Calibration

Calibration is the act of comparing an instrument's measurement accuracy to a known standard. The standard may be maintained by a national or international organization.

For physical constants, weights, and measures, there are known and agreed values in the International System of Units (SI). Such constants include the length of the meter, the mass of the kilogram, and the volume of a liter.

In the USA, a governmental organization called the National Institute of Standards and Technology maintains standards and is considered the arbiter and ultimate authority for values of SI units and industrial standards. NIST also defines traceability, by which an instrument's accuracy is established in an unbroken chain relating an instrument's measurements through one or more derivative standards to a standard maintained by NIST.