10_errors03

10_errors03 - ERROR ANALYSIS (UNCERTAINTY ANALYSIS) 16.621...

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1 ERROR ANALYSIS (UNCERTAINTY ANALYSIS) 16.621 Experimental Projects Lab I
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2 TOPICS TO BE COVERED Why do error analysis? If we don’t ever know the true value , how do we estimate the error in the true value? Error propagation in the measurement chain How do errors combine? (How do they behave in general?) How do we do an end-to-end uncertainty analysis ? What are ways to mitigate errors? A hypothetical dilemma (probably nothing to do with anyone in the class) When should I throw out some data that I don’t like? Answer: NEVER, but there are reasons to throw out data Backup slides : an example of an immense amount of money and effort directed at error analysis and mitigation - jet engine testing
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3 ERROR AND UNCERTAINTY In engineering the word “error”, when used to describe an aspect of measurement does not necessarily carry the connotation of mistake or blunder (although it can!) Error in a measurement means the inevitable uncertainty that attends all measurements We cannot avoid errors in this sense We can ensure that they are as small as reasonably possible and that we have a reliable estimate of how small they are [Adapted from Taylor, J. R, An Introduction to Error Analysis; The Study of Uncertainties in Physical Measurements ]
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4 USES OF UNCERTAINTY ANALYSIS (I) Assess experimental procedure including identification of potential difficulties Definition of necessary steps Gaps Advise what procedures need to be put in place for measurement Identify instruments and procedures that control accuracy and precision Usually one, or at most a small number, out of the large set of possibilities Inform us when experiment cannot meet desired accuracy
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5 USES OF UNCERTAINTY ANALYSIS (II) Provide the only known basis for deciding whether: Data agrees with theory Tests from different facilities (jet engine performance) agree Hypothesis has been appropriately assessed (resolved) Phenomena measured are real Provide basis for defining whether a closure check has been achieved Is continuity satisfied (does the same amount of mass go in as goes out?) Is energy conserved? Provide an integrated grasp of how to conduct the experiment [Adapted from Kline, S. J., 1985, “The Purposes of Uncertainty Analysis”, ASME J. Fluids Engineering , pp. 153-160]
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6 UNCERTAINTY ESTIMATES AND HYPOTHESIS ASSESSMENT 0 100 200 300 400 500 600 0 20 40 60 80 100 120 Mass [g] Distance [cm] 0 100 200 300 400 500 600 0 20 40 60 80 100 120 Mass [g] 0 100 200 300 400 500 600 0 20 40 60 80 100 120 Mass [g]
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7 HOW DO WE DEAL WITH NOT KNOWING THE TRUE VALUE? In “all” real situations we don’t know the true value we are
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This note was uploaded on 11/08/2011 for the course AERO 16.61 taught by Professor Earlmurman during the Spring '03 term at MIT.

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10_errors03 - ERROR ANALYSIS (UNCERTAINTY ANALYSIS) 16.621...

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