Editing Calibration (section)

===Quality of calibration===
To improve the quality of the calibration and have the results accepted by outside organizations it is desirable for the calibration and subsequent measurements to be "traceable" to the internationally defined measurement units.  Establishing [[traceability]] is accomplished by a formal comparison to a [[Standard (metrology)|standard]] which is directly or indirectly related to national standards (such as [[NIST]] in the USA), international standards, or [[certified reference materials]]. This may be done by national standards laboratories operated by the government or by private firms offering metrology services.

[[Quality management system]]s call for an effective [[metrology]] system which includes formal, periodic, and documented calibration of all measuring instruments.  [[ISO 9000]]<ref name="iso9001">ISO 9001: "Quality management systems — Requirements" (2008), section 7.6.</ref> and [[ISO 17025]]<ref name="iso17025">ISO 17025: "General requirements for the competence of testing and calibration laboratories" (2005), section 5.</ref> standards require that these traceable actions are to a high level and set out how they can be quantified.

To communicate the quality of a calibration the calibration value is  often accompanied by a traceable uncertainty statement to a stated confidence level.  This is evaluated through careful uncertainty analysis.
Some times a DFS (Departure From Spec) is required to operate machinery in a degraded state. Whenever this does happen, it must be in writing and authorized by a manager with the technical assistance of a calibration technician.

Measuring devices and instruments are categorized according to the physical quantities they are designed to measure. These vary internationally, e.g., [[National Institute of Standards and Technology|NIST]] 150-2G in the U.S.<ref>{{Cite journal|url = https://www.nist.gov/nvlap/upload/hb150-2g-1.pdf|title = Calibration Laboratories: Technical Guide for Mechanical Measurements|last1 = Faison|first1 = C. Douglas|date = March 2004|journal = NIST Handbook 150-2G|access-date = 14 June 2015|last2 = Brickenkamp|first2 = Carroll S.|publisher = [[NIST]]|archive-date = 12 May 2015|archive-url = https://web.archive.org/web/20150512215554/http://www.nist.gov/nvlap/upload/hb150-2g-1.pdf|url-status = dead}}</ref> and [[National Accreditation Board for Testing and Calibration Laboratories|NABL]]-141 in India.<ref>{{Cite web|url = http://www.fcriindia.com/national-training-2/metrology-pressure-thermal-electrotechnical-measurement-calibration/|title = Metrology, Pressure, Thermal & Eletrotechnical Measurement and Calibration|access-date = 14 June 2015|publisher = Fluid Control Research Institute (FCRI), Ministry of Heavy Industries & Public Enterprises, Govt. of India|archive-url = https://web.archive.org/web/20150614205726/http://www.fcriindia.com/national-training-2/metrology-pressure-thermal-electrotechnical-measurement-calibration/|archive-date = 14 June 2015}}</ref> Together, these standards cover instruments that measure various physical quantities such as [[electromagnetic radiation]] ([[RF probe]]s), [[sound]] ([[sound level meter]] or [[noise dosimeter]]), time and frequency ([[intervalometer]]), [[ionizing radiation]] ([[Geiger counter]]), light ([[light meter]]), mechanical quantities ([[limit switch]], [[pressure gauge]], [[pressure switch]]), and, thermodynamic or thermal properties ([[thermometer]], [[temperature control]]ler). The standard instrument for each test device varies accordingly, e.g., a dead weight tester for pressure gauge calibration and a dry block temperature tester for temperature gauge calibration.