ASME B89.7.3.2-2007 pdf download

ASME B89.7.3.2-2007 pdf download.Guidelines for the Evaluation of Dimensional Measurement Uncertainty.
Correlations can exist between uncertainty sources; however, most uncertainty evaluations involve uncorre- lated uncertainty sources. Consequently, correlation effects are omitted in this document, except for some guidelines to identify when they are present and hence more advanced methods (beyond the scope of this docu- ment) are needed. Accordingly, this guideline has the following two assumptions: (a) Uncertainty sources are not assigned any degrees of freedom (i.e., no attempt is made to evaluate the uncertainty of the uncertainty). Hence, it is assumed that the expanded (k p 2) uncertainty interval has a 95% probability of containing the true value of the measurand. (b) All uncertaintysources are assumed to be uncorre- lated. Finally, for simplicity, all input quantities of the uncertainty budget are packaged in quantities that have the unit of the measurand (i.e., length). This avoids the issue ofsensitivity coefficients thattypically involve par- tial differentiation. 3 BASIC CONCEPTS AND TERMINOLOGY OF UNCERTAINTY The formal definition ofthe term “uncertainty ofmea- surement” in the current International Vocabulary of Basic and General Terms in Metrology (VIM) [7] (VIM entry 3.9) is as follows: uncertainty (ofmeasurement): parameter, associated with the result of a measurement, that characterizes the dis- persion of the values that could reasonably be attributed to the measurand. This can be interpreted as saying that measurement uncertainty is a number that describes an interval cen- tered about the measurement result where we have rea- sonable confidence that it includes the “true value” of the quantity we are measuring.
influence quantity: any quantity, other than the quantity being measured, that affects the measurement result. Constructing the list of influence quantities is one of the first steps ofan uncertainty evaluation. This list includes not only obvious sources of influence such as the uncer- tainty in the value of a reference standard, or the value of a force setting on an instrument, but also nuisance quantities such as environmental parameters or gauge contamination (dirt). (See Nonmandatory Appendix C.) input quantity: a specific “line item” in the uncertainty budget that represents one or more influence quantities combined together into one quantity. That is, all signifi- cant influence quantities must be included (i.e., “pack- aged”) in some input quantity. Different uncertainty budgets developed by different metrologists might use different input quantities, but all budgets include (in some input quantity) all the significant influence quanti- ties. The selectionofthe inputquantities is usuallybased on the type of the data available about the influence quantities. For example, if a long-term reproducibility study using a check standard has been conducted (e.g., measuring the same feature on a gauge once a week, for several years), then the effects of many influ- ence quantities such as temperature, different operators, recalibration of the instrument, and other factors, are all combined in the observed variation of the check stan- dard results. In this example, a very large number of influence quantities are combined into a single input quantity (i.e., the reproducibility of the check standard results). 1 correlation: refers to a relationship between two input quantities. Correlation between two input quantities means that these two quantities are notcompletely inde- pendent.
measurand: the particular quantity subject to measure- ment. It is defined by a set of specifications (i.e., instruc- tions) that specifies what we intend to measure; it is not a numerical value. It represents the quantity intended to be measured. Itshould specify, as generically as possible, exactly the quantity of interest, and avoid specifying details regarding experimental setups that might be used to measure the measurand. For example, measur- ands specified by ASME Y14.5 [8], such as the diameter of a feature of size or the concentricity of two bores, do not attempt to describe the measurement procedure in detail. 2 Ideally the measurand should be completely independent of experimental measurement details so that different measurement technologies can be used to measure the same measurand and get the same result. Indeed, the measurand is an idealized concept and it may be impossible to produce an actual gauge, artifact, or instrumentexactly to the specifications ofthe measur- and. Consequently, a well-specified measurand provides enough information, and is generic enough, to allow different techniques to be used to perform the measure- ment. The more completely defined the measurand, the less uncertainty will (potentially) be associated with its realization. A completely specified definition of the measurand has associated with it a unique value, and an incompletely specified measurand may have many values, each conforming to the (incompletely defined) measurand. The ambiguity associated with an incom- pletely defined measurand results in an uncertainty con- tributor that must be assessed during the measurement uncertainty evaluation.