thermisor calibration ADC values Marlin 2

I believe I have 100k beta 3950 1% thermistor (4.7k pullup) thermistor but I am not actually sure what thermistor came with my hotend. After searching for a bit I found that I should most likely choose

#define TEMP_SENSOR_0 11 // 11 : 100k beta 3950 1% thermistor (4.7k pullup)

In Marlin 2. To test and calibrate the thermistor I have removed the nozzle and stuck thermocouple connected to multimeter. After connecting with pronterfece I got this:

I am totally confused. The temperature shown by pronterface is almost 200*C, but the ADC value is 557. If the reading is 200*C shouldn't reported ADC be 141 as it is in the table for thermistor_11.h ?

temperature measured with the thermocouple was actually 187*C

I thought that I would just replace ADC values and temperatures in thermistor_11.h with the actual temperatures measured with thermocouple and corresponding ADC values reported by pronterface, but the differences are so big (at 25*C reported ADC is 3900, but termistor_11.h table shows that it should be about 950) that I am no longer sure if what I am doing makes sense.

Being able to measure the actual temperature of the hot end how do I actually go about calibrating the thermistor?

Edited 1 time(s). Last edit at 05/09/2020 11:35AM by witor.

As already answered on discord.

You're on a platform with a 12 bit ADC - you're getting a reading of 557 out of 4096. This gets adjusted to 10 bits (i.e. divided by 4) to give 139 out of 1024 which is what is being looked up in the table.

Thank you. I asked here before I discovered marlin discord server.

Was more for anyone else who had the same question, now they also have the answer.

Nothing worse than looking for an answer and finding the same question with no answer.

So just to make things absolutely clear, In my case (I am using SKR 1.3) I need to divide ADC values reported by pronterface by four and then enter the result into thermistor_11.h, Is that correct?

Do I have to have the same number of rows of ADC values and temperatures as in the original thermistor_11.h?

It counts the size so no need to keep same number of rows.
but do keep number under 255 entries.

Anyone who has a cheap Chinese clone of V6 with what they claim to be 100 kOhm, beta25 = 3950 K, 4.7 kOhm pull-up thermistor might find this useful.

I have calibrated my thermistor and the results are shown on the graph below. Blue line shows default Marlin thermistor_11.h values, orange line shows my results. As you can see at 200*C there was more than 10*C difference between temperature read by the printer and the temperature measured with thermocouple.

I have modified one of the thermistor files in marlin with my results (thermistor_11.h - attached)

If you have a similar hotend and possibly thermistor and if you suspect that your temperature readings are not accurate you might give my results a go. Make sure to back up the original thermistor_11.h so you can revert to it if needed.