Adjust M305. What am I missing?

  • hi,

    after read all info that you have pass me in previous post... and try with little success adjust. I have done a test area.
    Although now I am very close, or at least more than before, it does not end well. I put here the method that I have used, so that you tell me that it is wrong, or that I can do better

    What have used:

    • Old heatblock
    • 50W 24v Heatcardrige
    • Termistor (for control heatcardrige), now T1
    • Termistor, that i want configure in DUET, now T2
    • Kitchen termometer, only reach 250 degrees
    • Multimeter


    01- First drill in old chinesse Volcano heatblock a hole for T2
    02- Mount in heatblock
    02.A- 50w 24v Heatcardrige, connected to DUET
    02.B- Termistor, T1. Connected to DUET
    02.C- Termistor, T1. Conected to Multimeter
    02.D- Kitchen termometer, in nozzle hole
    Test setup.jpg
    03 - Power on Duet, and connect to DWC with a PC
    04 - I measure the temperature and the resistance (with Kitchen termometer and multimeter), at rest.
    Real temperature = 28 - Resistance 84900 Omh
    05 - With DWC raise temp, and measure resistance, and do a table

    DWC Temp            Real Temperature       Resistance
                                     28                    84900
    65                               73                    11900
    120                             132                     2200
    181                             201                      431
    201                             220                      275
    220                             237                      188

    Cant raise more temp, because the thermometer only reaches 250 degrees

    06 - With these info, go to Steinhart thermistor model, and put 3 temps, lower, one midrange, and higher, the web gives me B=4285, change temperature resistance, and the result its similar


    07 - Put these valour in config.g M305
    08 - Check if the temp its ok, and its near, but no, the new results are:

    DWC Temp            Real Temperature       Resistance
                                     28                    84900
    180                              199                     468
    236                              244                     165

    Whats its wrong? Maybe C parameter in M305, but wiki say thats its by default 0, and i have see in some config.g... and its near 0. How affect?

    Cnnn If the sensor is a thermistor, this is the Steinhart-Hart C coefficient, default 0

  • Steinhart–Hart is only used if you specify C. With C=0 the beta model is used.

  • What model its better? and how can find C?

  • Steinhard hard will give better results over a wider range.

    go to
    click on the beta value. Select custom and put in your temperatures and resistance values.

  • @Veti


    Strange values...

    RRF configurator

    R25 : 95437 Ω
    β : 6908 K
    C : 3.606903e-7

    put these values in config.g M305,

    M305 P1 T95437 B6908 R4700 C3.606903e-7	


    DWC___________Measured TEMP
    220 _____________ 241
    178______________ 199

    more temperature difference betewn measured and showed in Duet

  • @peirof said in Adjust M305. What am I missing?:


    there might be a insecurities in your measuring equipment because this number should be 100000 or very close to it.

  • Moderator

    How are you actually getting this setting to be used? Resetting the board? You can send the M305 setting direct via the console, and this should take effect immediately.

    The temperatures reported don't look like they're changing much after you change the settings, ie within the variation of measurement. If you're putting these figures in config.g, do you have a config-override.g that is being called at the end of config.g, and an old setting is in that? You also didn't post what the thermistor setting originally was.

    I get different results to you using the RRF configurator and the SRS Thermistor Calculator, but then they also differ from each other!
    Screenshot 2019-09-25 at 12.07.12.png
    Screenshot 2019-09-25 at 12.07.51.png

    However, evidence would seem to point to you having a 100k thermistor, beta of 4285. That should be enough to use the Beta model, so send this at the gcode console and test:

    M305 P1 T100000 B4285 R4700

    R4700 assumes you have a Duet Wifi/Ethernet rather than Maestro.


  • @droftarts said in Adjust M305. What am I missing?:

    However, evidence would seem to point to you having a 100k thermistor, beta of 4285.

    from his previous post he has 3 semitec and 2 B3950 thermistors.

  • Moderator

    @Veti thanks, I didn't see the previous post.
    @peirof Remember you can send M305 P1 (with no other parameters) to check what the thermistors are actually set to, to confirm that any changes you make are in use. Check for config-override.g being used, and over-riding the settings you're putting in config.g.


  • Thanks to all,

    Tomorrow try and give feedback... I al not in home

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