As-Left Data

As-left data documents the state of an instrument after all calibration adjustments, cleaning, and corrections have been performed. It confirms that the instrument meets its accuracy specifications and is fit for return to service. If no adjustment was needed (as-found data was within tolerance), the as-left data may be identical to the as-found data, or a fresh set of readings may be taken to confirm.

The as-left data serves as the starting-point reference for the next calibration cycle. It establishes the instrument's known good condition at the beginning of the new calibration interval. When the instrument returns for its next calibration, the new as-found data will be compared against the previous as-left data to assess how much the instrument has changed (drifted) during the interval.

For calibration management, both as-found and as-left data must be recorded and retained on every calibration record. The pair of datasets tells the complete story: as-found shows how the instrument performed in service, and as-left shows its condition as returned. This dual-record approach is required by ISO 17025, ISO 9001, and most quality standards. It enables trend analysis, interval optimization, and provides the documentation needed for audits and regulatory inspections. Calibration software should facilitate recording both datasets and flagging any as-left readings that remain outside tolerance.