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I am using T type thermocouple wire. I am using it in a situation where I don't want the sensor to fail hence I thought it is a good idea to solder the ends together instead of only twisting them to prevent them from separating. On the internet there are mixed thoughts about soldering thermocouple wire together. Can someone help me out?

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  • \$\begingroup\$ How does the manufacturer suggest extending the leads? They usually say. \$\endgroup\$
    – Solar Mike
    Dec 10, 2021 at 8:47
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    \$\begingroup\$ they should be welded together, not soldered. \$\endgroup\$ Dec 10, 2021 at 8:52
  • \$\begingroup\$ @SolarMike I don't want to extend the wire. I want to solder the exposed end of my wire (the part that measures the temperature) together instead of only twisting them. \$\endgroup\$
    – Rens
    Dec 10, 2021 at 8:53
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    \$\begingroup\$ I've soldered the actual thermocouple (k type device) to the legs of a zener diode for testing it's temperature and this worked. \$\endgroup\$
    – Andy aka
    Dec 10, 2021 at 8:54
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    \$\begingroup\$ Apply "Seebeck Effect" to more than 2 metals used as usual. Some specifications will change. en.wikipedia.org/wiki/Thermoelectric_effect#Seebeck_effect but see this fr.wikipedia.org/wiki/Effet_Seebeck because of some more information (table ) \$\endgroup\$
    – Antonio51
    Dec 10, 2021 at 9:15

3 Answers 3

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You can form a junction by joining a pair of thermocouple wires together with a third metal. As long as both junctions are at the same temperature, and at the temperature of the thing you are sensing, the voltage you read will be the same as if you had a simple junction between the two metals.

Being able to solder thermocouple wires is a different problem. I've only worked with K-type thermocouples, and ordinary flux-cored electrical/electronic solder does not work, the flux isn't active enough to break through the surface oxides. However I have some multi-core solder intended for use on aluminium, which does work on K-type wires. It has a vicious flux. It can be used to solder the ends together, or to tin them so that ordinary solder can then be used to complete the joint.

T-type thermocouples use copper-constantan. You'll obviously have no trouble with the copper part. Constantan will not solder using ordinary solder, and I've not tried constantan wire with my aluminium solder, so can't advise you whether that particular method will work. However, find an active enough flux, and you'll probably be able to do it.

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  • \$\begingroup\$ ...and be sure to clean that evil flux off when done soldering, using the proper method for the applied flux. \$\endgroup\$ Dec 10, 2021 at 13:25
  • \$\begingroup\$ also, also, there's lots of papers on flux migration up wire, and under insulation. If you're concerned about longevity and durability, be sure to mind where the active flux ends up, and how to get rid of it. \$\endgroup\$ Dec 10, 2021 at 13:44
  • \$\begingroup\$ I believe the thermocouple principle that says this is OK (if you can do it) is the law of intermediate metals. \$\endgroup\$ Dec 10, 2021 at 14:06
  • \$\begingroup\$ It feels odd to me that you can't wet constantan with tin solder; are you sure about that? Both nickel and copper are highly wettable, so I would expect an alloy of the two to also be easily soldered. \$\endgroup\$
    – Hearth
    Dec 10, 2021 at 20:21
  • \$\begingroup\$ @Hearth I have a number of resistance wires in my reels bin, some will wet with ordinary solder, most won't. It's possibile I've mis-identified some. \$\endgroup\$
    – Neil_UK
    Dec 10, 2021 at 20:47
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Apart from the fact that the soldering brings additional metals into the connection and thus shifts the thermoelectric properties into the unknown, there are other disadvantages.

The connection can definitely only be used below the temperature at which the solder melts. The mass is much larger than that of a point-welded connection, which makes the sensor slow and negates the advantage of the thermo-elements. Then rather take an RTD. That is much more accurate!

Overall not recommended.

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    \$\begingroup\$ Surely below the point at which the solder melts... \$\endgroup\$
    – Solar Mike
    Dec 10, 2021 at 19:29
  • \$\begingroup\$ Of course! @SolarMike :-) \$\endgroup\$
    – arnisz
    Dec 11, 2021 at 21:33
  • \$\begingroup\$ Need to be clear with what you state... \$\endgroup\$
    – Solar Mike
    Dec 11, 2021 at 21:46
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Copper-Constantan is reasonably solderable. In fact you can use the Constantan for DIY shunts (low value resistors). It's not as good as Manganin for tempco but it is much more solderable.

Yes, you can solder the junction, and it's better than twisting the wires. Even if it becomes liquid it will still work, but it would be good to stay below that temperature. The 'hot' junction will be essentially distributed out wherever the solder and wires are in contact.

Molten metal temperature can be (and is sometimes) measured by sticking two individual probes made of thermocouple materials into it.

Ideally you would want to inert-gas weld the two wires together in a controlled manner, which is really easy if you have a TIG welder, but soldering is a reasonable approach for low temperatures.

A note of caution: T type thermocouples are often used in applications near or in food processing and other biological applications such as storage of 'samples' and so on- so be careful not to introduce toxic solder materials into the proximity of food or other biological processes.

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