National Metrology Institute of Japan (NMIJ)

About Metrology

Standards for the SI Base Units

The International System of Units (SI) is structured based on the seven base units: the meter (m), the kilogram (kg), the second (s), the ampere (A), the kelvin (K), the mole (mol) and the candela (cd). These are clearly defined units for seven dimensionally independent quantities such as length, mass, time, electricity, thermodynamic temperature, amount of substance and luminous intensity. There are other units besides the base units, as shown in the figure below. They are called derived units, which are formed by a combination of the seven base units.


What is "Being Traceable"?

Measuring devices are calibrated by standards.
Those standards are, in turn, calibrated by standards with higher accuracy (lower uncertainty).
When you repeat this procedure over and over, you eventually end up to national standards.
When a measuring device is guaranteed to trace a calibration chain up to a national standard, you can say, "This device is traceable to the national standard".

The same applies to analytical apparatuses. When a reference material that is used to calibrate an analytical apparatus can climb up to a reference material acknowledged as a national standard at the end, the apparatus is considered to be traceable to the national standard.

Development of National Standards

In order to make all measuring devices and analytical apparatuses that we need traceable to national standards, all corresponding national standards must be developed.
NMIJ is making vigorous efforts in the development of national standards to meet this requirement.
In Japan, national standards are designated as primary measurement standards or primary reference materials by the Measurement Law, and each national standard is stipulated by government decrees from the Ministry of Economy, Trade and Industry.

Accreditation of Calibration Laboratories (Japan Calibration Service System, JCSS)

To ensure traceability to national standards, it is essential to maintain the reliability of calibration, in which a standard device is calibrated by a higher standard device, between national measurements and ones used at various locations such as factories and research institutes.
In order to calibrate standard devices or to produce reference materials, specific skills and equipment are necessary, and also a quality system is essential to maintain such skills and equipment. Laboratories are accredited by the National Institute of Technology and Evaluation (NITE) when they meet such requirements and pass the examination conducted by NITE.
Standard devices carried by accredited laboratories are called secondary measurement standards, and calibrated by primary measurement standards (or primary sub-measurement standards).
Similarly, reference materials produced by them are called secondary reference materials, and calibrated by primary reference materials.
This system is called "the Japan Calibration Service System" (JCSS).
For further information, visit the JCSS web site.


"Traceability", derived from its adjective form "traceable", is assured when standards that calibrate measuring instruments are guaranteed to trace up to national standards. When traceability is assured, general users can have their measuring instruments calibrated by accredited laboratories without any concern about the calibration chain between national standards and their instruments.

(Further information) The origin of "traceability"

The world's first launch of an artificial satellite by the then Soviet Union brought a tremendous shock to the United States.
This incident was named "the Sputnik Shock," the expression that is still used until the present. After the Sputnik Shock, the United States designed a carefully planned space program and immediately started to execute it, determined to outstrip the Soviet Union in the field of space.
The plan included the reliability assurance of measuring instruments and scientific data.
All measuring instruments were required to be traceable to then NBS, which is now called NIST; Data centers for each scientific field were established for the evaluation and distribution of various scientific data.
The present organizations do not necessarily share the original purposes, goals and scientific fields in the past, but the conviction still remains unchanged.


"Uncertainty" is a newly developed scale to convey the reliability of measurement data and has been used since 1990s. In the past, the concepts of "error" or "accuracy" were used to express the reliability of measurement. However, the practical application of these concepts widely varied, depending on the fields of technology and countries. Thus, the International Committee of Weights and Measures (CIPM) took the initiative in developing an integrated method for evaluating and describing the reliability of measurement data. The collaborated efforts resulted in a 1993 guide book co-published by the seven main international organizations of metrology --- "the Guide to the Expression of Uncertainty in Measurement." This guide book is often called "GUM" taking initial letters from the original title.

GUM defines "uncertainty" as a mean to express the degree of ambiguity in our knowledge gained by measurement and explains the detailed procedures of quantitative evaluation. An uncertainty value is based on two different ways of evaluation. The Type A evaluation uses the conventional statistical analysis of the standard deviation. The Type B evaluation uses the estimation of a value comparable to the standard deviation from a variety of information other than measurement data. The results from those two types of evaluation are then put together to get a combined uncertainty.

"Uncertainty" is now used in various technical and academic documents in which the reliability of measurement data play a significant role. The evaluation of uncertainty is an imperative requirement for standard specifications such as ISO 9000 (for quality systems) and ISO 17025 (for general requirements on the capability of calibration and testing laboratories).