Metrology References and Resources

Following are references, resources, and definitions that provide additional details on metrology topics.

Common Acronyms

• A2LA: American Association for Laboratory Accreditation, a non-governmental organization that provides services for laboratory accreditation based on ISO 17025.
• ANSI: American National Standards Institute.
• DAkkS: Deutsche Kalibrierdienst, an association of calibration laboratories and entities in Germany.
• ISO: International Organization for Standardization, chartered to implement quality standards such as ISO 9001.
• NCSLI: National Conference of Standards Laboratories International, an independent industry organization.
• NIST: National Institute of Standards and Technology, U.S. government standards institute; maintains U.S. measurement artifacts.
• NPL: National Physical Laboratory, United Kingdom's metrology institute.
• PTB: Physikalisch-Technische Bundesanstalt, Germany's metrology institute.

Metrology Glossary

Accuracy
Conformity to fact, exactness. A measure of the closeness of an instrument's reading to the actual value of the parameter it is measuring (e.g. 99.998% accurate). Accuracy specifications often include stipulations as to range and other variables that affect accuracy.

Artifact
A reference standard to which subordinate standards are traceable. Typically maintained and supervised in a national metrology institute such as NIST in the United States. For example, a particular standard 10 volt artifact is presumed to be the most accurate available representation of 10 volts of electromotive force in that country.

Calibration
To check, adjust, or determine by comparison with a "standard" the readings from a measurement device. Ideally the standard is a device whose accuracy is traceable through a succession of higher standards. (See Traceability defined below or go to our Traceability page for more details).

In calibration, the intent is to determine how far the unknown instrument deviates from the standard. Calibration can also include adjustment of the unknown instrument readings to bring them closer to those obtained from the standard.

Measurement
The execution of a pre-determined set of operations on a device with the intent of determining some physical property of same.

Metrology
The science of weights and measures. The field of knowledge concerned with all measurements.

Precision
The state or quality of being precise; exactness. The measure of the ability of an instrument to repeat a measurement and get the same answer. Note that this does not necessarily imply the correct answer--that is a function of accuracy.

Repeatability
Closeness of the agreement between the results of successive measurements of the same measure and carried out under the same conditions of measurement.

Reproducibility
Closeness of the agreement between the results of measurements of the same measure and carried out under changed conditions of measurement.

Resolution
The fineness of detail that an instrument can present. For example, a mechanical clock with two hands (hours and minutes) has a resolution of one minute. Adding a hand for seconds improves the resolution to one second. However, higher resolution is no guarantee of accuracy. The clock with one-second resolution may be two hours slow.

Tolerance
The allowable variation from a standard. The limits of the range of values that apply to a properly functioning instrument.

Traceability
The "pedigree" of a calibration. The documented proof that the measurements taken during a calibration can be traced back to an accepted standard. This means that every measurement made during a calibration was made by an instrument whose calibration is certified as being performed correctly using proper standards that are traceable to a national standard. Go to Traceability for more details.

Uncertainty
The estimated amount by which an observed or calculated value may differ from the true value. The term "uncertainty" (e.g, uncertainty of +/-300 ppm + 10mV) is preferred over "accuracy" when describing measurements. Traditionally, uncertainty is reported in terms of the probability that the true value lies within a stated range of values.