This analysis a coverage factor of k 2 should be used

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this analysis, a coverage factor of k = 2 should be used. This coverage factor provides a confidence level of approximately 95 %. X2.4.4 The measurement bias B of the hardness machine is the difference between the expected hardness measurement values as displayed by the hardness machine and the “true” hardness of a material. Ideally, measurement biases should be corrected. When test systems are not corrected for measure- ment bias, as often occurs in Rockwell hardness testing, the bias then contributes to the overall uncertainty in a measure- ment. There are a number of possible methods for incorporat- ing biases into an uncertainty calculation, each of which has both advantages and disadvantages. A simple and conservative method is to combine the bias with the calculation of the expanded uncertainty as: U 5 ku c 1 ABS ~ B ! (X2.5) where ABS ( B ) is the absolute value of the bias. E18 - 15 32
X2.4.5 Because several approaches may be used to evaluate and express measurement uncertainty, a brief description of what the reported uncertainty values represent should be included with the reported uncertainty value. X2.5 Sources of Uncertainty X2.5.1 This section describes the most significant sources of uncertainty in a Rockwell hardness measurement and provides procedures and formulas for calculating the total uncertainty in the hardness value. In later sections, it will be shown how these sources of uncertainty contribute to the total measurement uncertainty for the three measurement circum- stances described in X2.1.2 . X2.5.2 The sources of uncertainty to be discussed are ( 1 ) the hardness machine’s lack of repeatability, ( 2 ) the non- uniformity in hardness of the material under test, ( 3 ) the hardness machine’s lack of reproducibility, ( 4 ) the resolution of the hardness machine’s measurement display, and ( 5 ) the uncertainty in the certified value of the reference test block standards. An estimation of the measurement bias and its inclusion into the expanded uncertainty will also be discussed. X2.5.3 Uncertainty Due to Lack of Repeatability (u Repeat ) and when Combined with Non-uniformity (u Rep& NU )— The repeatability of a hardness machine is an indication of how well it can continually produce the same hardness value each time a measurement is made. Imagine there is a material, which is perfectly uniform in hardness over its entire surface. Also imagine that hardness measurements are made repeatedly on this uniform material over a short period of time without varying the testing conditions (including the operator). Even though the actual hardness of every test location is exactly the same, it would be found that due to random errors each measurement value would differ from all other measurement values (assuming sufficient measurement resolution).

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