Trying to configure my thermistor, please help



  • I built a CoreXY with a Duet 0.8.5 board.

    I am trying to do adjust the M305 for the hot end and bed heater and if I turn off my printer, unplug my sensor, I can't get the error… I used M305 H-100 and I remain at -2 to 0...

    Correcting the temperature calibration of Duet 0.6 and 0.85. boards
    The ADC on the Duet 0.6 and 0.8.5 is not calibrated automatically, so you may need to calibrate it in order to get accurate readings, especially at low temperatures.Here's how:

    With the machine cold, see what temperatures the are displayed for the bed and hot end(s), and compare that with room temperature.
    Add H parameters in the M305 commands to get readings close to room temperature. If the readings are too low, use a positive H parameter to bring the readings up to the correct value. If the readings are too high, use a negative H parameter. All heater numbers should need the same value of H parameter - any residual differences you see are probably caused by thermistor tolerances. You should never need to use an H parameter below -100 or above 100, and it is rare to need a value outside the range -30 to 30.
    Very important! Check that if you disconnect the thermistors, the temperature in the web interface reads as error. If instead it reads a very low temperature, reduce the H parameters until it does report error, then reduce them a bit more.

    Also, what do you use to measure the temperature of the hotend at high temperature (for the L)?

    Thank you


  • administrators

    In most cases you can leave H and L set to zero. An H value of -100 is outside the normal range.



  • My method, especially for an unknown thermistor, is to measure the hotend actual temp using a k-type thermocouple probe which comes as an accessory with a decent multimeter. Use a drop of thermal compound on the end of it. Place it against the heater block or if you have a spare hole in the block shove it in there.

    Now set 190 deg C and see how close you are. If you are a few degrees either way then your B value is probably fairly close. Now try 240 deg C, again if you are within a few degrees, then you are there.

    If you aren't close then you can either do:
    a) the proper way. Chose two temperatures, 190 and 240 will do as these are temps you're likely to print at. Get the actual temp up to 190 (not the temp on the duet/web interface) and then measure the resistance of the thermistor at that temp, write it down. Now do the same at 240 deg C actual temp.
    Now use a calculator to derive a B value this spreadsheet will do it for you https://1drv.ms/x/s!Apv79JfGbPIwgv9DIbujMxNtMwHQpg enter the resistance in ohms for each temperature setpoint. Now use the B value you get in M305. See if it gets close enough at the temperatures you want to use.
    b) the other way, get up to a temperature and then send M305 P1 Bxxxx start at 3950, and see how it affects how close the temperature displayed is to the actual measured temperature, it takes a few seconds to stabilise after changing it. In this way you can get a fairly close match, but you have to try it at two different temps and set a B value that gets you closest at each.



  • Thank you for the suggestion, will do the testing, I actually have a PID controller with a K-type Thermocouple probe for a different project.

    Very important! Check that if you disconnect the thermistors, the temperature in the web interface reads as error. If instead it reads a very low temperature, reduce the H parameters until it does report error, then reduce them a bit more.

    How much of an issue is it IF I can't bring error while trying the Very important check above?


 

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