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Questions and answers

What is a National Metrology Institute?

Providing and maintaining the UK’s measurement infrastructure

Most countries have their own National Metrology Institute (NMI) which is recognised globally and is part of an international system for ensuring the comparability of measurement standards and measurement capabilities.  There is only one NMI in each country, and it maintains that country's national standards and provides traceability to the International System of Units (the SI) at stated levels of confidence - often called measurement uncertainty.

NPL is the UK’s National Metrology Institute. It realises, maintains and develops the UK's primary measurement standards and uses them to provide calibrations, measurement services and testing facilities for users.  

There are other measurement laboratories called Designated Institutes (DIs) which assist the NMI by providing expertise in areas not covered by the NMI. In the UK this network is part of the National Measurement System (NMS) which maintains the UK's measurement infrastructure, represents the position of UK measurement internationally and influences the development of documentary standards. 

Metrology is not all NPL does, but it is fundamental to the description of NPL as the UK’s NMI.

Why are measurement and metrology important?

Measurement is important because most things in everyday life, practical science, technology, engineering and medicine involve measurements. It is the process of experimentally obtaining values that can reasonably be attributed to a quantity or property.

Metrology is important because it is the discipline that ensures the stability, comparability and accuracy of those measurements. Metrology, the ‘science of measurement’, is about examining and scrutinising the processes of measurement and its application.

Do you understand these terms?

Measurement traceability is the unbroken chain of comparisons between a given measurement device and national or international standards. It gives you confidence that all measurements are stable, comparable and correct within their associated measurement uncertainty.

Calibration is the comparison of measurement values delivered by test equipment or a test sample with those of standard equipment or a standard sample of known accuracy. It can be used to determine how accurate a measurement instrument is. Measurement uncertainty necessarily increases along the sequence of calibrations.

Measurement uncertainty is the doubt that exists about the result of any measurement. Explained another way, it is the range of values within which the true value of a measurement lies. All measurements are subject to uncertainty and a measurement result is complete only when it is accompanied by a statement of the associated uncertainty.


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