Thermistor calibration

leadinglights

Is there a way to adjust the calibration of a thermocouple sensor? My readings have a slope of about 4% too low with an offset about 3.6°C low: At a hotend body temperature of 100° Repetier reports 92.3°C and at 220°C Repetier reports 207.8°C
Specifics are: RAMPS/Mega2560, Two hotends with type K thermocouples and Adafruit AD8495 thermocouple conditioners, Repetier Firmware 0.92.9

Thanks for any help or advice you can give,

Mike

Repetier

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

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

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

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