ASME B89.7.3.2-2007 pdf download

ASME B89.7.3.2-2007 pdf download.Guidelines for the Evaluation of Dimensional Measurement Uncertainty.
ABSTRACT The primary purpose of this Technical Report is to provide introductory guidelines for assessing dimen- sional measurement uncertainty in a manner that is less complex than presented in the Guide to the Expression of Uncertainty in Measurement (GUM). These guide- lines are fully consistent with the GUM methodology and philosophy. The technical simplifications include not assigning degrees of freedom to uncertainty sources, assuming uncorrelated uncertainty sources, and avoiding partial differentiation by always working with input quantities having units of the measurand. A detailed discussion is presented on measurement uncer- tainty concepts that should prove valuable to both the novice and experienced metrologist (Nonmandatory Appendices A and B). Potential influence quantities that can affect a measurement result are listed in Nonmanda- tory Appendix C. Worked examples, with an emphasis on thermal issues, are provided in Nonmandatory Appendix D. The bibliography is located in Nonmanda- tory Appendix E. and at best represents a slight refinement of the uncer- tainty statement. Indeed, even in the determination of fundamental constants the practice of using degrees of freedom has been abandoned [6]. 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 uncertainty sources 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 1 SCOPE These guidelines address the evaluation of dimen- issue of sensitivity coefficients that typically involve par- tial differentiation. sional measurement uncertainty. Emphasis is placed on simplified methods appropriate for the industrial prac- 3 BASIC CONCEPTS AND TERMINOLOGY OF titioner. The introductory methods presented are consist- ent with the Guide to the Expression of Uncertainty in Measurement (GUM), the nationally [2] and internation- ally [1 ] accepted method to quantify measurement uncertainty. The use of these guidelines does not pre- clude the use of more advanced methods in the uncer- tainty evaluation process. UNCERTAINTY The formal definition of the term “uncertainty of mea- surement” in the current International Vocabulary of Basic and General Terms in Metrology (VIM) [7] (VIM entry 3.9) is as follows: uncertainty (of measurement): parameter, associated with the result of a measurement, that characterizes the dis- 2 SIMPLIFICATIONS IN THE EVALUATION OF MEASUREMENT UNCERTAINTY persion of the values that could reasonably be attributed to the measurand. This can be interpreted as saying that measurement To simplify and focus the uncertainty evaluation pro- cess in an industrial setting, issues associated with the effective degrees of freedom of the uncertainty statement and correlation between uncertainty sources are consid- ered less important when compared to problems associ- ated with underestimating or omitting uncertainty sources. The issue of effective degrees of freedom fre- quently confuses beginning uncertainty practitioners 1 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. expanded uncertainty (with a coverage factor of 2), U: a number that defines an interval around the measure- ment result, y , given by y ± U , that has an approximate 95% level of confidence (i.e., probability) of including