International Traceability (Metrology)

25.5.
As markets open up and trade takes, place at global level, all products need to be manufatured and tested for international acceptance. The method of testing and calibration need to have international traceability. National standards need to be traced to international standards. Obviously National Metrology Institutes have to play an important role in maintaining the measurement traceability to international systems. The precision measurement is related to the uncertainty in the measurement and traceability to SI units. Modern instrumentation requires various measurements with a reproducibility that exceeds the uncertainty of the realisation of the SI units. For this purpose metrologists have developed artifacts and experiments that can generate various parameters that are stable and reproducible to a level approaching 0.001 parts in 106.
National Physical Laboratory, India is playing a dominant role for the realisation of the standards of physical, chemical and electrical measurements with highest levels of accuracy and traceability to international measurement system. Considerable efforts are needed in
building the confidence that measurements made in one location in the world are compatible or equivalent to those made in other locations on the same or related products. For this, the measurements have to be applicable, comparable
and traceable to both national and international standards.
Following techniques are involved in validating these efforts : (i) Mutual Recognition Arrangement
(ii) Accreditation Procedures
(iii) Audit Processes
(iv) Proficiency Testing
(v) Measurement Assurance Programmes
(vi) Intercomparison and Calibration
(vii) Achieving Traceability and Timeliness in Traceability.
Traceability means to make the measurements traceable to SI units. Hierarchical chain follows following orders : SI units, primary standards, secondary standards, reference standards, working standards, and measuring standards. The definition and realisation of SI units are given by CIPM. The recommendations of CIPM give standards (primary) of highest metrological quality. As per latest norms, most of these standards based on quantum phenomena need to be intercompared before establishing these as primary standards. The intercomparison of primary standard establishes a relationship between primary standards used by various National Metrology Institutes. (NMI). This is known as equivalence of standards. Thus the intercomparison establishes traceability among standards of same quality (horizontal traceability). The vertical traceability is achieved through calibration. Both vertical and horizontal traceability, i.e. calibration and intercomparison are ways to establish traceability and to ensure reliable measurement of parts to be compatible worldwide.
The primary standards maintained at various NMIs have to be equivalent, there being no hierarchical relationship between them. The measurement capabilities of various NMIs are demonstrated to be equivalent within a certain range of uncertainty. Equivalence is declared by signing Mutual Recognition Arrangement (MRA) and declaring measurement capabilities on the basis of quality systems and intercomparisons/key comparisons among NMIs.
The assurance of laboratories technical competence to perform measurements by interlaboratory comparison is achieved by proficiency testing (PT). Participation in PT helps in building confidence among staff and provides independent and objective evidence to the measurement capability and quality assurance to participating labs.
Timeliness in traceability, i.e. having calibration at smaller periods (3 to 6 months) to take care of drifts helps to guarantee maximum reliability in measurement.