# SI Units and Physical Standards

The SI units, more commonly referred to as the metric system are the basis for modern calibrations.

When measuring length we refer to the meter. The history of the meter dates back to the 18th century in France. Yes, there is an actual meter physical standard. As with any physical standard, it is subject to environmental effects. In fact, the original calculation of the meter was incorrect due to a miscalculation and over time the need for a more stable definition became apparent. This lead to a definition of the meter as follows:

The meter is the length of the path traveled by light in a vacuum during a time interval of 1/299 792 458 of a second.

There are 7 base SI units:

SI units were chosen as they are considered independent of dimension. The kilogram is the last standard to still be tied to an actual object.

Why does this matter and what impact does it have on measurement and calibration?

A key consideration in any measurement is measurement uncertainty. This is the +/- in any measurement. Think of using a ruler that has 1/32 inch divisions. While you can guess where a measurement might fall in between the divisions, you only really know 1/32 of an inch. If you could measure to 1 millionth of an inch you would have a smaller measurement uncertainty than 1/32 of an inch.

The desire for smaller measurement uncertainty has lead scientists to research involving SI units for measurement. As advances in science occur it is likely that measurement uncertainty will decrease.

Take a look at the picture below from the UK.

NIST in the United States offers a similar picture.

The advances in measurement science are celebrated yearly on World Metrology Day.

Calibration laboratories that are accredited to ISO 17025 must demonstrate competence by establishing an unbroken traceability chain to the SI unit.

ISO/IEC 17025:2005 Clause 5.6.2
Measurement Traceability

• 5.6.2.1 “For calibration laboratories, the program for calibration of equipment shall be designed and operated so as to ensure that calibrations and measurements made by the laboratory are traceable to the International System of Units(SI) (Système international d’unités).”

• A calibration laboratory establishes traceability of its own measurement standards and measuring instruments to the SI by means of an unbroken chain of calibrations or comparisons linking them to relevant primary standards of the SI units of measurement. The link to SI units may be achieved by reference to national measurement standards. National measurement standards may be primary standards, which are primary realizations of the SI units or agreed representations of SI units based on fundamental physical constants, or they may be secondary standards which are standards calibrated by another national metrology institute. When using external calibration services, traceability of measurement shall be assured by the use of calibration services from laboratories that can demonstrate competence, measurement capability and traceability. The calibration certificates issued by these laboratories shall contain the measurement results, including the measurement uncertainty and/or a statement of compliance with an identified metrological specification (see also 5.10.4.2).

NOTE 1 Calibration laboratories fulfilling the requirements of this handbook are considered to be competent. A calibration certificate bearing an accreditation body logo from a calibration laboratory accredited to this handbook, for the calibration concerned, is sufficient evidence of traceability of the calibration data reported.

NOTE 2 Traceability to SI units of measurement may be achieved by reference to an appropriate primary standard (see VIM:1993, 6.4) or by reference to a natural constant, the value of which in terms of the relevant SI unit is known and recommended by the General Conference of Weights and Measures (CGPM) and the International Committee for Weights and Measures (CIPM).

NOTE 3 Calibration laboratories that maintain their own primary standard or representation of SI units based on fundamental physical constants can claim traceability to the SI system only after these standards have been compared, directly or indirectly, with other similar standards of a national metrology institute.

NOTE 4 The term “identified metrological specification” means that it must be clear from the calibration certificate which specification the measurements have been compared with, by including the specification or by giving an unambiguous reference to the specification.

NOTE 5 When the terms “international standard” or “national standard” are used in connection with traceability, it is assumed that these standards fulfill the properties of primary standards for the realization of SI units.

NOTE 6 Traceability to national measurement standards does not necessarily require the use of the national metrology institute of the country in which the laboratory is located.

NOTE 7 If a calibration laboratory wishes or needs to obtain traceability from a national metrology institute other than in its own country, this laboratory should select a national metrology institute that actively participates in the activities of BIPM either directly or through regional groups.

NOTE 8 The unbroken chain of calibrations or comparisons may be achieved in several steps carried out by different laboratories that can demonstrate traceability.

### Phil Wiseman

Phil Wiseman is Chief Marketing Officer at Alliance Calibration. He earned a B.S. in Chemical Physics from Centre College. Phil is an ASQ Certified Quality Auditor and ASQ Certified Manager of Quality/Organizational Excellence.

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