K-Type thermocouple reading 0.0°C



  • Hi All,

    I've got an interesting challenge. When I use glass shielded k-type thermocouple wire I get a accurate temperature reading, as soon as I switch to k-type thermocouples with steel braiding I get a 0.0°C temp reading. This has happened with three different types of thermo couples now.

    IMG_2261.JPG
    Glass shielded thermocouple wire.

    IMG_2260.JPG
    Steel braided thermo couple wire.

    I've tested them by using a multimeter which has a temperature reading function and they all work fine, each one of them gives the same reading as the next including the glass shielded thermocouple.

    Has anyone else come across this before? If so did you manage to rectify the problem?

    Any help would be greatly appreciated.

    Regards,

    Andre



  • Looking at the documentation it states the thermocouple must be electrically insulated from the hotend. Are you using the appropriate daughterboard to hook it up?

    https://duet3d.dozuki.com/Wiki/Connecting_thermocouples



  • I'm using the standard daughter board supplied by Duet with the latest MAX31856 chip on it. I saw the line in the documentation where it refers to the electrical insulation however that is only if using a bare tip thermocouple which neither of these are.

    I've tried grounding the shielding as well and that does not make a difference.


  • administrators

    Have you checked with a multimeter that there is no continuity between the thermocouple wires and the steel case?



  • Good call, I have not. I'll have a look tomorrow.



  • DC42, I checked and there is indeed continuity between the thermocouple and the steel case. Is there a way around it? I would really like to be able to use these thermocouples.



  • JamesM: You were right all it took was some insulation in the end, i covered them with Kapton and away we go. Thank you very much Jean-Marc for the Kapton suggestion.


  • administrators

    @Andre said in K-Type thermocouple reading 0.0°C:

    DC42, I checked and there is indeed continuity between the thermocouple and the steel case. Is there a way around it? I would really like to be able to use these thermocouples.

    The only way around it that I know of is to use a digital isolator on the SPI bus between the Duet and the thermocouple board; and even that might suffer from noise issues. It would have to be one that provides power as well as data. I have previously built these (not for this application) using one ADUM6401 chip and one SN74LVC1G125.



  • @dc42, I'll have a look into that thank you. For now just insulating the entire thermocouple seems to have done the trick.


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