ASME B89.7.3.3-2003 pdf free download.GUIDELINES FOR ASSESSING THE RELIABILITY OF DIMENSIONAL MEASUREMENT UNCERTAINTY STATEMENTS
3 THE NATURE OF DISAGREEMENTS IN UNCERTAINTY STATEMENTS
3.1 General In an ideal situation, customers and suppliers will address the issue of measurement uncertainty when they discuss the product specifcations. Agreeing on the measurement plan, the corresponding magnitude of the measurement uncertainty, and the decision rule (if applicable), will avoid future disagreements regarding the acceptance/rejection of a product. However, it is recognized that two experts can produce two different uncertainty statements often varying as much as 25% due to differing assumptions and data (as described in section 5). Resolving these differences at the contract stage is potentially less contentious than doing so after an argument develops over the acceptance or rejection of the product.
3.2 Disagreements Involving Single Measurement Systems In many situations there is only a single measurement system; e.g., a customer agrees to accept the supplier’s measurement results provided that the supplier uses stringent acceptance with a 100% guard band (i.e., the guard band equals the expanded uncertainty). In this example, a disagreement may arise if the customer feels the supplier has underestimated the measurement uncertainty. Although there is a single measurement system, the supplier and the customer have developed differing uncertainty statements.
3.3 Disagreements Involving
Multiple Measurement Systems In some situations, a customer and supplier both make measurements, each having their own measurement system and uncertainty statement. There are two cases to consider: frst, when a product characteristic is being measured to assign it a value, e.g., the length ofa gauge block, and second, when a product characteristic is being measured to determine whether it conforms with specifcations. In the frst case, a best estimate of the value of the product characteristic is being sought.
Two measure- ments, from different measurement systems, will give a better estimate when their results are appropriately combined than will each system independently, provided the uncertainty statements associated with the measurement systems are valid. It is unlikely that the measure- ments performed by the supplier and the customer will yield exactly the same value; however, agreement between the measurements is obtained by some extent of overlap of the uncertainty intervals. The extent of overlap should be specifed in order to clearly identify when the parties are in disagreement. (This avoids disagreements on what constitutes a measurement dis- agreement.) There are several possible cases of metrological signifcance as shown in Fig. 1. Let x s and x c be the measurement results ofthe supplier and customer, with respective expanded uncertainties of U s and U c (both using a coverage factor of k p 2). Let ? p |x s − x c | be the absolute value of the difference between the measurements. Figure 1 illustrates this case with fiv different pairs ofmeasurements. The measurements are considered to be in disagreement when > U s + U c and in agreement when is less than the minimum of either U s or U c . In laboratory round robins, measure ments are generally considered to agree when is less than or equal to the root sum of squares (RSS) of the two expanded uncertainties and in disagreement if is greater than this quantity. Sometimes two different measurement systems are used to determine if a product is in conformance with specification and the outcomes of the two measure- ments differ, i.e., the acceptance or rejection of the product is in dispute. This case can be separated into issues involving the decision rule and issues involving the reliability of the uncertainty statement. When two different parties each perform measurements on the same product, potential disagreements can arise due to inherent conflic in the decision rules. For example, if both the supplier and customer apply stringent acceptance then the party with the larger guard band will typically reject more of the product. This concerns the decision rule selection, not the uncertainty statements, and consequently is outside the scope of these guidelines.ASME B89.7.3.3 pdf download.ASME B89.7.3.3-2003 pdf free download
