Thermistor calibration

Comments

• Repetier

  October 2020

  For thermocouples it is in extruder.cpp line 2555
  

  case 60: // AD8495 (Delivers 5mV/degC vs the AD595's 10mV)

  #if CPU_ARCH == ARCH_AVR

  currentTemperatureC = ((float)currentTemperature * 1000.0f / (1024 << (2 - ANALOG_REDUCE_BITS)));

  #else

  currentTemperatureC = ((float)currentTemperature * 660.0f / (1024 << (2 - ANALOG_REDUCE_BITS)));

  #endif

  break;

  case 61: // AD8495 1.25V Vref offset (like Adafruit 8495 breakout board)

  #if CPU_ARCH == ARCH_AVR

  currentTemperatureC = ((float)currentTemperature * 1000.0f / (1024 << (2 - ANALOG_REDUCE_BITS))) - 250.0f;

  #else

  currentTemperatureC = ((float)currentTemperature * 660.0f / (1024 << (2 - ANALOG_REDUCE_BITS))) - 250.0f;

  #endif

  break;

  
  You see here is no correction. It is just conversion of voltage to a linear equation from the sensor type.
  
  But that would be the place where you can modify currentTemperatureC with some equation after computation. Then the corrected temperature would be used instead.
• leadinglights

  October 2020

  Thank you Repetier.
  I have now checked out the second hot-end/thermocouple/AD8495 plus a spare and I get a bit of a scatter from 4% low to 1% high on slope though somewhat more consistent on offset. This seems to be within the believable sum of errors for the class 2 thermocouple, Adafruit conditioning board, and Mega2560 voltage reference.
  I think the best answer is to go old school and build replacement conditioning boards with slope and offset trimmers.
  
  Mike
  
• Repetier

  October 2020

  I forget one option. You can add gain/bias support to calibrate it. Requires latest dev version as I just fixed a bug there. Then you mod temperature like
  newTemp = bias + oldTemp*gain
  
  Default is bias 0 and gain 1.
  You need to have TEMP_GAIN 1 defined in configuration for this.
• leadinglights

  October 2020

  Thank you Repetier, that now works better than I could have hoped. Adjusting the gain and bias through the EEPROM I was able to set up within minutes and it now is within 1 degree from 150 degrees to 250 degrees on both hotends.

  
  

  Mike