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Author Topic: thermocouple: is it ok if it actually touches the metal surface?  (Read 1309 times)
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hi! long-time lurker here, time for my first question!
the title pretty much says it all, I want to use a thermocouple for measuring the temperature of a metal surface (nichrome wire, or the soldering iron's tip) and I want to ask if it's ok for it to actually touches the surface under measurement
will this interfere with the voltage it generates which is translated into temperature measurement?
well of course it touches something anyway, even if it's in the air, but that's not a circuit which may influence it's generated voltage, in contrast to a nichrome wire that carries current...
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Holland MI
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Yes. That is perfectly okay as long as BOTH wires touch the SAME surfaace. We use them where I work, and it is routine here to fork out the two TC wires like the letter Y and separately spot weld each wire directly onto the survace whose temperature we wish to measure. This has been common practice in the aerospace industry for ever and ever. The 'junction' is electrically connected through the base metal. It is the same base metal for both TC wires (Chromel and Alumel for type K). So techincally two junctions, but equal and opposite and at the same temperature. The electrons don't care. From the perspective of the TC cable's other end where you connect the instrument it is electrically the same.
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wow thank you very much Aplonis for your clear answer!
I have another question, obviously stupid but since I can't understand it I need an explanation smiley
how come the base metal to which the two thermocouple wires are soldered doesn't set them to the same potential? so their voltage output would be zero all the time?


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Grand Blanc, MI, USA
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wow thank you very much Aplonis for your clear answer!
I have another question, obviously stupid but since I can't understand it I need an explanation smiley
how come the base metal to which the two thermocouple wires are soldered doesn't set them to the same potential?

Of course they are at the same potential, at the point they are joined!

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so their voltage output would be zero all the time?

The voltage is actually due to the thermal gradient along the conductors. Different metals develop different voltages. See http://en.wikipedia.org/wiki/Thermoelectric_effect#Seebeck_effect
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