How to extend thermocouple to reach duet wifi controller?



  • This is my first time, I am working with a thermocouple and duet wifi controller. I read various forums and I found out extending thermocouple is not advised. I was wondering if I can place the thermocouple board (from duet 3d) somewhere around the thermocouple and extend it's output wires to the duet wifi controller, thermocouple I am using is Ktype thermocouple.

    For rest of the electronics, I am connecting the extensions using soldering, should I use male female plugs instead of soldering?


  • administrators

    If you extend the thermocouple wires using ordinary copper wire, you move the cold junction from the thermocouple daughter board to the place where you make the join. So the temperature reading will be off by approximately the difference in temperatures between the daughter board and the joint.

    It should be OK to move the thermocouple board a short distance from the Duet. We haven't tested it, but my guess is that 200mm would be OK.

    It's possible to get thermocouple connectors and extension leads that are matched to the thermocouple. Or find a thermocouple with longer leads. Or if the temperature you want to read is no higher than about 400C, use a PT100 sensor instead.

    HTH David



  • thanks David for a great answer.
    I need about 1m extension. its a big machine.
    is it even worth trying to place thermocouple daughter board that far away from controller or should I just browse for longer thermocouple (I live in India, its not easy to find custom sized thermocouple)

    and also for other electronics, I have right wires, I am going to solder the extensions and heat shrink over it. sounds good to you? I don't know much about electronics.


  • administrators

    Don't try to put the daughter board that far away from the thermocouple. One option would be to buy another K-type thermocouple, cut the tip off, then use the wires to extend the real thermocouple (green wire to green, white wire to white). Twist the bare wires together pairwise, then clamp them in a 2-way terminal block to make sure they stay connected. Or you can buy a K-type thermocouple extension cable and do the same thing.



  • I have been making my own thermocouples for years, any length up to around 2 meters, no extensions. Simply create hot junction by twisting the bare ends tightly together then use small wire cutters and snip off all but about two twist. Sometimes I epoxy the hot end to the surface I want to measure (JB Weld). I have even soldered the hot end so it doesn't separate, doesn't seem to make any difference in readings. I am a air conditioning & heating instructor and needed an inexpensive way to do temperature measurements in our lab. I got these recommendations from the engineer I was using to make our thermocouples, he said there may be some accuracy lost but very minimal. I got a 1000 foot roll of K wire, still have some left. If you have some scrap thermocouple wire check it out.

    Ed



  • I work in an aerospace tooling company, and we regularly build thermocouple extensions up to ~25', and many of the thermocouples themselves are already 10-15' long, so your printer shouldn't have an issue with wire length.

    Regarding making them, as Edkirk was saying, just make sure you have a good connection between the two wires, but ONLY at the end, where you want to measure the temperature.
    My work uses a specialized TIG welder to fuse the wires, but there are many methods for field expedient joints.

    Whatever you do, make sure that you have the right type of thermocouple.
    The boards are designed to work with Type-K thermocouples, so plugging in a Type-J, for instance, would throw the readings off completely.

    One last thought: Also, if you are making, or buying, a complete new thermocouple, make sure to buy stranded wire if it will be moving. The cheaper solid wire does not like flexing, and will break in short order.


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