09_measurement03

09_measurement03 - EXPERIMENTAL MEASUREMENT: METHODS AND...

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EXPERIMENTAL MEASUREMENT: METHODS AND METHODOLOGY Strategies and Tactics for Measurements in 16.62x 16.621 Experimental Projects Lab I

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BASIS FOR MEASUREMENT Whatever exists, exists in some amount Our goals: Measure the appropriate quantity, Measure with the appropriate accuracy, to allow us to assess our hypothesis Some questions: What data do we need? What do we measure? How do we measure? How well do we need to measure? What do we do with the data? 2
ERROR AND UNCERTAINTY IN MEASUREMENT This is the subject of a future class, but the idea is so important that I mention it today as well 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”] 3

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SOME GENERAL MEASUREMENT CONCEPTS The measurement exists in some context We want to do this measurement because ……. The context (and objective) shapes the measurement strategy and tactics [“Begin with the end in mind”- Steven Covey] You generally do not directly measure the quantity you want Most measurement devices are systems which translate the measured quantity (voltage, for example) into the quantity you want (pressure) All measurements have some uncertainty , or error , connected with them It is important to know not only the result of the measurement but also the fidelity of the result (how big is the uncertainty, how much can I trust the answer?) 4
ELEMENTS OF A MEASUREMENT SYSTEM Measured Medium Observer Variable Manipulation Element Primary Sensing Element Variable Conversion Element Data Transmission Element Data Storage/Playback Element Data Presentation Element Sensing Signal Conditioning Readout Adapted from Ernest O. Doebelin, Measurement Systems: Application and Design , McGraw Hill 5

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MEASUREMENT ELEMENT DEFINITIONS Primary Sensing element Retrieves energy from the measured system Produces some form of output
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09_measurement03 - EXPERIMENTAL MEASUREMENT: METHODS AND...

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