Calibration is the process of checking the performance of a measuring instrument or device against some commonly accepted standard. A watch, for example, has to be calibrated so that it keeps correct time, agreeing with the international standard. The dials on a radio must also be calibrated so that the correct frequency or station is actually being received. Calibration provides consistency in a variety of applications. Because rulers and calipers are calibrated, for instance, a 0.39 in (10 mm) nut made by one factory will fit a 0.39 in (10 mm) screw machined by another halfway around the world. Without calibration, such standardization and inter-changeability would not be possible.
Laboratories exist that provide official calibration of various instruments. In addition to clocks, such instruments and tools as electrical meters, laser beam power analyzers, torque wrenches, thermometers, and surveyors' theodolites all need calibration to an accepted standard to be useful. Calibration is performed by comparing the results of the instrument or device being tested (the value you actually get) to the accepted standard (the value you should get), and adjusting the instrument/device being tested until the two agree.
Frequency of calibration varies according to the device being calibrated and the applications. A clock or a common ruler for home use, for example, will only be calibrated at the time it is manufactured. A torque wrench on a NASA project, on the other hand, may require calibration every year. Some sophisticated electronic instruments for such projects may require calibration every few months.
In the United States, calibration of instruments or devices for high precision applications is generally traceable to standards established by the National Institute of Standards and Technology (NIST). In other words, if a laboratory is calibrating a meter stick for distance measurement, they need to prove that the standard they are measuring against has also been calibrated against the NIST definition of the meter. NIST keeps standard definitions of mass, length, temperature, etc. Historically, standards have been based on a "magic measure" such as the platinum-iridium bar initially used as the standard for the meter. The trend now is away from physical expressions of standards and toward standards based on some physical constant. One of the official definitions of the meter, for example, is based on the wavelength of light emitted by calcium atoms under certain specific conditions. Such a definition can be recreated at need and does not depend on the physical existence of a slab of metal. International standards have been agreed upon, simplifying international trade and science.
Traceability to NIST is not in and of itself a guarantee of accuracy, however. All measurements have some uncertainty associated with them. Common sense should thus be used when measurement accuracy requirements for an instrument or device are formulated. Accuracy is limited by calibration, and even calibration has its limits.
See also Caliper.