Highly erratic temperature readings above certain temperature

  • Hi forum,

    first of all let me say thank you to all the help you've unknowingly provided to me just by running this forum, especially to dc42, you're doing an amazing job at supporting the Duet and everything around it.
    I've just about finished building a printer around the Duet Ethernet, and it actually earned me my master's degree in engineering just yesterday. It's a high temperature machine, thus i've had to account for insulation, heating of the build volume, cooling etc. in rather unusual ways. It's the second printer I've built, first was around a smoothieboard but Duet seemed to be the way to go for #2, and yes it was!
    I've also worked as a development engineer in additive manufacturing, so I more or less know my way around FDM/FFF and also DLP printers.

    My problem today is one that I've not been able to solve by browsing the forum. Unfortunately it comes at a highly unfavorable time as we need the printer to be up and running in the coming weeks.

    My setup is "E3D everything" -- standard v6, 40W heater, copper block, copper nozzle, silicone sock, type K sensor with matching daughterboard on the Duet. Additionally, there's a PT100 daughterboard which monitors the heatbed and enclosure heater. The enclosure will reach well above 100°C and is suited to do so, so nothing to worry about there. The leads from the print head are guided in glass fiber sleeving. I've set this up just this morning, so the parts aside from the heater were all new (I changed out a PT100 sensor from the hotend as its insulation started to get crispy at around 400 °C and just crumbled away - you shouldn't use these at this high a temperature, it seems).

    My problem is as follows; temperature will rise just fine on the hotend up to about 280-290°C. From then, readings will start to jitter and then flip out, giving me the occasional 2000°C which means a short to something else (iirc).

    0_1523959720112_heater fault 1.PNG

    so the heater goes into fault, from then the reading will slowly and jittery creep to the actual temperature and just be fine again (see above).
    So i cleared the fault, tried again and well, it did the same thing.

    1_1523959720112_heater fault 2.PNG
    (second and third time)
    I then tried to tune the heater to 350°C which did not fit the controller model as it seems;

    4_1523959720113_heater tuning 2 - fault.PNG

    tried again at S250

    5_1523959720113_heater tuning 3 - succ.PNG

    which worked okay, but i could see what i suspected until now, that the misreadings are somewhat in connection to the heater being on or off -- as you might be able to tell from the S350 run, as soon as the heater is turned off in the graph above readings become jittery for a few seconds and then smooth out.

    I decided to give it a go and preheated the machine, the chamber heater's reading is straight from the heater itself, don't be alarmed! Actual chamber temperature is dotted red line.

    2_1523959720112_heater fault 3.PNG

    What puzzled me is that temperature drops to "straight" values such as 300°C, 250°C while it's spazzing out (see above). Next try to no avail:

    3_1523959720113_heater fault 4.PNG

    And again what's highly curious is that the reading slowly creeps up to the actual temperature after it's been all over the place. I can't imagine that the sensor would change its resistance in this way.

    The heater faults to "short to other wiring" which I didn't check yet but find rather unlikely as I've just put this all together. I've checked that the leads of the heater and the sensor don't touch the cartridge because that would cleary mean a short of sorts...
    I'm just puzzled by the repeatability of the thing, that it would start to spazz out just below 300°C...

    any pointers?

    thanks in advance for your time!

    Best regards, Niklas

  • I was having vary similar issues as this with a E3D sensor, it was not always a spike but a 20-30c error at times then other times it would spike to the max. Replacing it solved all issues even though it was a new barely used sensor.

  • I had similar issues with my E3D PT100 sensor.
    Had to remove the molex connector and crimped a new one and now its working fine.


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