ASME B89.7.6-2019 pdf download

ASME B89.7.6-2019 pdf download.Guidelines for the Evaluation of Uncertainty of Test Values Associated With the Verification of Dimensional Measuring Instruments to Their Performance Specifications.
Introduction ASME B89 standards describe the testing protocols used to specifythe performance ofdimensional measuringinstru- ments. Verification of an instrument to its MPE is based on a decision rule (e.g., see ASME B89.7.3.1) that determines whether a test value (obtained per the testing protocol) is within its MPE specification. differenttestvalueuncertainties,e.g.,duetodifferenttestingtemperatures ordifferentuncertainties ofcalibratedgauges used in the verification. It is important that the test value uncertainty not be confused with the measurement uncertainty associated with.Theuncertaintyofaworkpiecemeasurement,havingameasurandsimilartothatusedin theASME B89 testingprotocolandobtainedwithintheratedconditions oftheinstrument, is primarilydeterminedbythe instrument’s MPE. In contrast, the testvalue uncertainty is associated with uncertainty ofthe testvalues obtained when verifying the MPE; it is independent of the MPE and hence has nothing to do with the accuracy of future workpiece measurements. An inaccurate instrument, with a large MPE, mighthave a relativelysmall testvalue uncertainty. The test value uncertainty does not describe the accuracy of the instrument’s measurement results, but rather describes the accuracyofthe testvalues obtained in a verification test. Figure 4.1-1 illustrates the distinction between the instrument under verification that produces an error ofindication and the verification system that includes a calibrated reference quantity.
The evaluation ofthe uncertainty associated with a testvalue obtained from an instrumentunder verification follows theusualmethodologyofJCGM 100 (GUM); however,itis importantto clearlyidentifytheinstrumentas thedeviceunder verification and the calibrated reference standards (e.g., gauge blocks) as part ofthe verification system performing the test. Itis the verification system thatis measuringthe error ofindication ofthe instrument, and when the testconditions are fully within the rated operating conditions, the test value uncertainty arises solely from the verification system, not from the instrument being verified. Factorsaffectingonlytheperformanceoftheinstrumentaremanifestedinthetestvalueandarenotcontributorstothe test value uncertainty. The resolution of the instrument under verification is not an uncertainty source; it is an error source, and a low-resolution instrument under verification will generally have a larger testvalue than a high-resolution instrument. For example, if a caliper with a 1-cm resolution were tested with a 4-mm calibrated gauge block and an indicated value at0 cm, a –4-mm errorwould be reported as the testvalue; however, the testvalue uncertaintywould be very small and mostly due to the uncertainty of the calibrated value of the gauge block. Similarly, the lack of repeatability of the instrument, e.g., the instrument generates different test values when measuring the same reference quantity under nominally the same conditions, is not a test value uncertainty source. Each “repeated” test value corresponds to a different test, and each test value is compared to its MPE using the decision rule. For example, Figure 5.1-1 shows a hypothetical testing protocol that requires five repeated measurements of a calibrated gauge and the associated MPE for this measurement. Each test value corresponds to a different test with a different indicated value, and each must be within the MPE for a conforming instrument.
There are several subtle issues to consider when the test conditions are outside the rated operating conditions. Since anMPE specificationofthe instrumentdoes notexistoutside the ratedoperatingconditions, testvalues obtained outside the rated operatingconditions cannotdirectlybe used to verifythe instrument’s MPE specification. The testcan proceed onlyifthe testingprotocol (within an ASME B89 standard) specificallyallows testvalues to be obtained outside the rated operating conditions. In this case, corrections are applied to obtain a corrected indicated value that corresponds to what the instrument under verification would have indicated at a specified condition within the rated operatingconditions. Since all corrections to the indicated value have an associated uncertainty, this mustbe included in the evaluation of the test value uncertainty. 5.3.2 Test Conditions Outside the Rated Operating Conditions Due to the Tester-Supplied Coefficient of Thermal Expansion (CTE) Value. Some instruments under verification include a means ofperforming a thermal expansion com- pensation, e.g., an instrument that includes a temperature probe as part ofits normal operation and yields an indicated value thatis compensated for thermal expansion.