Have you sent your instrument out for recalibration and 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 is the comparison of a measurement device (an unknown) against an equal or better standard.  We consider a standard in a measurement the reference.  It is the one in the comparison taken to be the more correct of the two.  Calibration finds out how far the unknown is from the standard.

A typical commercial calibration uses the manufacturer’s calibration procedure.  They perform it with a reference standard at least four times more accurate than the instrument under test.

What As-Found Calibration Means

As-Found data is what a calibration lab finds when it performs a calibration on an as-received device.  Equipment like a calibrator, gauge, or sensor, prior to 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

If the As-Found data is found to be outside of published specifications, the device needs to be adjusted to bring it within spec.  If this process consists of zero, or offset, and span adjustments, and sometimes the data will be linearized.  Sometimes limits narrower than we apply the published specs.  They do this to account for uncertainties in the measuring system, which is known as guard banding.  Another possibility is that the device needs to be repaired.  No matter the 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 sensor’s 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 to conform to expectations in accuracy.

Calibration Certificates Terms

• As-Found Calibration – This shows the as-received device data points. If this data falls outside of the published specs, or guardband if applicable, they will issue it as an as-found certificate.
• As-Left Calibration – 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. They issue this data as an As-Left data certificate.
• As-Found and As-Left Calibration – In this case, they found the device 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.  They issue only one certificate, marked as both As-Found and As-Left.

What Happens if the Instrument is Damaged or New?

If the device cannot have an as-found calibration run perform 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 repairment may not be the actual 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 as an An-Left certificate.

This would also apply to new sensors and calibrators.   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.