Every measuring instrument or device requires calibration at periodic intervals, which is part of preventive maintenance (PM). It is because, every instrument or device undergoes drift in calibration with time.
Let us discuss As-found data and As-left data in this article.
In calibration sheet with “As-found” data is most important because it will determine the validity of any tests run with the instrument in the past. As-found data furnish a history of the drift characteristics over time. It is also an indication of how often the instrument or device should be calibrated.
Engineers can plan for preventive maintenance or routine checking. Also to assess how far it is currently meeting the manufacturer’s recommended calibration interval specification.
As-left data is also important because it (calibrator) gives the operator assurance that future tests run using the device conform to expectations in accuracy.
“As Found” & “As Left” – example in the sheet below.
The labels on the calibration certificates
1. As-Found Calibration
In specific cases, if operational people suspect a small error in measurement, the instrument is sent laboratory to confirm the same. Instrument calibration is checked by the lab and if calibration data is within specifications, no need of any kind of adjustment. It does not require its calibration again. In such a case, the device is sent out with an as-found certificate.
2. As -Left Calibration
For example, it is confirmed by the maintenance team that device calibration is drift is confirmed and out of specifications. It requires a thorough calibration. In the lab, device calibration was carried out and sent out with an as-left certificate.
3. As -Found & As-Left Calibration
In this case, the was as-received, the device violated neither the specifications nor the guard band limits, so it did not require any adjustment. In consequence, the instrument is not adjusted, and the calibration procedure is not re-run. Hence, the as-left data is the as-found data. Only one certificate is issued by marking both as-found and as-left.
In what instances, a calibration certificate is issued with only an “As-left”?
- Once in a while, there will only be an as-left certificate in case the instrument is damaged. In such cases of damage, the instrument cannot have an as-found calibration run performed as received.
- Repair work to the device may affect the calibration data. So, any calibration data run once the instrument is repaired may not be the actual calibration data. Because that would have been found if the device could have been calibrated as received. In such cases, a lab will only do an as-left calibration and supply only an as-left certificate.
- Additionally, new devices or calibrators would have data “as-left” since these devices are only calibration in the factory after they have completed testing and quality procedures, so the calibration will not need to adjust. This would also apply to new devices and calibrators. In calibration certificates, there will only be “as-left” data, as these devices are only calibrated once they have completed testing and quality procedures in the factory.
So they further need not be adjusted by the calibration lab. In such instances, an instrument calibration certificate showing only “as-left” data will be issued.