Standard (metrology)
In metrology (the science of measurement), a standard (or etalon) is an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity.[1] Standards are the fundamental reference for a system of weights and measures, against which all other measuring devices are compared. Historical standards for length, volume, and mass were defined by many different authorities, which resulted in confusion and inaccuracy of measurements. Modern measurements are defined in relationship to internationally standardized reference objects, which are used under carefully controlled laboratory conditions to define the units of length, mass, electrical potential, and other physical quantities.
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]
A machine shop will have physical working standards (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.