Calibration is about being able to make meaningful, confident measurements which are documented so they can be traced to national standards. Any manufacturer, or in the case of the calibration laboratory, instrument repairer has to be able to ensure that work is carried out to the required standards. Design and development of any product depends on being able to prove /measure performance characteristics. The reliability and stability of instrumentation used for this purpose must be proved.
However the scene is relentlessly changing. New issues arise e.g. the concern about electronic circuits interfering with one another, this is known as electromagnetic compliance (EMC). More stable standards are continually being developed by physicists at national laboratories. This reflects in commercial instruments whose  manufactures often use better specifications as a selling point.
Occasionally the question gets asked why is all this accuracy necessary ? particularly when modern control systems are designed to accommodate uncertainty in sensors by using artificial intelligence. Potentially thousands can be saved by asking simple questions like " what do we need to measure ?" and "to what accuracy ?"  Many users of measuring equipment prefer to ignore this issue by insisting upon equipment being inside its commercial specification when returned from a calibration facility.
This entails adjustment which may take some time to settle. User's insisting upon adherence to the commercial specifications have to shoulder the expense of sending the instrument to the manufacturer and then back to the calibration laboratory for re-calibration. Here it is hoped that there is compliance between the measurements made by the manufacturer and the calibration laboratory. Effectively two calibrations and one adjustment with a lot of transit costs. Asking for measured values is a more enlightened approach provided the instrument is not too far out-of-tolerance.

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