PT100 temperature too low on duet 0.85



  • Aaaargghh:

    Because thermocouple didn't work out on 0.85 i changed to a pt100 with a MAX31865 breakout.
    I have a pt100 with 3 leads. i changed the jumpers on the MAX31865 breakout board as explained in the manual.
    https://cdn-learn.adafruit.com/downloads/pdf/adafruit-max31865-rtd-pt100-amplifier.pdf
    page 19

    connected the pt100 as explained here: https://www.duet3d.com/wiki/Connecting_PT100_temperature_sensors
    CS 27

    config m305 P1 X200

    when i connect to the duet it gives me a reading of 0,9 degrees.
    The moment i set the temperature of the extruder to 110 degrees, it jumps directly to 1 degrees and stays there. Some seconds later i get an error which states the temperature is rising too slowly.

    What is going wrong?

    It would be nice to finally get this tempreading working.


  • administrators

    The page on connecting PT100 sensors says to use a PT100 interface board with a reference resistor of 400 ohms. The board you have purchased uses a reference resistor of 430 ohms.

    Fortunately for you, the latest 1.20beta firmware allow you to configure the value of the reference resistor, using parameter R in the M305 command. So add R430 to your M305 P1 X200 command, and upgrade to the latest beta firmware.



  • I installed the beta and now i got some strange readings.
    At first everything looks ok. i got an initial reading of 21,7 which is ok.
    The moment i start heating the extruder it starts fluctuating between its effective temp and 2000 degrees (which is an error code)
    arround 50 degrees it stops heating and keeps fluctuating.
    I have to power off the printer to be able to start heating the extruder again.

    Has this anything to do with the beta or with the pt100?

    EDIT: I expect that interference is the culprit. I tried M18 to no avail, i will now try to lead it separately to the deltahead and see if this gives an improvement.



  • Still spiking:

    • I reconnected the leads from the pt100 to be sure they were correctly connected

    • l redirected the pt100 lead so it would nowhere go alongside one of the other leads (extruder-stepper / leveling servo)

    • i started with M18

    Every time i got at 30 degrees i got my first spike, then it intervals (like a square block signal, with longer times at 2000 then at the normal temp) around 42 degrees the heater stops heating and i can't reactivate it without power cycling the printer.



  • I got most spikes solved by hanging the leads from the pt100 straight down. This is not feasable for working but makes clear it is probably an interference problem. I have rather long leads (2m) i will try to shorten them as much as possible and look for shielded cable for al the stepper, servo and heater cables.

    BUT his still doesn't solve my problem of the extruder heater disabling itself.
    Every time when i start the heater, it starts heating untill a temperature around 40 degrees (it varies) and then will stop heating. This happens also when i don't get temp spikes. Could this because of the beta?

    Do i have to change also some configfiles on the SD-card when i install the beta? i come from version 1.17.


  • administrators

    When the heating stops, do you get an error messages and does the heater to into the Fault state? If so, then either the interference is too great, or you haven't auto-tuned the heater.



  • Thanks David,

    finally got everything ok.

    It was a combination of a faulty heater and forgetting to autotune again.

    Changed the 3-wire pt100 to the 2-wire pt100 from e3d. Solved almost all the interference
    changed the heater from the extruder. It was faulty and didn;t heat enough.
    Changed to a new one and added a silicone cover.
    And finally after doing the auto-tuning it finally worked.
    Hopefully tomorrow back in business again.

    Next project: adding the delta effector 🙂



  • Just reading this thread, I wondered if I might ask how do we feel about using the R parameter to "offset" the temperature so that nozzle temp is achieve rather than pt100 temp.

    For clarity what I mean is measuring the temperature at the nozzle with a type k thermocouple probe (and transfer paste), and then adjusting the R value so that the actual nozzle temp matches the indicated temp in DWC/paneldue?

    I changed my R value to 390 to achieve 250 deg C nozzle temp for 250 deg C indicated? Where as prior to that 250 indicated was only 240 nozzle temp.


  • administrators

    The PT100 temperature reading should be accurate, but the difference between PT100 temperature and nozzle temperature will depend on several factors, including how high the temperature is, how much airflow the nozzle is in, whether a silicone sock is fitted, and how fast you are melting plastic. So IMO it's better to just make a note that on this printer under the conditions you are using, the filament needs an indicated temperature of 250C rather than the manufacturer's recommendation of 240C.



  • Thanks, given that unlike thermistors the response of the pt100 is linear, and given that nozzle temp is the temperature we are interested in (although I suspect filament manufacturers recommended temps are for typical hotend setups) I'll give this a try and report back.


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