How to Read APL Certificates

Glossary of Terms in APL Cerificates

If you are not required to read and interprit instrument calibration reports on a regular basis, many of the terms may not be immediatly intuitive.

Although our focus is on the best possible performance assessment for your instruments, we also want to ensure our reports are clear and digestible for all end users.  Please find below a basic glossary for some of the terms found on our reports:

Corrections: This is the quantity to add the instruments reading or output to determine the true/expected value, to compensate ("correct") for any errors that we have observed in the instrument performance.  Some laboratories quantify the error observed in your instruments, but we prefer to state the correction that you need to apply at each point.  The correction and error are typically the same size but opposite direction from each other (correction = -1 x error).

As Found Readings: The performance of the instrument before any adjustments have been performed, i.e. the performance of the instrument as it was received by APL.  These can also be called "before adjustment" readings.

As Left Readings/Corrections: The performance of the instrument after adjustments have been completed, i.e. the performance of the instrument as we left it at the end of calibration.  These can also be called "after adjustment" readings.

If adjustments have been performed, APL will always provide the As Found results and As Left results, so you can determine how the instrument was performing before we adjusted it, which enables you to track the drift (change in performance with time) of the instrument between calibrations.

Accuracy/Specification: On APL reports, references to accuracy and/or specification, and the corresponding column in our results table, refers to the accuracy specification from which we are assessing your instrument to fall within.  This is typically the manufacturers accuracy specification unless you have specified or agreed to a different specification.  This is also commonly referred to as the "tolerance".

Uncertainty: All APL reports include an uncertainty value expressed at each calibration point.  This is the band within which we are confident that the corrections (or errors) reported for your instrument are true, with 95% confidence.  This includes many components, such as how well we know the true applied quantities from our reference equipment (i.e. the uncertainty inherent in our reference equipment and environmental factors) as well as the resolution and repeatability of your instrument during the test.  In general terms, this is a measure of the quality of the correction, or how sure we are about the correction and readings that we have reported for your instrument compared to the true quanties.  APL works very hard to offer you the lowest possible uncertainties (highest possible quality) by utilising low uncertainty reference equipment and carefully handling all components that contribute to the uncertainty.  This is the ultimate challenge for us metrologists, the quest for the lowest possible uncertainties, to get as close as possible to realising the true SI quantities and pass them on to our customers.

If you are unsure about any aspects of our reports, please don't hesitate to contact us.