Adjust bed temperature



  • Is it possible to adjust the bed temperature such that when at present it says 65 it would report 60. This has to do with the thermistor being in the heater and the top surface never gets to 60 when I set it to 60 -- always around 55-58.



  • why not set the bed temperature to 65 then?


  • administrators

    @veti said in Adjust bed temperature:

    why not set the bed temperature to 65 then?

    That's exactly what I do: use a higher set point to allow for the temperature drop across the glass.

    You can play with the B value in the M305 P0 command to change the reading if you like, but you won't get accurate readings over the whole temperature range that way.



  • @dc42 I'm asking because i have a similar issue (but in reverse --bed is hotter than thermistor says) on another machine which runs smoothie, and there I simply defined the thermistor with 4 pairs of resistance and temperature values -- that defines a curve and it's accurate now all across the board. Wondering if there is a way to do that (basically marlin's version of a temp table)



  • @kazolar said in Adjust bed temperature:

    I'm asking because i have a similar issue (but in reverse --bed is hotter than thermistor says) on another machine which runs smoothie,

    According to the duet3d gcode documentation , M305 accepts A,B,C parameters that it plugs into this resistance to temperature formula https://en.wikipedia.org/wiki/Steinhart–Hart_equation

    I would think that if you have a few calibration resistance-temperature pairs, you can compute A,B,C values that best achieve those calibration points and plug them in your M303 command.



  • @kazolar However you do it, it's only going to be a "guestimate" because you are trying to get a predicted measurement of a point that is some distance away from the point of measurement. How are you measuring the top surface temperature and how do you know that's an accurate reading? It's very difficult to do.

    If you have thick aluminium plate, then you could always do what I do and drill a deep hole in the side of the plate, as close as possible to the upper surface, and put the sensor in there.



  • @deckingman I used a few different sensors on the top surface and see that the bed is close to target 60, but about 5c below on average. When set to 65, it's right, and that seems to be the magic temperature for everything I've printed so far -- at least the size of the build plate. May just set 65 in the S3D profiles and forget about it...



  • @kazolar
    since every machine is different you need to find the right temperature for each filament for your printer.
    there is no right temperature for everyone.


  • administrators

    @zapta said in Adjust bed temperature:

    @kazolar said in Adjust bed temperature:

    I'm asking because i have a similar issue (but in reverse --bed is hotter than thermistor says) on another machine which runs smoothie,

    According to the duet3d gcode documentation , M305 accepts A,B,C parameters that it plugs into this resistance to temperature formula https://en.wikipedia.org/wiki/Steinhart–Hart_equation

    I would think that if you have a few calibration resistance-temperature pairs, you can compute A,B,C values that best achieve those calibration points and plug them in your M303 command.

    If you go to https://configurator.reprapfirmware.org/Heaters, click on the R25 value for the thermistor and then select "Custom" from the list, it will do the calculation for you from three resistance/temperature pairs.


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