Have you sent your instrument out for recalibration, gotten the certificate back, and seen “As-Found Calibration” or “As-Left Calibration” and wondered what this means? Here is a guide to understanding what these terms mean.

The Definition of Calibration

Calibration compares a measurement device (an unknown) against an equal or better standard. 

A standard in a measurement is considered the reference. It is the one in the comparison taken to be the more correct. Calibration finds out how far the unknown is from the standard.

A typical commercial calibration uses the manufacturer’s calibration procedure. It is performed with a reference standard at least four times more accurately than the instrument under test.

What As-Found Calibration Means

As-Found data is what a calibration lab finds when it calibrates on an as-received device, like a calibrator, gauge, or sensor, before any adjustments or repairs.

What As-Left Calibration Means

As-Left data is the calibration data when the instrument leaves the lab.

As-Found and As-Left Data Importance 

Suppose the As-Found data is outside of published specifications. In that case, the device must be adjusted to bring it within spec. For example, the data will sometimes be linearized if this process consists of zero, offset, and span adjustments. Sometimes limits narrower than the published specs are applied to account for uncertainties in the measuring system, known as guard banding. Another possibility is that the device needs repairing. No matter the situation, the calibration is re-run. The data generated during this recalibration is the as-left data.

As-Found data is crucial because it will determine the validity of any tests run with the device in the past. In addition, as-Found data provides a history of the sensor’s drift characteristics over time. It also indicates how often the sensor should be calibrated and if it meets the manufacturer’s recommended calibration interval specification.

As-Left data is also essential because it assures the operator that future tests using the device conform to expectations of inaccuracy.

Calibration Certificates Terms

• As-Found Calibration – This shows the as-received device data points. If this data falls outside the published specs, or guardband if applicable, it will be issued as an as-found certificate.
• As-Left Calibration – If the As-Found data falls outside 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 as an As-Left data certificate.
• As-Found 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.

What Happens if the Instrument is Damaged or New?

If the device cannot perform an as-found calibration run as received if the device is damaged, then there will only be an As-Left Certificate. Repair work to the instrument may affect the calibration data, so any calibration data run once the unit is repaired may not be the data that would have been found if the instrument could have been calibrated as received. In this case, a lab will only do an As-Left calibration and supply only an An-Left certificate.

This would also apply to new sensors and calibrators. However, there will only be as-left data as these instruments 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. So, the calibration certificate issued will be As-Left data.