Measuring instruments that are sent off for recalibration will return with a calibration certificate full of important data about the device. On these certificates, you will see the terms 'As-Found Calibration,’ ‘As-Left Calibration,’ or sometimes both. What do these terms mean, and when will you see them on a calibration certificate?
As-found data is what a calibration lab finds when it performs a calibration on an as-received device like a calibrator, monitor, gauge or sensor, prior to any adjustments or repairs. As-left data is the calibration data for the device when it leaves the lab.
If the as-found data is found to be outside of published specifications, the device needs to be adjusted to bring it within spec. Often this process consists of zero, or offset, and span adjustments, and sometimes the data will be linearized. Sometimes limits narrower than the published specs are applied to account for uncertainties in the measuring system, which is known as guardbanding. Another possibility is that the device needs to be repaired. In either situation, the calibration is re-run. The data generated during this recalibration is the as-left data.
As-found data is important because it will determine the validity of any tests run with the device in the past. As-found data provides a history of the sensors drift characteristics over time. It is also an indication of how often the sensor should be calibrated, and if it is currently meeting the manufacturers' recommended calibration interval specification.
As-left data is also important because it gives the operator assurance that future tests run using the device conform to expectations in accuracy.
Example as found vs. as left data with limits shown
On a calibration certificate, the terms have the following meanings:
This shows the as-received device data points. If this data falls outside of the published specs, or guardband if applicable, it will be issued as an as-found certificate.
If the as-found data falls outside of the published specs or guardband, adjustments to bring it within spec/guardband limits are made, and the calibration is re-run to confirm it is now within requirements. This data is issued an as-left data certificate.
As-Found Calibration and As-Left Calibration
In this case the device was found to be within the spec/guardband limits as-received, so no adjustment is necessary. Consequently, the device is not adjusted, the calibration data is not re-run and the as-left data is the as-found data. Only one certificate is issued, marked as both as-found and as-left.
Note: Occasionally there will only be an as-left certificate, which happens if the device cannot have an as-found calibration run performed as received (e.g., the device is damaged). Repair work to the device may affect the calibration data, so any calibration data run once the unit is repaired may not be the actual data that would have been found if the device could have been calibrated as received. In this case a lab will only do an as-left calibration and supply only an as-left certificate.
This would also apply to new sensors and calibrators. 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 will not need to be adjusted by the calibration lab. Therefore a calibration certificate showing only as-left data will be issued.