Duet 0.8.5 temperature accuracy

  • I am trying to tune the temperatures for my hotend and heated bed and have had a little trouble getting what appear to be accurate values when cool. I am coming from smoothieware, so the differences are just enough that I am getting confused. I don't know whether my board is/was stickered "4.7K", but changing the R1000 values to R4700 makes the ambient tempertures of the hotend and heated bed more realistic. With R1000 the temperaturess are 50 and 54 C, With R4700 they are about 16 and 18 C. My basement is cool, but not quite that cold.

    I have had to use H30 in each of the M305 lines to get the readings closer to what I measure with a thermocouple. I thought the H value was an offset in degrees- is it a percentage or some other kind of coefficient?

    ; Thermistors
    ;*** If you have a Duet board stickered "4.7K", change R1000 to R4700 to the following M305 commands

    M305 P0 T100000 B3950 C0 R4700 H30 L0 ; Put your own H and/or L values here to set the bed thermistor ADC correction

    M305 P1 T100000 B4725 C7.060000e-8 R4700 H30 L0 ; Put your own H and/or L values here to set the first nozzle thermistor ADC correction

  • administrators

    All Duet 0.8.5 boards use 4700 ohm series resistors. It is only older Duet 0.6 boards and older DueX4 expansion boards that use 1000 ohm series resistors.

    On the older Duets (0.6 and 0.8.5) it is not unusual to need to adjust the H parameter to get accurate results at low temperatures, and the L parameter to get accurate results at high temperatures. The H value is not an offset in degrees, it is the offset in ADC counts. A value of 30 is within the typical range. In theory, all thermistor inputs should need the same value.

    The generation 2 Duets have self-calibrating ADCs and do not normally need any corrections at all.

    HTH David

  • Thanks for the help. If I hadn't reversed the wire connection on my heatsink fan last night (I have been doing way too much of that lately) and gotten melted filament jambed up in the heat break, I think I might be up and running. I will put everything back together today before I get too tired to make solid decisions.

Log in to reply