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I want to attach a thermocouple to my 40Watt soldering iron in order to make it temperature controlled. The thermocouple I'm going to buy is a K type MAX6675 thermocouple that looks like this: enter image description here

But the thing is it has this bulky metal head attached to it which doesn't seem to be detachable. Since I want to attach the thermocouple to my soldering iron the smaller it is the better for my application.

I was wondering if I could just cut off the metal head of the MAX6675, twist the heads of the wires together and turn it into something like this: enter image description here

Will the thermocopule stay intact if I do such thing?

And if not, is there any other way I can get rid of that bulky head? (Buying another thermocopule is not an option)

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    \$\begingroup\$ If you can weld the two wires together at the end, you can re-create the thermocouple junction. Just twisting them together is not likely to work. \$\endgroup\$ – The Photon Mar 28 '18 at 16:37
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    \$\begingroup\$ Regardless of the question, an add-on thermocouple approach is impractical from a mechanical/heat transfer perspective if you want to do meaningful soldering with the result. If you want to make a thermocouple controlled soldering iron, buy a replacement tip for a design that includes a thermocouple along with the heater in the tip. 3rd party tips compatible with the Hakko T12 series seem among the most inexpensive, and even the genuine ones aren't that costly, but probably best to overtemp an imitation while getting your controller sorted out. \$\endgroup\$ – Chris Stratton Mar 28 '18 at 16:49
  • \$\begingroup\$ @ChrisStratton why is an add-on thermocouple impractical? I'm planning to turn the iron off when it passes a certain temperature and turn it on again when it falls below a certain temperature. \$\endgroup\$ – Pouria P Mar 28 '18 at 18:42
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    \$\begingroup\$ The practical performance of a soldering iron is heavily tied to having the temperature sensor rapidly respond when the working part of the tip looses heat. This means that the sensor needs to be very close to the tip, not buried in something larger with a lot of thermal mass. For a "science-fair" type project you could perhaps try cross-drilling the base of a tip and embedding the sensor somehow, but if you want to make a usable tool, buy a tip with a sensor embedded near its working part. Otherwise no matter how much trouble you go to with the electronics, you'll be getting bad data. \$\endgroup\$ – Chris Stratton Mar 28 '18 at 19:17
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    \$\begingroup\$ I'm very happy with the chinese T12. Wish it was available 30 years ago. Remember that an external thermocouple will have to be insulated from heat loss or ambient airflow. So it will have to be wrapped on the tip with some thermal conductor and then wrapped with some thermal insulator. Makes it all bulky, and in the end the poor regulation may not be worth it. The smaller the tip and heater, the less it loses heat. So, even a manually controlled variable wattage micro soldering pen works out much better than a big high wattage iron. \$\endgroup\$ – Indraneel Mar 28 '18 at 21:03
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Yes, you should be able to cut the wires and reattach them without losing calibration. I wouldn't necessarily just twist them up, though. A weld would be best, and brazing second best (http://eleceng.dit.ie/gavin/Instrument/Temperature/Laws%20of%20ThermoC.html -- the law of intermediate metals can be used to show this). A blob of solder is sort of like a braze. Even a bad solder is likely good enough.

Twisting will give you some life, but it may not be gas-tight. I'd be concerned about oxidation eventually causing intermittent connections. Crimping should work too, in a pinch, and there are gas-tight ways to crimp.

https://www.physicsforums.com/threads/okay-to-twist-thermocouple-wires.793193/#post-4982730

Reference https://www.physicsforums.com/threads/okay-to-twist-thermocouple-wires.793193/

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  • \$\begingroup\$ Citations from Internet can give a lot of confused opinions, like this total delirium from an alleged professional site, capgo.com/Resources/Temperature/thermocouple/thermocouple.html The site alleges that the output of a thermocouple depends on gradient of temperature along the wires near the joint, citing Thomson Effect. This would be true if the measurement schema would use any significant current flow. In reality the measuring micro-voltmeters have very high input impedance, and currents along the wires are zero. Careful what do you read on Internet. \$\endgroup\$ – Ale..chenski Mar 29 '18 at 6:44
  • \$\begingroup\$ However, in fact, the Thermocouple reading do depend on longitudinal gradient, but not because of Thomson effect, but because if the bulk of wires is exposed to a different temperature of ambient, the heat will flow along the wires, and change the actual temperature of the thermocouple tip. So the first rule of thermocouple placement on objects is to lay them along isotherms as much as you can. \$\endgroup\$ – Ale..chenski Mar 29 '18 at 6:49
  • \$\begingroup\$ The only point I'm trying to cite from that site is the Law of Intermediate Metals, which is a Law \$\endgroup\$ – Scott Seidman Mar 29 '18 at 10:22
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You can do anything with thermocouple wires as long as you can figure out how to WELD the tip of thermocouple. Soldering is a bad option.

You shouldn't worry much about "calibration", especially for the soldering iron application, because the voltage output is determined by metal alloy compositions of the two wires, and by nothing else.

If you would require results to a fraction of a degree C, then it is always wise to get your own calibration curve.

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  • \$\begingroup\$ Soldering should be good enough in a pinch. \$\endgroup\$ – Scott Seidman Mar 28 '18 at 21:46
  • \$\begingroup\$ Obviously, if it falls apart, life is compromised. It's the mechanical connection that counts, though, not the electrical. \$\endgroup\$ – Scott Seidman Mar 28 '18 at 21:51
  • \$\begingroup\$ @ScottSeidman, soldering can be fine as long as the wire alloys stick with solder alloy (wet surface, can't find proper term), which is usually VERY hard to achieve. Much easier and reliable is to design a micro-welder. Especially if this is a high-temperature soldering iron application. \$\endgroup\$ – Ale..chenski Mar 28 '18 at 21:59
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Will the thermocopule stay intact if I do such thing?

I'd argue that "intactness" is broken the moment you cut something off.

Functionally: the contact of the two different metals is indeed what gives the thermovoltage.

The thermocouple I'm going to buy is a K type MAX6675 thermocouple that looks like this:

(Buying another thermocopule is not an option)

(emphasis by me)

I call nonsense on that. You need a differently sized thermocouple head. You can buy hundreds of others out there.

So, get a different thermocouple.

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    \$\begingroup\$ I agree. Buying the correct thermocouple is priority, and you'd better MAKE it an option. \$\endgroup\$ – DerStrom8 Mar 28 '18 at 16:49
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    \$\begingroup\$ I've made ad hoc thermocouple probes by twisting the wires together and securing the twist with a solder blob. The thermocouple worked. Its accuracy didn't degrade. So, I can't confirm your point about not introducing other metals. (How well solder would stick to thermocouple materials? Does solder lower the practical temperature at which the thermocouple assembly can work? Those are valid but separate questions.) \$\endgroup\$ – Nick Alexeev Mar 28 '18 at 16:51
  • \$\begingroup\$ @NickAlexeev I can see how the Lead1-solder-Lead2 double-junction should effectively have the same voltage as the Lead1-Lead2 junction, thinking about this. But I'm afraid that an oxide (or flux) layer between lead1 and 2 would actually be pretty detrimental, at the very least to the current sourcing capability of the junction, so I'm aware that I'm wrong, but kiiiiinda hesitant to say "sure, just go ahead". \$\endgroup\$ – Marcus Müller Mar 28 '18 at 17:02
  • \$\begingroup\$ @MarcusMüller Thanks for your answer. That's the only inexpensive thermocouple I could find online where I'm from. Also I haven't calibrated anything, I haven't bought it yet. How much error are we talking about? I don't need very precise measurements. \$\endgroup\$ – Pouria P Mar 28 '18 at 19:05
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    \$\begingroup\$ A thermocouple is a thermocouple is a thermocouple. Every thermocouple of the same type has the same calibration (though different time constants due to thermal mass). I wouldn't just twist them up. Solder -- even a bad solder -- should be OK, by the Law of Intermediate Metals. \$\endgroup\$ – Scott Seidman Mar 28 '18 at 21:46
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Yes, theoretically that will work; mechanical issues aside.

Marcus Müller's answer warns that metal contamination of the junction will effect the thermocouples accuracy; but that's not correct. If you introduce a dissimilar metal you're basically creating 2 thermocouple junctions at the point: A-B-C. Well it turns out that if both junctions A-B, and B-C are at the same temperature the effect of B cancels out, and it's equivalent to A-C. So no, a little metal from your pliers will not ruin the thermocouple junction.

You can check out this document for more in depth information (see page 5): https://www.omega.com/temperature/Z/pdf/z021-032.pdf

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