Electronics > Beginners
Thermocouple reading backwards
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Peabody:
I ordered a cheapass K-type thermocouple with digital readout - a TM-902C from Ebay - and it arrived today.  The problem is that it reads cold objects as hot and vice versa.  I've taken it apart, and there's no way anything could be reversed on the board, so my guess is that the two wires are reversed inside the thermocouple connector.  Does that make sense?  If I put my digital voltmeter on the connector, with the red lead on the plus pin, and turn on my hair drier, I should get a positive voltage, right?  And if it's negative, which it is, that means the wires are reversed?

Will I be able to reverse the wires using regular solder, or do they have to be welded or something?

Of course I can send it back, but if it's a simple fix, I'd rather avoid the delay in getting something that works.

ratio:
I ordered an eBay Special two thermocouple thermometer a few years ago. Seems it operated on °C internally; when displaying T1-T2 it would read the temperature of the thermocouples, convert the temp to °F, subtract T2 from T1, convert the temp to °F, & display the temp. :palm:
den:
Ideally they should be welded or screwed together, because ideally even the connector pins are made of the same material, as wires  (very questionable, if you bought it for a dollar). When you solder it, you make additional thermocouples in the connection, but since the soldered connection is small and localized we can assume it has a uniform T, so the voltages cancel out.
Nusa:
Have you opened the connector body? I suspect the answer to the OP's question will be obvious once you do that.
Peabody:
The wires are connected inside the connector body using little screw terminals.  So I was able to reverse the wires without soldering.  And now it works properly.  Well, it's probably not the most accurate thermometer, but reacts to changes quite rapidly.  The ambient reading is 22.2C, which is about right, and holding the tip between my fingers, I get 33.6C, which may be just a bit high.  The operator's manual is in Chinese.

What I don't know yet is whether the thermocouple will work up to 220C for reflow use with leaded solder paste.  The meter goes up to 1300C, but the included thermocouple is a TP01, which the package says is "suitable" for measuring air temperature from -50C to 204C.  I don't know what happens if you exceed 204C, and measure a PC board.  Banggood sells one that looks exactly the same, and they say it will go to 650C.  But feedback says otherwise - the shrinkwrap at the end starts to melt at 210C.  So I suspect it may be necessary to spend a bit more for a thermocouple that works.  Adafruit has one for about $10 that is used in a number of the toaster oven projects on Youtube.

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