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Temperature reading is about twice actual temp

Posted by ivanstein 
Temperature reading is about twice actual temp
February 06, 2021 01:27PM
Greetings all,

Ender 3 pro
SKR Mini E3 V2
MicroSwiss hot end.

About two weeks ago I had an extrusion overflow and meltdown that looked like a mushroom growing on the hot end.
After cleaning it, I had to replace the heater cartridge due to broken wires. I also replaced the thermistor at the time.
Since that change, the displayed temperature is about right at room temp (25C = 25C) but as the requested temperature rises, the actual temperature lags behind.
Thus, at 200C displayed temp, the resulting hot end temperature is about 98C.
The most I can get out of it is 125C at max printer set temp of 275C.

Steps taken
1. I have tried about every table applicable in the firmware. I didn't use any of the PTC ones though.

2. I have tried to make my own table which results in less than one ADC count per 5C over about 150C. Doing this matches temperature well, but there isn't enough resolution above about 140C to get a stable temperature and the printer errors out. I cannot get to 200 using the table.

3. I have tried different thermistors from different vendors. I have tried different heater cartridges. I have replaced the SKR Mini board. I have replaced the hot end assembly to the original.

4. I have checked the calibration of the thermistor and have the following results (for all 10 of the spares, from different vendors nonetheless)
116200 ohm @ 18.5C
6750 ohm at 100C
1500 ohm at 200C
These result in a 3800 beta value per this thermistor calculator.
When putting the above values in that calculator, the slope of the curve at anything above 150C is nearly horizontal, explaining the reason I am where I am.

5. I have done multiple PID tunes, ensured I have reset the board after firmware flashes and saved to EEPROM, to assuage the impending "did you do a PID tune" questions.

6. I have verified 3.3V at the thermistor pins without the thermistor plugged in.

Further discussion:
I was able to resuscitate my original thermistor and try that. It resulted in the same issue.
I usually don't like fixing things by throwing parts at the problem, but my diagnostic capabilities with new electronics is low, and, to be honest, I don't have any more parts that I can throw at it.

Any help on this issue would be greatly appreciated. I am at a complete loss. It appears the board is assigning the wrong ADC counts to the resistance values, but I haven't figured out how to convert ADC back to ohms.

I have attached my configuration files and a screen shot of the temperature curve generated using my measurements.

Edited 1 time(s). Last edit at 02/06/2021 01:30PM by ivanstein.
open | download - Marlin_zip.zip (73 KB)
open | download - Untitled.png (47 KB)
Re: Temperature reading is about twice actual temp
February 06, 2021 06:27PM
It just sound like the thermistor you got is designed for a maximum temperature of 150C
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