country in which it is situated, such as the National Physical Laboratory in the UK. Quality 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 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 systems call for an effective metrology system which includes formal, periodic, and documented calibration of all measuring instruments. ISO 9000 and ISO 17025 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., NIST 150-2G in the U.S. and NABL-141 in India. Together, these standards cover instruments that measure various physical quantities such as electromagnetic radiation (RF probes), 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 controller). 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. Instrument calibration prompts Calibration may be required for the following reasons: a new instrument after an instrument has been repaired or modified moving from one location to another location when a specified time period has elapsed when a specified usage (operating hours) has elapsed before and/or after a critical measurement after an event, for example after an instrument has been exposed to a shock, vibration, or physical damage, which might potentially have compromised the integrity of its calibration sudden changes in