed and calibrated from the primary standards.These are sent periodically to the national standard laboratory for their calibration.These standard are kept by the measurement laboratories and the industrial organization
   
   
3. Working standard: These standard have an accuracy one order lower than that of the secondary standard.These are normal standard used by the workers and technician who actually carry out measurement.