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Jenkins" Subject: RE: Calibration Uncertainty Philosophy Ron, To begin with, lets establish some common understanding: Measurement Uncertainty: the uncertainty of the measurement data relative to "true value".
In calibration and testing this means the combined uncertainty of the traceable reference equipment including the uncertainty attributed to the applied process, environment etc. This also includes the contibutory traits of the subject device, such as its random error contribution and display precision as applicable , but does not include the subject units specifications.
Device Uncertainty: typically used to describe the uncertainty of a measurement device, expressed as either a containment value such as a specification or a certified value with applicable uncertainty.
To be realistic, both of these types must consider uncertainty growth over time since measurement and have some appropriate containment value. Typically when using a certified value we perform a regression relative to changes in value over time and predict the value for a given time since measurement based on a calculated drift slope.
It should be obvious that this process would also add uncertainty derived from the prediction. Adjustment of the "subject unit" device being calibrated will have absolutely NO effect on the measurement uncertainty of the data taken during the calibration before and after adjustment. But will have a definite effect on improving the "In-tolerance Probability" of the subject units performance relative to its "error containment" specifications over time calibration interval.
Type "A" assessment of measurement standards are based upon certified values over time and I agree, you should request that these devices NOT be adjusted as that might disturb the stability of the drift slope. Although it is rare for the typical user of calibrated equipment to use their equipment in this certified value manner.
This is usually reserved for passive or otherwise extremely stable reference standards. Most customers of calibration and users of measurement equipment desire that their equipment perform nominally within specifications and do not reference the measured value, rather they assume error containment by the specifications relative to displayed values.
They do not correct the displayed value by some known bias. Such as in a garage when the mechanic measures the battery voltage to be For this application to work reliably, a high in-tolerance probability is desired, hence so is the in-tolerance adjustment desirable as it can increase this in-tolerance probability.
When computing measurement uncertainty using devices in which we quote the specifications as error containment, we express this in-tolerance probability as the "Confidence Level" of the specifications adequately containing the error type B. And you are correct in recognizing that a laboratories Best Measurement Uncertainties as defined in their "Scope of Accreditation" do not apply directly to all data taken in subsequent measurements of varying subject units.
This "Best Measurement Uncertainty" is useful is assessing their abilities in making certain measurements and does NOT mean that all measurements made by this laboratory will have this uncertainty. What you desire is a "Specific Measurement Uncertainty" which is based on the interaction of your specific subject unit with their equipment and the applied measurement process.
To be in compliance to ISO Guide 25, a laboratory must be able to provide this specific measurement uncertainty upon request. But be prepared for applicable charges as this process can be time consuming as it includes a characterization of the specific subject unit. We have found 3 different types of analyses being reported by laboratories, Best, Specific, and Generic or Typical.
While we realize that the "Specific" is the most accurate and costly, the others can be beneficial and have value when taken in context to their origin and definition. Seeing that all three are currently being reported by labs it is our opinion that when using something other than a "Specific" it should be made clear to the recipient. In our classes we cover the various types of analyses, applications of measurement uncertainty and the concepts of the uncertainty growth principle.
We teach how to develop the laboratories "Scope" and compute a "Specific Measurement Uncertainty" for their customers, when required. We also cover the application of using an economical "Generic or Typical" measurement uncertainty and when it is advisable to report a "Best Measurement Uncertainty" for identifying the labs limits of measurement accuracy. Once one realizes the intended application of the measurement uncertainty, i.
ASTM E563 PDF
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