Lecture_TEMPERATURE

Lecture_TEMPERATURE - MAE 170 Lecture 5: Temperature...

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MAE 170 Lecture 5: Temperature Measurement October 23, 2006
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Today’s schedule In-class midterm Discussion of thermocouple operation Upcoming lab overview Sample data and analysis ideas for upcoming week Next week’s Labview – listen carefully
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Purpose of this week’s experiment Learn how a thermocouple works Use a thermocouple to take temperature measurements Calibration – Measure heating / cooling rate of metal spheres in H 2 O Use sets of data from a repeated experiment to assess experimental error Please read section 9.2, Introduction to Engineering Experimentation
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The Seebeck Effect Generation of a voltage in a circuit containing two different metals, or semiconductors, by keeping the junctions between them at different temperatures. Discovered by the German (Estonian) physician / physicist Thomas Seebeck (1770–1831) who (apparently accidentally) discovered the effect in 1821. Also called the “thermoelectric effect.” The basis of the Seebeck effect is electron mobility in conductors and semiconductors, which is a function of temperature http://www.its.caltech.edu/~jsnyder/thermoelectrics/demos_page.htm
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So then, what is a thermocouple? Electron mobility through a conductor changes as a function of temperature When two different metals are joined, relative difference in electron mobility makes electrons from the more “mobile” metal jump to the less mobile metal A potential difference is created between the two conductors In the absence of a circuit, this causes charge to accumulate in one conductor, and charge to be depleted in the other conductor.
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Example: Type K thermocouple Standard thermocouple gives 12.2 mV at 300 ºC But … we can’t connect it like this as the measurement leads, which are metals, introduce secondary thermocouple junctions! Voltage at secondary junctions would be erroneous if we put our DVM leads between the two sides of the TC here! Measure ?
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Wiring thermocouples – Cold Junctions As long as the connections of the measurement device with A are kept at the same temperature, the same voltage is generated at each measurement point and this cancels out T 2 must be a known temperature, historically, ice bath Tabulated voltages assume T 2 = 0 ºC Hot Cold, known Measure!
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Thermocouple construction, continued … What about the joining of A to B, the two thermocouple leads? If I solder this will it create a new TC junction? “Law of intermediate metals” states that a third metal, inserted between the two dissimilar metals of a thermocouple junction will have no effect provided that the two junctions (solder + A) and (solder + B) are at the same temperature – practically this is always the case. So soldering TC junctions is OK.
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This note was uploaded on 04/15/2008 for the course COMP CETU311 taught by Professor Butler during the Spring '08 term at Northeastern.

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Lecture_TEMPERATURE - MAE 170 Lecture 5: Temperature...

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