lecture_3_3_11 - Measurement uncertainty Introduction...

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Measurement uncertainty
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Introduction Caliper/micrometer Rotary/linear encoder Hall effect sensor Capacitance probe •L V D T Displacement measuring interferometer •V i b r o m e t e r Accelerometer Strain gage Thermocouple •C M M •S W L I SEM •A F M In a previous lecture, we discussed several measurement platforms for determining position, velocity, acceleration, strain, temperature, and part/surface dimensions. For any measurement, there is always uncertainty associated with the result. We cannot know the “true” value.
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Metrology term definitions ± ISO Guide to the Expression of Uncertainty in Measurement ± NIST Technical Note 1297 Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results 1. accuracy (of a measurement) closeness of the agreement between the result of a measurement and the (true) value Notes: Accuracy is a qualitative concept. Numbers should not be associated with it. Numbers should be associated with measures of uncertainty instead. The term precision should not be used for accuracy. 2. error (of measurement) result of a measurement minus the (true) value Note: Since the true value cannot be determined, in practice a conventional value is sometimes used.
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3. uncertainty (of measurement) parameter that characterizes the dispersion of the values that could reasonably be attributed to the measurement result Notes: Uncertainty can be characterized using statistical methods or from assumed probability distributions based on experience or other information. 4. resolution minimum detectable quantity 5. repeatability (of results of a measurement) closeness of the agreement between the results of successive measurements of the same parameter carried out under the same conditions of measurement (same procedure, observer, instrument, and location over a short time period) Metrology term definitions
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6. reproducibility (of results of a measurement) closeness of the agreement between the results of measurements of the same parameter carried out under changed conditions of measurement Common misuses of these terms include: ± precision is substituted for accuracy ± accuracy is used as a quantitative, rather than qualitative, descriptor. Metrology term definitions
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Measured value Error True value (don’t know this) Uncertainty interval Value of the measurement lies in the uncertainty interval with a stated level of confidence Relationships between definitions
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Metrology term definitions Assume that we’ve measured the yield strength (the stress at which plastic, or permanent, deformation will occur) for a two set of steel beams from two different manufacturers. YS Sample number YS Sample number Structure failure Mean value Both data sets have the same mean value, but three (maybe four) of the second group would yield for the anticipated loading conditions. Which manufacturer would you choose?
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lecture_3_3_11 - Measurement uncertainty Introduction...

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