Adjusting Thermocouple Temperature?

[sgraber]

Last week Jim was helping Phil adjust the temperature reported by his thermocouple on his RostockMAX.  Jim/Phil: how did you do this?  The reason I ask is that I put an old PrusaNozzle v.2 on my i3 and I don't have any idea what thermocouple they used. I know it's under-reporting by at least 50°C based on my thermocouple reading.  I'd like to adjust this in Marlin, but I'm not sure where or how to do this.  Any pointers?    

[ZionPhil]

You will want Jim to help you on this one, but I connected to the board through repetier to watch the temperature, and then Jim did a Gcode command to cause the hotend to go into PID tuning. From 3 readings of temps, they were averaged and then put into the EEPROM settings through Repetier where it wrote it live to the board. Then based on what we read from the temperature prob, we knew we where almost dead on. Seems like this has been reflected in my printing also. I am back to where I was before changing the entire hot end with thermistor.

[jimustanguitar]

We did 2 processes on Phil's printer. One was a PID autotune that more precisely matches the heater control PID to the heater itself, and the other process was changing the thermistor coefficient in the firmware to better match the actual temperature as measured with a thermocouple.

To adjust the themistor table, (#define GENERIC_THERM1_BETA 4450) you rig up a way to read the actual hot-end temp with a thermocouple, compare it to the Rambo's displayed temp, and calculate what percentage to change the 4450 value by in the firmware... Lather, rinse, repeat until you're happy with accuracy of the results.

For PID autotune, M303 S230 will start the routine (S230 refers to 503.15 Kelvin so you can adjust your target temperature as desired). You want to do this from a cold start with everything at room temp. A routine built into the firmware will run a heat/cool cycle up and down a few times to figure out what input equals what result with the heater. When it's done about 5 minutes later, you'll see 3 data sets in the serial log (a P, I, and D value) that you want to average together and enter into your EEPROM. I've found that doing this process 2 or 3 days in a row will really zero in on getting your heat just right.

[jimustanguitar]

And M303 P1 S80 (or whatever temp you choose) will do the PID autotune on the heated bed.

[sgraber]

I'll give the PID adjustment a shot tonight along with adjusting the thermistor reading. Interestingly enough, Marlin does not have #define GENERIC_THERM1_BETA as a firmware setting. That's strictly in Repetier.

[jimustanguitar]

It could be called something else. I copied verbatim what is in Repetier, but I'm sure there's some sailfish equivalent.