Measurement Standards

The International Prototype Kilogram (IPK) is an artifact standard or prototype that is defined to be exactly one kilogram mass.

Primary reference standards[edit]

Main article: Primary standard

A set of gauge blocks are used as a working standard to check the calibration of measurement tools such as micrometers.

An example of a primary standard is the international prototype kilogram (IPK) which is the master kilogram and the primary mass standard for the International System of Units (SI). The IPK is a one kilogram mass of a platinum-iridium alloy maintained by theInternational Bureau of Weights and Measures (BIPM) in Sèvres, France.

Another example is the unit of electrical potential, the volt. Formerly it was defined in terms of standard cell electrochemical batteries, which limited the stability and precision of the definition. Currently the volt is defined in terms of the output of a Josephson junction,^[3]which bears a direct relationship to fundamental physical constants.

In contrast, the reference standard for the meter is no longer defined by a physical object (as the former international prototype meter(IPM) or originally the mètre des Archives). In 1983, the standard meter was redefined as the distance light travels in a vacuum during 1/299,792,458 of a second.^[4]

Secondary reference standards[edit]

Secondary reference standards are very close approximations of primary reference standards. For example, major national measuring laboratories such as the US's National Institute of Standards and Technology (NIST) will hold several "national standard" kilograms, which are periodically calibrated against the IPK and each other.^[5]

Working standards[edit]

Although the SI definition of the "meter" is based on a laboratory procedure combining the speed of light and the duration of a second, a machine shop will have a physical working standard (gauge blocks for example) that are used for checking its measuring instruments. Working standards and certified reference materials used in commerce and industry have a traceable relationship to the secondary and primary standards.

Working standards are expected to deteriorate, and are no longer considered traceable to a national standard^[6] after a time period or use count expires.^[7]

Laboratory Standards[edit]

National organizations provide calibration and private industrial laboratories with items, processes and/or certification so they can provide certified traceability to national standards. (In the United States, NIST operates the NVLAP program.^[8]) These laboratory standards are kept in controlled conditions to maintain their precision, and used as a reference for calibration and creating working standards.^[9] Sometimes they are (incorrectly) called "secondary standards" because of their high quality and reference suitability