Have you ever reversed the current across one of these and noticed the junction gets cold?
[email protected] wrote: The generally accepted theory is that the voltage arises due the temperature gradient along the length of the wire from the hot junction to the cold junction. The two different materials for the wires generate different voltages for the same temperature gradient, and this difference is measured to determine the temperature difference between the junctions. >From a physical point of view, the gradient argument makes sense. The generated voltage is electrical energy. This energy comes from heat flow. There is no heat flow across the welded junction of the two wires (they are at the same temperature), only along the length of the two wires between the hot and cold junctions. Donald Borowski Schweitzer Engineering Labs Pullman, Washington, USA "Ralph McDiarmid" xantrex.com> To Sent by: [email protected] cc Subject 08/23/2007 12:25 RE: thermocouples PM I say that the voltage is generated at the junction of the wires, but the wires should conduct some heat away from the junction. I expect that the error is typically small, but it and other effects (like air flow and bead-to-surface bonding) might produce an error on the order of a few degrees C. Ralph McDiarmid, AScT Compliance Engineering Group Xantrex Technology Inc From: [email protected] [mailto:[email protected]] On Behalf Of John Woodgate Sent: Thursday, August 23, 2007 11:34 AM To: [email protected] Subject: Re: thermocouples In message <[email protected]>, dated Thu, 23 Aug 2007, "Tarver, Peter" writes: >With all due respect to 'the experts,' the article pointed to contains >inadequate and misleading description of what makes thermocouples work. It must be true - it's on the Web! For me, anything that denies that the thermal voltage is generated at the junction passes muster. I agree that the wording is stumbling. > For accurate information on the underlying principles of how >thermocouples work, perform searches on "Seebeck Effect." I just looked at the Wikipedia entry. Not terribly helpful, in my opinion. There's a long tale about this, which is OT, so I'll drastically précis. Two physicists are trying to find out whether the voltage is in fact generated at the junction. One shortens his wires progressively and finds that the voltage goes down, so the voltage must be generated in the wires. But the second argues that the short wires are hotter at the ends away from the junction, so that is why the voltage is less. He keeps those ends at a constant temperature as he reduces their lengths, even though when they get to 100 microns long, he needs liquid helium for coolant. The voltage stays constant. So does that mean that the voltage IS generated in the junction? Or is it doe to the temperature GRADIENTS in the wires? -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk There are benefits from being irrational - just ask the square root of 2. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc _____ Looking for a deal? Find great prices on flights and hotels <http://us.rd.yahoo.com/evt=47094/*http //farechase.yahoo.com/;_ylc=X3oDMTFicDJ NDllBF9TAzk3NDA3NTg5BHBvcwMxMwRzZWMDZ3JvdXBzBHNsawNlbWFpbC1uY20-> with Yahoo! FareChase. - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
