As accredited calibrations become more necessary and prevalent, we see a rise in requests for and misunderstandings of this term "uncertainties". I hope to explain some of this and how it effects you in this post. As always, feel free to contact us for more information.
There are a few types of uncertainties that surround calibration. There is "measurement uncertainty" and "test uncertainty". These two will most effect the end user, our customers, when looking at calibration results. The test uncertainty is the uncertainty of a calibration process, this means, any possible error that could accumulate to a measurable value that is explained as how uncertain we can be of a test result due to the imperfections in the process used. The measurement uncertainty is, in turn, the measurable amount of uncertainty within a particular measurement. Based on the accuracy of the standard used, effects of the environment, resolution of the device under test and other influences, a particular measurement will be given a measurable uncertainty. This is most often the item of most concern for our customers.
With an accredited calibration, customers not only receive point data and the associated tolerances, but also the reported measurement uncertainty for each point. Now, with all this information, our customers can not only see information about their instrument, but also examine all relevant information associated with the process involved. As the standards used in calibration have to themselves be calibrated there is a level of what we will call imperfection. Because this is imperfect, even though better than the instrument it is calibrating, the customer can now properly see how this might effect their use of the calibrated instrument.
Our customers can choose whether or not they further want to include uncertainties by requesting that their measurements be "guard-banded". This will essentially incorporate the uncertainties with the established tolerances. If an instrument had a tolerance of +/- 1V and an uncertainty of +/- 0.25V we would guard-band the results and make sure the instrument read within +/- 0.75V to meet the requirements of this calibration. Conversely, as this will certainly effect the compliance statement for your instrument, a customer can choose to not have the measurements guard-banded and will receive the uncertainties reported only.
Whether or not you are requesting uncertainties with your calibration it is good to understand how they could be effecting you and your calibrated instruments. No matter where you get your instruments calibrated you will have some amount of uncertainty in the calibration. Find an accredited laboratory to give you honest, trustworthy results that are held to a higher standard.