Calibration quantifies the risk associated with a measurement. It provides a user with an unbroken chain between the instrument in use, and its corresponding SI unit. Calibration also evaluates how an instrument drifts from the true value with time; thereby providing a means of determining if a device is stable or approaching the end of its useful life. There are two distinct types of calibration. “As Found” or “Post Adjust” (sometimes referred to as “As Left”).
“As Found” calibrations report the deviation of the unit under calibration (UUC) from the true value, in its existing, unadjusted state. This gives the end user a sense of how well their in-service equipment is performing, and is usually done when the UUC cannot be adjusted, or by client request. The “Post Adjust” calibration seeks to reduce the deviation between the UUC and the true value – thus the UUC has a more “accurate” state at the end of the calibration. “Post Adjust” calibrations are usually desirable; however, certain instruments simply cannot be adjusted. Some of these include: liquid in glass thermometers, reference masses, dial indicators and some self calibrating electronic instruments. In such circumstances, the end user will have to manually correct measured data using their calibration certificate. Thus the calibration certificate is not simply evidence of calibration and traceability, but a tool that the user must employ to attain accurate measurements.
To get more information on calibration, its philosophy, and to learn more about CARIRI’s Metrology Services, check out our interactive, on-line presentation: